B ULLETIN OF THE FLORIDA STATE MUSEUM BIOLOGICAL SCIENCES Volume 3 Number I COMMENTS ON THE OSTEOLOGY AND PHYLOGENY OF AMBYSTOMATID SALAMANDERS J. A. Tihen Af,'- i.~&*Al i UNIVERSITY OF FLORIDA Gainesville 1958 The numbers of THE BULLETIN OF THE FLORIDA STATE MUSEUM, BIOLOGICAL SCIENCES, will be published at irregular intervals. Volumes will contain about 300 pages and will not necessarily be completed in any one calendar year. WILLIAM J. RIEMER, Editor ROLAND F. HUSSEY, Associate Editor All communications concerning purchase -or exchange of the publication should be addressed to the Curator of Biological Sciences, Florida State Milseum, Seagle Building, Gainesville, Florida. Manuscripts should be ·sent to the Editor of the BULLETIN, Flint Hall, University of Florida, Gainesville, Florida. Published 20 June 1958 Price for this· issue $.55 COMMENTS ON THE OSTEOLOGY AND PHYLOGENY OF AMBYSTOMATID SALAMANDERS J. A. TIHEN 1 SYNOPSIS: The family Ambyhtomatidae is a strictly Nearctic family, probably derived from the Asiatic, Hynobiidae in the late Mesozoic or very early Tertiary. Osteological evidence indicates that the P.acme Coast forms Dicamptodon and Rhyacottiton were derived very early, and ihdependently.· from the proto-amby- stomatid stock. These forms are so distinct from each-~ other and from the re- mainder of the family that recognition of three separate subfamilies is proposed. These are the Dicamptodontinae, including Dicdmptodon and, presumably, the fossil genus Ambystomichnus; the Rhyacotritoninae, including only the genus Rh!/- acotriton; Fand the Anibystomatinae, includidg all other forms. Two well-defined genera are recognized in the subfamily Ambystomatinae.. One of these, Rhyacosiredon, contains. only four species. The other, Ambystoma, is in turn divided into three: subgendra, Ambystoma, Bathystredon, and Linguae- lapsus. Four imperfectly delineated species groups are recognizable in the sub- genus Amb!/stoma-the mexicanum group (which includes the fossil species kansense), the tigrinum group, the opacum group, and the maculatum group. It is postulated that Dicamptodon.and Rh!/acotriton developed 'from a branch of the original prot6-an',bystomatid stock that established itself in the western part of the continent very early in the history of the family, while the primary center of dispersal for the remainder of the family was in the northeast. Estab- lishment Of. this eastern group probably preceded the Miocene, with the various species groups being developed not later than the early Pliocene, and many of the extant species by or in the early PleiStocene. ' , INTRODUCTION The family Ambystomatidae 2 is a strictly Nearctic group of sala- manders, ranging geographically from southern Alaska and extreme 1 The author' is, at present a Research Associate in the University of Florida' School of Medicine, under an Atomic.Energy Commis'sion contract. He has been particularly interested in the paleontological history of. living amphibian and reptile groups._ The present paper is an outgrowth of attempts to identify, and detefnine the relationships of, ambystomatid salainanders represented by remains recovered . from yarious Pliocene and PIeistocene fossilifefous depdsits. Manuscript submitted i8 May 1958.-ED. ' This emendation of the familiarname Ambystomidae is mandatory under Paragrabh 50 (1), Sections (a) and (b), of the 1958 "Copenhagen Decisions on Zoological. Nomenclature" (page 84). Section (a) prescribes the method of forming Family-Group names based on generic names of Greek or Latin origin; its provi- sions require in the present instance that -stomat- be used as the root to which the ending -idae is affixed. Section (b) requires "automatic" correction of a Family-Group name which, when first published,.was incorrectly formed in con- travention of (a). The emendation Ambystoma~inae must similarly be used for the subfamily name, Though the "Copenhagen Decisions" made these provisions specifically applicable only to Family-Group nkimes,-from subfamily to superfamily, : consistency requires that the subordinal name also be emended, to Ambystomatoi- dea. 2 BULLETIN FL~RIDA STATE MUSEUM Vol. 8 southern Labrador southward to the southern part of the central plateau of Mexico, and temporally from possibly the late Mesoz6ic, certainly from the very early Tertiary, to the present. No fossil records are known from outside of the present range of the family; it has probably been confined to North America throughout its entire geo- logical history. The family, including fossil forms, contains between 30 and 85 currently recognized species. Certain broad relationships have gen- erally been recognized at various supraspecific levels, but there has been little effort to define these supraspecific categories accurately, to delimit them clearly, and to determine their relative significance. The present study will treat such supraspecific groupings as tliey are indicated by their osteology. Such a restricted approach naturally has its limitations, and the conclusions reached must be considered tentative, Subject to modification on the basis of evidence from other sources. On the whole, however, I believe the interrelationships to be rather accurately recbgnizable on the basis of osteology. No at- tempt has been made to investigate problems bearing on the status. or- definition of individual forms; only the supraspecifc relationships will be considered. Subspecific or specific status in the listings given is based for the most part only, on the apparent current consensus, and not on any additional evidence derived from the present study. In this respect and in matters such as synonymies, ranges, genotypes, and references to Original descriptions I have drawn freely on the check lists of Schmidt (1958) and Smith and Taylor (1948). I wish to acknowledge my indebtedness to the following persons for assistance .in procuring the -skeletal material for study: C. M. Bogert of the American Museum , of Natural History; Herndon G. Dowling of the University of Arkansas; Denzel E. Ferguson of Mis- sissippi State College; Coleman J. Coin and Arnold B. Grobman of the University of Florida; Wilfred T. Neill of the Ross Allen Reptile , Institute; the late Karl P. Schmidt of the Chicago Natural History Museum; Hobart M. Smith of the University. of Illinois; Edward H. Taylor of the University of Kansas. C. W. Hibbard of the University of Michigan Museum of Vertebrate Paleontology and Robert Wilson of the University of Kansas Museum of Vertebrate Paleontology have kindly permitted the 6xamination of fossil material in the collections 6f their respective institutions. I am also grateful to Goin, '.Smith (who also provided fadiographs of certain Mexican forms), Taylor, and to Walter Auffenberg.of the University of Florida for their assistance in numerous other respects during the course of this study. 1958 TIHEN: AMBYSTOMATID SALAMANDERS 8 In order to facilitate the preliminary discussion, the classification that I have adopted is outlined below. An asterisk following the name of a form indicates that I have seen only immature (nonneo- tenic) larval specimens of that form; a double asterisk indicates that I have not been able to obtain any specimens for examination, a dag- get (t) indicates that no skeletons have been seen, but that soft-ray tadiographs have been examined. Subspecies are not included in this preliminary classification. ' Suborder: Ambystomatdidea Ambystoma Buvinatum** Family: Ambystomatidae Ambystoma g,·anulosumt Subfamily: Dicamptodontinae Ambustoma hibbardi Genus: Ambystomichnus Ambystoma lacustris 00 - - Ambystomichnus Ambystoma ordindrium montanensis** Ambystoma rosaceum .0 Genus: Dicamptodon Ambystoma subsalsum Dicamptodon ensatus Ambystoma tigrinum Subfamily: Rhyacotritoninae The opacum group , Genus: Rhyacotriton Ambystoma opacum Rhuacotriton olympicus Ambustoma tatpoideum Subfdmily: Ambystomatinae The maculatum group Genus: Rhyadosiredon , Ambystoma gracile RhyacoNredon altamirani Ambystoma ie#ersonianum Rh!/acosiredon Zeorae Ambystoma Zaterdle Rht/acosiredon noutaris Ambustoma, *wcrodactylum Rht/acosiredon zempoalensis *0 Ambustoma nwcutatum Genus: Ambystoma Subgenus: Linguaelapsus Subgenus: Ambystoma Ambystoma annulatum The mexicanum group Ambystoma cingulatum Ambvatoma kansense Ambystoma mabeei Ambystoma Zermaensis Ambystoma schmidtit Ambystoma mexicanum Ambystoma texanum The tigrinum group Subgenus: Bathysiredon Amb!/stoma ambZucephalum0 Bath{/siredon dumeriti Ambustoma bombupellum* The family is characterized by the rather frequent occurrence of partial or complete',neoteny. Ne6teny may occur in occ~sional indi- viduals of many of the species, in large populations of some forms, and is apparently universally obligatory in certain forms. Some dis- cussion of certain of the morphological consequences seems desirable. In general, the term neoteny is used here to refer to a situation in which sexual maturity is attained, even though the individual involved retains the external gills and other essentially larval features. L6ss of the gills is frequently used as the sole criterion of metamorphosis, but it is; of courses only one of many aspects of the metamorphic process. All systems of the· body undergo extensive metamorphic 4 BULLETIN FLORIDA STATE MUSEUM Vol. 3 - changes. Retention of any larval feature, even though the individual may be typically adult in all other respects, can be considered as a sort of partial neoteny. It is often assumed, probably correctly, that a number of morpho- logical features of the larvae of existing forms are at least roughly compardble to the adult condition in some ancestor of these forms. For example, in the Hynobiidae and some primitive Ambystomatidae, the columella is free,, not fused with the otic elements. 3 The same condition occurs in the larvae of all Ambystomatidae, but in adults 6f most forms 0 fusion with the otic does occur. , This fusion is dlearly a characteristic which has developed phylogenetically within the family and develops ontogenetically within individuals of certain f6rms. Other similar examples could be cited.. Since neoteny, or partial neoteny, is itself usually a secondary de- velopment-a specialized characteristi~, so to speak-it is easy to conceive of situations in which retrograde evolution of a particular feature might appear to occur. In the example given, if some form of Ambystoma should exhibit partial neoteny only in respect to this one feature, then, in that particular case, the failure of the columella to fuse with the otic represents an evolutienary stage beyond the fused condition. - In general, when neoteny and partial neoteny occur in a group, the direction of evolution of individual features of this sort (i.e., those in which ontogeny more or less parallels phylogeny) cannot be definitely determined per se, but must be inferred from evidence from other sources concerning the direction of evolution in the 5eries of forms being compared. A form appearing to be primitive may actually be so, with progressive modification leading to more special- ized conditions in related forms. But it might also be possible, that the apparently most specialized form is actually, as regards this partic- ular group, the:most primitive, with progressive partial neoteny lead- ing to the conditions found in the other forms. As a further compli- cation, it is even conceivable that an inte~mediate condition is an- cestral, with progressive partial neoteny leading along one line to the apparently more, primitiye conditions, while progressive modification leads along another line to the more specialized conditions. Because of these considerations, features in which there is, or is presumed to be, a very cloke parallel between phylogenetic history and ontogenetic history do not of themselves provide much informa- tion concerning the phylogeny of the forms inVolved. Conclusions concerning phylogeny should be based, insofar as possible, on char- , acteristics that can be shown to be relatively independent of this prob- lem. For example, the parasphenoid varies from one group to another 1958 TIHEN: AMBYSTOMATID SALAMANDERS 5 within the family; in other words, there has been phylogenetic modifi- cation of the shape of that element. However, in individuals of any given group, the parasphenoid assumes its characteristic shape even in very young larvae; there is no corresponding ontogenetic' modifica- tion in the developmedt of the individual. A neotenic individual, or form, of a group would, therefore, still retain the parasphenoid shape characteristic of that group, rather than apparently reverting to a condition con*arable to that in ancestral fcirms. Despite the extensive and productive experimental studies of vari- ous physiological and chemical factors involved in neoteny and normal metamorphosis, the interaction of different environmental and genetic influences to produce neoteny in nature is still largely unexplained. And it is by no means certain that the factors involved in partial neoteny are always identical with those involved in complete neoteny. In the Ambystomatidae, neotenic populations seem to be concen- trated in regions of high altitude or low temperature or both, and perhaps in regions of iodine deficiency. The phenomenon is not, how- ever, limited to such regions nor universal within them. That genetic factors are involved has been clearly demonstrated (e.g., Geyer and Freytag, 1949; Twitty, 1941). It is equally clear that environmental factors also are frequently involved. In many instances, at least in certaint subspecies of tigrinum, metamorphosis can be induced simply. by transference of individuals from the natural environment to an aquarium. In Other cases, and other forms, it is very difficult or ini- possible to induce metamorphosis by any experimental means. Within a limited area in southwestern Kansas, it appears that the changing environments of the glacial and interglacial stages during the Pleisto- cene were accompanied by corresponding changes in the mode of living of populations of tigrinum inhabiting that area. + Populations from deposits associable with major glacial advances were apparently neotenic, while those associable with the interglacial stages underwent normal metam6rphosis (cf. Tihen, 1955). Large neotenic populations are pretty much limited to the A. tigrinum - mexicanum - Bath!/sireddn 'complex, but situations 'inter- pretable· as representing partial neoteny occur throughout the family. Occasional neotenic individuals have also been reported in several other forms, such as Dicamptodon (DeMarco, 1952), A. gracile (Slater, , 1986),and Rhyacosiredon (Taylor and Smith, 1945).It is not certain whether the. individuals reported in these three forms would have remained neotenic throughout their life cycle under natural conditions, or whether they represent instances of what might better be termed 6 BULLETIN FLORIDA STATE MUSEUM Vol. 3 delayed metamorphosis, perhaps passing one extra season in the unmetamorphosed form but still with the expectation of normal meta- morphosis to folfow. DISCUSSION OF CHARACTERS Several of the characteristics that vary from group to group within the family exhibit only two possible conditions; for example, an inde- pendent lacrimal is either present or it is not. In such cases, and in the case of features relatively uniform throughout the family except- - ing in one particular group, the characteristics are best treated in con- nection with the group or groups involved. In 6ther instances, where the variation is of a progressive, continii- ous, or diversifed nature, a preliminary discussion of the conditions found is preferable to a piecemeal presentation by individual groups. The piresent section is limited to features exhibiting variation of this latter sort. Detailed descriptions need not then be repeated in con- ' nection with the various groups; mention of the condition will be made, and further details can be obtained by reference to this pre- liminary section. The composition of the various supraspecific categories, as used in this discussion, has been outlined in the introduction. , Teeth In animals such as these, having continuous tooth replacement, there will be some variation in the form of individual teeth due to their varying stages of development. There is also some slight vari- ation according to the position in the jaw. But the great majority of teeth in any given (mature) individual are of a form characteristic of the taxonomic category to which it belongs (fig. 1). The teeth of the Hynobiidae are conical, rather sharply pointed, with the- tips slightly bifid but scarcely or not at all expanded. A similar condition occurs in immature larvae of all Ambystomatidae, although advanced larvae usually exhibit an approa6h towards the adult condition in , their respective species. It is presumed that the teeth' of the ances- tral ambystomatids were similar to those of the hynobiids. Whether these teeth are best described as having a bifid tip or as developing a lateral cusp is debatable, but that question seems primarily a matter, of terminology. In what I have assumed to be the primitive condi- tion, the two portions (lingual and labial) of the tip seem nearly equal, but the lingual is p6rhaps slightly more fully developed. 1958 TIHEN: AMBYSTOMATID SALAMANDERS 7 a> , I ./ j r N. El .. ....... A B CD E r.... C.{ 1 .:1 -764B t, 1 Pl FGHIJ Figure 1.-Characteristic forms of 'individual teeth in various ambystomatid groups. ~ (A) A typical hynobiid„ Batrachuperus pinchoni, for comparison. This general form _of tooth is also characteristic of the developing larvae of all ambys- tomatids. (B) Dicaniptodon ensatus. Antero-lingual aspect. (C) Rhuacotriton olympicus. Anterior aspect. (D-E) Rhyacosiredon altamirani; showing some vari- ation in form. (F-I) Anterior (F), lingual (G, H) and Iabial (I) aspects of the type of tooth found in all members of the subgenus Ambustoma. (J) Ambystoma (Linguaetapslis) texanum. Antero-lingual aspect. In Rhyacotriton and Rhuacosiredon the inner cusp is usually some- what elongated, forming the actual tooth tip, the outer one being rep- resented by only a slight projection. The tooth tip itself is usually - slightly, ·'sometimbs rather ' strongly, curved or hooked inward. In Dicamptodon the teeth are strongly laterally compressed, with a sharp, sin*le tip. This gives the tboth the general form of a slightly cuived double-edged blade, quite different from the teeth of any other memlber of the family. The typical, form for other members of the family is a bluntly rounded, definitely bifid tip; many have each of the two cusps, par- ticularly the inner, expanded into a disclike structure. The, tooth as a wh6le thus exhibits a spade-shaped, or at least a peg-shaped, form. Such a form is characteristic for adults (and mature neotenic larvae) of all members of the subgeneta Ambystoma and Bathysiredon. In the subgenus Linguaelapsus, the tooth form is rather variable. In annulatum the essential form seems similar to that of the subgenus Ambystoma, but the tooth itself is rather sh6rt and the tip, though bifid, extremely blunt; the arrangement of the teeth is, however, characteristid of Linguaetapsus. In mabeei, most of the maxillary- premaxillary teeth are of the typical Ambystoma type, while maily of the mandibular teeth exhibit the pointed, hooked condition described 8 BULLETIN FLORIDA STATE MUSEUM Vol. 8 bdow for texanum;.the teeth are arranged almost entirely in a single row on each dentigerous element, as in the subgenus Ambystome In texanum the inner fork, or cusp, of the tip is long, sharply pointed, and strongly hooked inwards. There is apparently some geographic variation within the species in. respect to the extent of development of the medial and lateral cusps. The species cingulatum is unique in having a capstan-shaped tooth, with the tip exhibiting a nearly Hat surface. - This tip may have been develop.ed from the lateral cusp of a tooth such as that in texanum, the inner cusp having been reduced to the point of elimination. There is no indication of a bifid tip in this spe- dies, nor of the development of secondary cusps. B C D E AV F G H I Figure 2.-Anterodorsal aspects Of the premaxillae of various adult ambys- t6matids and hynobiids, (A) A typical hynobiid, Batrachuperus pinchbni. (B) Rhyacotriton olympicits. (C) Rhgacosiredon altamirani. (D) Ambystoma tigrinum, (E) Anibystoma taipoideum, (F) Ambystoma opacum. (G) Ambystoma macu- tatum. CH) Ambystoma mabeei. (I) Ambystoma cingulatum. The neotenic larvae of the A. mexicanum - tigrinum - Bathysiredon complex have,the same type of teeth as do nietamorphosed adults, so the ontogenetic development of this character appears there to be a matter of age of the individual rather than of metamorphosed or non- metamorphosed condition. 1958 TIHEN: AMBYSTOMATID SALAMANDERS 9. Premaxillae In the hynobiids the nasal processes of the premaxillae are rather short and are well separated from each other; in all ambystomatids, these processes are elongated (figs. 2 and 3). In larval ambystomatids the processes are widely separated, em- ~A*.2-bracing a large fontanelle; in adults of all members of the family excepting Rhyacotriton and R/wacosiredon the , processes are closely appr6ximated in the midline, nearly or completely 0--..3. \ obliterating the fontanelle. It is like- ~- O,·.:U> n , ly, though. not certain , that ·the per- . B sistence of a broad fontanelle in the - Figure 3.-Medial aspects of two genera named represents in each the premaxillae of Amb!/stem€: case an. example of partial neoteny, maculatum (A) and A, cingulatum rather than retention of a primitive (B), showing the flat-bladed char- conditi6n. acter of the spine in the former, The nasal processes vary greatly and the vertical extension in the latter, characteristic of the more ·in proportions, from the very broad specialized members of the sub- and relatively short processes of the genus Linguaetapsus, Dicamptodon premaxilla to the very long, slender ones of Rhyacotriton and A. texanum. Their propor- tions are, to some extent, correlated with the general proportions Of the cranium. Typically, the hasal process is a simple, flat blade. In members of the subgenus Linguaelapsus a vertical thickening occurs just above the dentigerous portion, forming a sort of partial septum in the nasal cavity. Both the longitudinal and vertical extent of this thickening vary from form to form within the group, and it may be represented either by a thickening of the entire blade, or only by the development of a lamina along the medial edge. In the region where this lamina or thickening develops, the lateral exte'nt (that is, the surface exposed dorsally) is usually greatly reduced. Customarily there is n6 balatal process 6f the premaxjlla, and thus no sutural connection between the premaxillae and prevomers. In Dicamptodon, however, a definite palatal shelf is present, and there is an extensive, firm, sutural connection-between this shelf and the prevo- mer (fig. 913). This contributes to the general solidity and rigidity gf the Dicamptodon skull; particularly the palate; this matter will be dis- cussed more fully in connection with that genus. , 10 BULLETIN FLORIDA STATE MUSEUM Vol. 8 Prevomers In adults of both the Hynobiidae and Ambystomatidae the prevo- mers (figs. 4 and 5) are rather extensive bones, approaching but usually , lacking firm sutural connections withthe premaxillae and maxillae on the floor of the skull. Rhyacosiredon is unique in ha*ing prevomers in the larval position and of only slightly greater than the usual larval extent. The prevomerine teeth in that genus are, in contrast to the larval c6ndition, reduced to a single row. The situation is comparable with a metamorphic stage in other f6rms, rather-than with a true larval stage; it is almost certainly the result of a partial neoteny, since no sim- ilar condition occurs in any known adults of either family. - The evidence that the palatinb bone contributes to the adult prevo- mer is tenuous, but there is a process (usually dentigerous) in the · adults of -most ambystomatids forming part of the posterior choa~al border and extending lateral to the choana. It is probable that this process represents the palatine bone; whether it is actually homolo- gous with the palatine or not, it is conveniently referred to as the palatine and such usage is followed here. In what is considerdd to be the most primitive condition, both the palatine and the prevomer prop- er bear teeth, those of the palatine being directly continuous with those. of the prevomer. In many forms of the genus Amb!/stoma there is a very definite diastema between the palatine and the prevomerine teeth; occasionally the palatine teeth are completely lacking, and in some forms of the subgenus Linguaelapsus the palatine itself is greatly reduced or lacking. In hknobiids the prevomerine teeth pf each side are usually in a curved series, eonvex anteriorly, 56 that the teeth of the two sides form a somewhat M-shaped pattern. In most ambystomatids the medial end of the tooth row on each side is usually located at least as far anteriorly as the portion lateral to it, so that the pattern is transverse, A-shaped, or W-shaped. In Dicaniptodon and Rhyacotriton there is a tendency, not strongly marked, towards the M-shaped pattern of the hynobiids. In adults of most ambystomatids the prevomerine teeth are situated along the extreme posterior edge of that element; in Dicamptodbn, Rhyacotriton, the tigrinum group, and A. mabeei there is a small postdentigerous process extending a short ,distance onto the - ventral surface of the parasphenoid. The anterior, medial, and posteromedial borders of the choanae are formed by the prevomers, with the palatine forming most of the poste- rior border. In hynobiids and the presumably more primitive ambys- tomatids there is no bony lateral border. In some groups of the genus 1958 TIHEN: AMBYSTOMATID SALAMANDERS 11 ,. BA C D .:il.> G E Figure 4.-Prevomers and parasphenoids of vari6us ambystomatids and hyno- biids. (A) Batrachimerus pi irchoni. (B) Hynobius Zeechi. (C) Rhyacotriton 04/rn- picus. (D) Rh!/acosifedon altamirani. (E) Ambt/.stoma mabeet. (F) Ambystoma annulatum. (G) Ambustoma. cingubitum. L 12 BULLETIN FLORIDA STATE MUSEUM Vol. 8 Ambystoma there is a projection of the posterolateral corner of the prevomer, forming a partial lateral border for the choana; in some forms the bony border is nearly complete. BA /46,:4 r -:.k{* CD E Figure 5.-Prevomers and parasphenoids in members of the subgenus Ambus- toma. (A) Amblistoma tigrinum. (B) Ambystoma opacum. (C) Ambustoma macrodactylum. (D) Amb!/stoma maculatum. (E) Ambystoma fellersonianum. 1958 TIHEN: AMBYSTOMATID SALAMANDERS 13 B C DE - Figure 6.-Examples of variation in the form 6f the parasphenoid in certain groups. (A) Rhyacosiredon altamirani (compare with fig: 4D). (B) Ambystoma ambluctphalum (larval). (C) Ambustoma Zermaensis. (Tliis represents as extreme a departure from the more typical tigrinum form.as will normally be encountered in specimens from either the tigrinum or mexicanum groups. Variation is intra- specific; the figured specimen is not typical of the species termaenss.) (p) Ambys- toma kansense, showing ~ clear affiliity with the mexicanum-tigrinum complex. (E) Ambustoma kansense. Examble. of variation. Parasphenoid In hynobiids the parasphenoid is ~a relatively straight-sided ele- ment, with the part underlying the otic region markedly expanded (figs. 4,5, and 6). The parasphenoid of Dicamptodon is virtually identical with these; that 6f Rhydcotriton is markedly expanded under the· otic region, but the ·sides diverge more or less· regularly from the · , anterior end posteriorly, and the expansion underlying the otic ilegion is 'less marked. In Ambystoma the amount of expansion of tke posterior part of the parasphenoid varies but is, in general, decidedly less marked *an in the hynobiids, Dicamptodon, and Rhyacotriton. In most f6rms the sides of. the parasphenoid anterior to the expanded posterior portion are distinctly cencave. In the tigrinum and mexicanuin groups there is a good deal of individual variation, but typically the posterior portion is only slightly or not at all expanded, and the sides of the element are nearly parallel rather than concave. As has been mentioned, the characteristic form of the parasphenoid can be discerned relatively early in larval development; there is little ontogenetic variation. 14 BULLETIN FLORIDA STATE MUSEUM Vol. 8 Hyobranchium The hyobranchium is consistently cartilaginous in larvae; in adults (and mature neotenic larvae) the os triangulare is consistently ossified. This is the only ossified portion of the hyobranchiuin in most members of the family, excepting for occasional small, scattered and isolated centers of ossification in one or mord of the other elements. In the Hynobiidae, in Dican*todon, and in Rhyacosiredon virtually the en- tire hyobranchium is ossified. The fact that it is ossified in hynobiids, supported by the fact of its 6ssification in Dicamptodon; leads to the conclusion that, insofar as the Ambystomatidae are concerned, the bony hyobranchium is a more primitive type than the tartilaginous. There is considerable infraspecific variation in the form of the os triangulare. There is a tendency for this element to be equally triradi- ate, or to have the anterior-posteri9r branch elongated, in the tigrinum and mexicanum groups and Rhyacosiredon. In the other forms the three arms are. frequently equal, but departures from this form are in the direction of a shortening of the anterior arm, and a more trans- verse orientation of the two posterior arms. The majority of specimens I have examined have been ~iried skele- tons, with the cartilaginous hyobranchium missing or, at best, shrunken and distorted. Various accounts in the literature do not seem to be in complete agreement.as regards details, but there appears to be rather general /agreement as to the presence of an annulus, or ring-shaped otoglossal, in certain forms. This 'was the primary basis of Cope's (1887) attempt to define the ambystomatid genera. He there retained in the genus Ambystoma, defined in part as having an annular otoglos- sal (in adults-no larvae possess an annular otoglossal), the species maculatum, talpoideum, opacum, tigrinum, and macrodactylum. There · is fairly general agreement that this element is typically present in form~ that I have referred to the genus Ambystoma-excepting,of course, those forms that are always neotenic and, possibly, A. gracile. An annular otoglossal also occurs in Rhyacotriton, according to both the original definition given by Dunn (1920) and the figure provided by Hilton (1946). - The two genera Chondrotus and Linguaelapsus differed,«according to Cope, in details of hyobranchial structu*e, but at least were in agreement in respect to the absence of an annular otoglossal. Since he placed «lepturus" (=cingulatum) in the latter genus and the un,. questionably very closely related "microstomum" (=texanum) in the former, it is clear that the other features involved in his definition must be of minor significance. In these two genera were included, beside the above two forms, ensatus, annulatum, and suprisingly. 1958 TIHEN: AMBYSTOMATID OALAMANDERS 15 ' gracile. · The absence of an anhular otoglossal in ensatus (i.e., Di- camptodon) has been supported by a number of subsequent observa- tions. I know of no other recorded observations on the condition in annulatum, cingulatum, or texanum, but my own' observations of the absence of an annulus in inabeei lends support to the conclusion that this is indeed a dharacteristic feature of the subgenus Linguaeldpsus. I am inclined to qu¢stion Cope's observation concerning A. gracile ufltil it is confirmed. In neotenic members of. the mexicanum and tigrinum groups. the hyobranchium is of typically larval form, and so lacks an annulus. Such an element is present in adult tigrinum, and would presumably be present in adults of forms at present known only as neotenics. Ac- cording to Hilton (1946) there' is no annulus in Rhyacosiredon. This could conceivably be the result of partial neoteny, but the hyobranch- ium is otherwise of typical adult form. , The osseous condition of the . hyobranchium in that genus definitely represents a primitive, rather than a neotenic, condition; this is probably also true of the absence of id aftnulus. Vertebrae The vertebral .column consists of a singld cervical vertebra, or atlas, a varying number of trunk vertebrae (this number being con- ' stant for a given species within narro*-lrmRs of variation), a single sacral vertebra, and a varying number of caudal vertebrae (this num- ber being quite variable even intraspecifically). -The-·number-of -trunk vertebrae is very closely correlated with costal groove count (High- ton, 1957). Since the latter is a readily determinable external'featurei the costal groove count (provided the method of counting is standard- ized) is a more utilizable characteristic than the direct vertebral count. Because of different methods employed in making the costal groove count, bowever, the literature records cannot always be taken as di- rectly comparable with each other. A116wing for this factor, it is still possible to determine costal groove numbers very closely in the various species and to compare them throughout the family. Variation in the number of costal _ grooves is relatively slight. Twelve seems to be a fairly typical num- ber, with nearly all forms having, most frequently, between 11 and 18. Exceptions include A. talpoideum which may have as few as 10, Rhy- acotriton with 14, and members of the subgenus Linguaelapsus in which the number varies from 18 to 15.. Occasional individuals of the tigrinum group may have as many as 14, but this number is not typical for any species. 16 BULLETIN FLORIDA STATE MUSEUM Vol. 3 I : '. A 1 B E©.:AS · ·4.. » 1,4 -3:.14 .~·..7 .:1 : /S·9* ~·~rA·~:39f./All #-di Figjre 7.-Dorsal and ventral views of vertebrae of various ambystomatids. (A) Rhyacosiredon altantitani. (B) Ambystoma tigrinum, as typical of all mem- bers of the tigrinum and mexicanum groups. (C) Ambustoma opacum. Note that general proportions are similar to the above. 1958 TIHEN: AMBYSTOMATID SALAMANDERS 17 There is little variation in basic structure of the trunk vertebrae fr6m different portions of the column within a given species, but there is usually a decided positional difference in proportions. The - vertebrate of the anterior portibh of the trunk are relatively high, broad, and short, becoming progressively lower, narrower, and rela- tively longer in the posterior portion of the trunk. In Rhyacotriton the proportiolis are essentially similar throughout the entire series (Stokely and Hdlle, 1958). Certain phylogenetic trends in vertebral proportions can be noted, but because of the linear variation just mentioned, along with individ- ual intraspecific variation, a much larger number of specimens than , have been available to me must be examined before any accurate statis- tical comparisons can be made. Some of the more marked trends are fairly obvious, though the extent of overlapping variation is not accu- rately determined. The veriebrae of Rhyacosiredon, Rhyacotriton, and, , the mexicanum and tigrinum groups of Ambystoma are similar in pro- portions. Using this as a comparative basis, those of the other mem- bers of the family are elongated to a greater or lesser extent, the elonga- tion being most marked in the maculatum group of Ambystoma (figs. 7 and 8).,A number of comparative ratios might be given, but two Will be probably the most useful. One is a comparison of the length , of the centrum with its width at the anterior end; figures are pre- sented in table 1. The range of variation' could probably be les- sened if only one specific vertebra of each specimen were used, but also, so would the usefdlness of the character. In general, I have used vertebrae only from the middle part of the trunk, excluding only the anteriormost three or fdur and a similar number posteriorly. In respect to this particular ratio, it would appear from the figures in the table that the vertebrae of cingulatum and texanum are reldtively short and wide; other vertebral measurements make it clear that this results from a tendency towards flaring of the ends of the centrum rather than a broadening of the Vertebra as a wh6le. The other ratio used is that of combined zygapophyseal width as compared with the zygapophyseal length. The term "combined zygapophyseal width" is used to denote the distance between the lat- eral borders of the prezygapophyses plul the distance betweeen the " lateral borders of the postzygapophyses. Zygapophyseal length" is the longitudinal distance between the anterior tips of the prezyga- pophyses and the posterior tips of the postzygapophyses. In general, the proportions as reflected by this ratio correspond, from one group to another, with those.reflected By the central measurement; the 18 BULLETIN FLORIDA STATE MUSEUM - Vol. 8 X .... lilli B f ; C Figure 8.-Dorsal and ventral views of vertebrae of various ambystomatids. (A) Ambystoma gracile. (B) Ambustoma maculatum. (C) Ambystoma texanum. 1958 TIHEN: AMBYSTOMATID SALAMANDERS 19 elongation of the vertebrae of texanum and cingulatum is clearly c shown. TABLE 1 VERTEBRAL RATIOS IN VARIOUS GROUPS OF AMBYSTOMATIDS Length of centrum Combined zygapophysial widthWidth of centrum at Forms anterior end Zygapophyseal length 'Rhyacosiredon 2 .0 - 2 . 2 1 .4 - 1 . 7 Dicamptodon 2.2- 2.5 1.4 - 1.6 A. inexicanum group 1.9 - 2.2 1.8-1.6 A. tigrinum group 1.8 - 2.3 ' 1.8 - 1.7 A. opacum group 2.0 - 2.6 1.3 - 1.5 A. maculatum group 2.2 - 2.9 1.1 - 1.4 A. mabeei and annulatum 2.8 - 2.7 1.0 - 1.8 A. texanum and cingulatum 1.9 - 2.8 1.0- 1.8 In most forms the posterior end of the neural arch projects defi- nitely posterior to the level of the posterior border of the postzyga- pophyses. In the niaculatum group and the subgenus Linguaelapsus, at least in the posterior portion of the trunk, the postzygapophyses extend at least as far, usually definitely farther, posteriorly than does the neural arch. An exception is found occasionally in A. gracile; in general, the vertebrae of that species seem to approach those of the tigrinum group more closely than do those of any of the other species in the maculatum group. In normal (i.e., not neotenic) larvae, and frequently in recently metamorphosed adults, the notochord is continuous. In mature adults and mature neotenic larvae a septum develops at the middle of each centrum, interrupting the notochord. From my 6bservations this fea- ture, depending as it apparently d6es on the age or state of maturity of the animal rather than on metamorphic condition, is a fairly re- liable gauge of the maturity or immaturity of the individual The single specimen of Bathysiredon that I have seen possesses a continu- ous notochord, and it is a large specimen, almost certainly sexually mature. All of the specimens of the mexicanum group that I have seen that are noted to be mature have the notochord interrupted, but two specimens of about the same size, with no notation as to whether or not they were mature, have a continuous notochord. The vertebrae of ambystomatids are, in general form, very similar to those of the hynobiids, except. that in the latter family the articular facet of the transverse process is often not sharply divided into dorsal 20 BULLETIN FLORIDA STATE MUSEUM Vol. 8 and ventral portions; the ribs are, coriespondingly, unicipital. This is not a diagnostic family characteristic; some hynobiids have trans- verse processes with the dorsal and ventral articular facets completely separated, indistinguishable from those of ambystomatids. I have seen neither a sufficient number of specimens nor forms of the hyno- biids to determine whether the variation in this respect is at an infra- specific, specific, or supraspecific level. ~YSTEMATIC ARRANGEMENT Suborder AMBYSTOMATOIDEA new emendation Ambystomoidea Noble, 1931, The biology of the amphibia, p, 471, DEFINITION.-Salamanders having the angular fused with the pre- articular; second epibranchial lost; premaxillary spines elongated; prevomers without extensive processes extendihg over the para- sphenoid region; vertebrae amphicoelous; skull without a frontosqua- mosal arch; fertilization internal. DISCUSSION.-This group is to some extent morphologically inter- mediate between the more primitive Cryptobranchoidea and the ·more specialized Salamandroidea. It appears to have been derived from one section of the former but is in no sense ancestral to the latter, which is believed to have arisen entirely independently from a differ- ent cryptobranchoid stock. The fusion of the angular and prearticular, the loss of the second epibranchial, the elongation and approach of the premaxillary spines, and internal fertilization are features shared with at least some salamandroids. The short prevomers, without ex- tensive posterior processes, the amphicoelous vertebrae, and the lack of a frontosquamosal arch are among the features distinguishing this group from various members of the Salamandroidea. Family AMBYSTOMATIDAE Hallowell Ambystomidae Hallowell, 1856, Proc. Acad. Nat. Sci.. Phila:, vol. 8, p. 11. DEFINITION.-Essential family characteristics the same as those of the suborder. DISCUSSION.-In the past, this family has been most frequently associated with the Hynobiidae and is almost certainly derived from a hynobiid stock that presumably migrated from Asia to North America via the Bering land bridge very early in the Tekiary, possibly ·even iri .the late Mesozoic. It is true that there is no direct fossil record of thefamily prior to the middle Pliocene, but trackways from the Paleo- cene of Montana have been referred to this family (Peabody, 1954): The absence of specimens from intervening geologic stages is not sur- 1958 TIHEN: AMBYSTOMATID SALAMANDERS 21 prising, even if we assume that the family was widespread and rela- tively abundant throughout mOst of the Tertiary. Wherever tech- niques for collecting microvertebrate fossils have been intensively applied to Pliocene and. Pleistocene deposits, and to many earlier ones, urodele remains have been recovered. It is to be expected in the - near future, as these techniques are developed and more extensively applied to older deposits, a much better fossil record of this and other salamander families will be obtained. Several features not mentioned in the definition of the suborder also serve to distinguish the Ambystomatiaae from the Hynobiidae. The universally bicipital ribs Of ambystomatids, as compared with the most frequently unicipital ribs of the hynobiids; have already been mentioned. The pterygoid and quadrate are fused with each other in all specimens I have seen of the Hynobiidae (cf. also Villiers, 1936), never in the Ambystomatidae. According to Dunn (1928) the Hyno- biidae possess an additional small carpal and tarsal element, possibly representing the vestiges of a prehallux and prepollex, not present in the Ambystomatidae. A number of minor differences can be ob- served, many of which may not be absolutely consistent. All of the members of the suborder Ambystomatoidea are included within this one family. Differences within the family, although in- volving only a limited number of species, are so extensive that I be- lieve recognition of three separate subfamilies to be desirable. Recog- nition of three 'separate subfamilies to accommodate only fve sep- - arate genera may appear to be rather drastic procedure. , Yet the great number of differences involved, and the magnitude of some of these differences can, in my opini6n, only be interpreted by assumingr a differentiatign into three distinct lines early .in the phylagenetic history of the family. Subfamily DICAMPTODONTINAE new subfamily DEFINmON.-Ambystomatids having an independent lacrimal bone; exoccipital and prootic independent; columella independent; skull exceptionally solid (see discussi6n for details); individual teeth compressed, having somewhat the shape of a curved, double-edged blade; nasal bones present; premaxillary spines short and broad, Zm- bracing no, or at most a small, fontanelle; a decided linear variation in the proportions of the trunk vertebrae; lungs, eyes, and ypsiloid cartilage normal. DISCUSSION.-It has long been recognized that Dicamptodon shares with Rhyacotriton several primitive features not Occurring elsewhere iii 22 BULLETIN FLORIDA STATE MUSEUM Vol. 8 the family. Most sign-meant of these are, in my opinion, the failure of the prootic and exoceipital to fuse into a single periotic bone on each side, the presence of an independent lacrimal, and a fairly close resemblance of the parasphenoid to' that of hynobiids. There are also other points of resemblance. Although the prevomerine tooth pattern .is greatly different, each shows a tendency towards an M-shaped - pattern, rathef than the A-shaped to W-shaped one of other members of the family. In both, the characteristic number of phalanges in the fourth toe is three, as compared with four in nearly all other forms. Both usually lack an enclosed procoracoid foramen in the pec- toral girdle (Stokely and Holle, 1954). In the larvae of both, the dor- sal fin is limited to the tail, not extending up onto the trunk. Despite these many resemblances each form, particularly Rhyaco- triton, has 56 many unique characteristics that the two are no more similar to each other than to the remainder of the family. All of these resemblances represent apparently primitive features; none of the specialized characteris~ics of either form are found in the other. The points of similarity. do not-then, denote any close relationship between the two, but simply indicate that the lines leading to these two forms were derived from the ancestral proto-Ambystomatidae at an early stage, prior to the development of the specializations characterizing the remainder of the family. The most distinctive feature of the skeleton of Dicamptodon is the solidity and rigidity of the skull. In the evolution of salamanders the tendency has been, gerierally, towards a less rigid skull. Whethet ihe condition in Dicamptodon represents the retention of a primitive con- dition, or represents a secondary development from a type of structure more nearly typical of other ambystomatids, is not certain. I am in- clined to the opinion that it is a secondary development, possibly as- sociated with habits or habitat, since a similar solidity is not encount- ered in any other known ambystomatid or hynobiid. Among the features contributing most strongly to this solidity, par- ticularly of the palatal region, are the firm, relatively extensive sutural connections of -the prevomers with the premaxillae and the maxillae. This involves the presence of a distinct palatal shelf on the premaxilla, not found in other forms, and of a more extensive palatal portion of the maxilla than is customary in other forms. Also, the pterygoid is heavy and extensive. In all individuals I have seen this element ac- tually abuts against the posterior end of the maxilla; some figures of the Dicamptodon skull (e.g., Hilton, 1946) indicate Only·that the ptery- goid approaches the maxilla Very closely, without aetually being in contact with it, so there may be some variation in this respect. All 1958 TIHEN: AMBYSTOMATID SALAMANDERS 28 of the bones of the skull are relatively heavy, with firm and solid sutures. The shape of the individual teeth is a uniqu'e and undoubtedly specialized feature. Dicamptodon seems to be an exceptionally vora- cious salaminder (ef. Graf, 1949), and it is possible that the tooth shape is to some extent correlated with its feeding habits. The exten- sive parieto-squamosal crests indicate a strongly developed jaw mus- culature, also perhaps associable with feeding habits. The hyobranchium is largely ossified, as in the Hynobiidae, and the 6toglossal cartilage is not annular. The carpus and tarsus are cartilaginous in larvae, ossified in adults; there are eight carpal elements. Genus Dicamptodon Strauch Dicamptodon Strauth, 1870, Mum. Acad. Sci. St. Pdtersbourg, sdr: 7, 16 (4) : 68. Type species ensatus Eschscholtz, 1888, by monotypy. Chondrotus Cope, 1887, Amer. Nat., 21 : 88. Type species tenebrosus Baird and Girard, 1852 = ensatus Eschscholtz, 1833. DEFINmON.-Essential generic characteristics the same as those of the subfamily (fig. 9). RANGE.-The Pacific coast, from southwestern British Columbia to Santa Cruz County, California; also known fr6m northern Idaho. Trackways referred to this form are known from the lower Pliocene of California (Peabody, 1954). ' DISCUSSION.-This genus is generally considered, correctly, I be- lieve, to be the most primitive existing representative of the family. The numerous primitive features shared with Rhimcotriton have already been mentioned. With the exception of the shape of the teeth and probably the rigidity of the skull, most of the points of difference between the two forms represent specializations in Rhyaco- triton, with Dicamptodon retaining a primitive condition frequently shared with other members of the family. REFERRED FORMS.-Dicamptodon ensatus (Eschscholtz), 1888, Zool. Atlas pt. 5 : 6, pl. 22. Genus Ambystomichnus Peabody Ambystomichnus Peabody,* 1954, Jour. Paleont., 28 (1) :,80. Type species monta- nensis Gilmore, 1928. DEFINITION.-Known only from trackways. I quote from the di- Kagnosis given by Peabody: ... Comparable in body form with . . . Dicamptodon, . . . but twice as large and slightly. longer in the 24 BULLETIN FLORIDA STATE MUSEUM Vol. 8 A ..fs-2. :%4 1:...: B Figure 9.-Skull of Dicamptodon ensatus. (A) Dorsal aspect. (B) Vential aspect. 1958 TIHEN: AMBYSTOMATID SALAMANDERS 25 trunk region. Details of th~ footprints agree closely with footprints of living and Pliocene ambystomids, and the bilobate palm of the manus is exactly as in living Dicamptodon ensatus. Stride 18 cm. maximum; pace angulation ...77 degfees maximum; manus 3.1 cm. wide, pes 3.7 cm. wide overall; both relatively distant from midline." RANGE.-Known only from the Paleocene of Bear Butte, Sweetgrass County, Montana. DISCUSSION.-Since this form is known only from trackways, ref- erence to any subfamily must be tentative. Peabody's discussion places emphasis on the bilobate palm of Ambystomichnus and Di- camptodon, and also mentions certain other items indicating an amnity between these two genera. Geographic and temporal considerations at least do not discredit ·such an association. REFERRED FORMS.-Ambustomichnus montanensis (Gilmore), 1928, Proc, U.S. Natl. Mus., 74 (5) : 1. Subfamily RHYACOTRITONINAE new subfamily DE~INITION.-Ambystomatids with an 'independent lacrimal bone; prootic and exoccipital independent; columella independent; skull not exceptionally rigid; individual teeth conical; nasal bones lacking; premaxillary spines long and narrow, embracing a fontanelle; only slight linear variation in the proporti6ns of the trunk veftebrae; eyes large, protuberant; lungs and ypsiloid cartilage much reduced. DISCUSSION.-The primitive features shared by this subfamily and the Dicamptodontinae have already been described. Despite these similarities, Rhyacotriton is so distinct, not only from Dicamptodon, but from all other members of the family, that recognition of a sep- arate subfamily seems essential. Rhyacotriton is unique in the absence of a nasal bone; to judge from appearances this element is actually lacking, not simply fused with one of the adjacent bones. This may be the result of a partial neoteny, since the nasal is one of the last bones to ossify during larval development, but the element is present in neotenic larvae of other members of the family. The extreme length of the premaxillary spines (==nasal processes) is approached only by some members of the sub- genus Linguaelapsus, in which the form of this bone is otherwise quite different, and which completely lack a fontanelle. The contrast with the short, broad nasal process of the Dicamptodon premaxilla is espe- cially marked. The pteryg6id is gfeatly reduced, and is less extensive than in any other member of the family; again this is in particularly strong contrast to the condition in Dicamptodon. The skull as a whole , 26 BULLETIN FLORIDA STATE MUSEUM Vol. 8 is less rigid, and with perhaps relatively less bone, than in other 'mem- bers of the family; again, the most extreme eontrast is with Dicampto- don. The abortive development of the lungs is unique among the Ambystomatidae; as in most other lungless salamanders, the ypsiloid cartilage is correspondingly reduced. The small size, body proportions, protuberant eyes, and coloration all contribute to a strikingly distinctive external general appearance. It is the only member of the family in which the carpus and tarsus remain cartilaginous in adults, and in which the number of carpal- elements is reduced to six (Hilton, 1948). It is distinctive in the almost complete lack of linear differentiation in the trunk vertebrae. In this' genus alone of the ambystomatids the musculus levator scapulae is incompletely differentiated anteriorly from the m. dorsalis trunci in the adult, though such a condition is found in the larvae of all forms (Dunn, 1941). Rhyacotriton is also unique in breeding habits; the eggs are large, laid singly; and the total complement averages only about 10 or 12 eggs (Noble and Richards, 1982; Stebbins and Lowe, 1951). In- liddition to these unique features, the Rhyacotritoninae differ from the Dicamptodontinae in several respects in which they resemble at least some of the Ambystomatinae. These include the conical shape of the teeth, which may be slightly hooked at the tip, the loss of the palatine teeth, the cartilaginous condition of the adult hyobranchium, the presence of an annular otoglossal cartilage, the presence of a premaxillary fontanelle, and numerous details of form and proportion. A number of the features of this group might be interpretable as the result of a partial neoteny, though such an interpretation might A B Figure 10.-Dentaries of typical ambystomatids. (A) Ambustoma gracile. This s6rt of dentary, with a prominent Range of bone on the lingual' surface, is typical of all forms of the subgenus Ambystoma, and ik also found in A. mabeei. (B) Ambystonw mexicanum, neotenic larva. This general type of dentary, lack- ing·the flange on the lingual surface, is characteristic of the larvae of all ambys- tomatids, and resembles that of adults of Dicamptodon, Rhjacotriton, Rhyacosite- don, and most members of the subgenus Linguaelapsus. V 1958 TIHEN: AMBYSTOMATID SALAMANDERS 27 well be 4uestioned in most of the instances. These features would include the presence of the premaxillary fontanelle, the relatively un- specialized teeth, the absence of the nasals, the cartilaginous cendition of the carpus and tarsus, the independent columella, and minor de- tails in the form of certain other elements, such as the dentary (fg. 10). In any event, whether or not ascribable to partial neoteny, the conditions in Rhyacbtriton differ in detail from those found in any other member of th6 family, larval, or adult. It is interesting to note that these features occur in a form that is largely aquatic, ahd is, in this aquatic environment, subject to relatively low temperatures. It is active at temperatures of from .5° to 10° C. Genus Rhyacotriton Dunn Rhydcotriton Dunn, 1920, Proc, New Engl, Zool. Club, 7 : 56. Type species olumpicus Gaige, 1917. DEFINITION,-Essential generic characteristics the same as those of.the subfamily. RANGE.-Pacific coastal region from the Olympic Mountains south • to southern Humboldt County, California. No fossil record. DISCUSSION:-As indicated above, this genus is the most distinctive member:of the family. The primitive features shared with, Dicampto- don indicate an early origin, but the subsequent modification has been extensivb. It cannot be considered a derivative of Dicdmptodon, as has sometimes been suggested. Eaton (1938) has described a condition which he interpreted as a sort of streptostyly in this genus and in the Pacific forms of Ambys- toma, and has postulated a derivation from A. macrodactylum or an extinct related form on that basis (1984). The existence of such a conditionjhas been strongly questioned by Villiers (1988a, 1988b) and others. There is no other evidence for such a derivation of-the genus, and a great deal of evidence that no close relationship exists between Rhyacotriton and the genus Ambystoma. I am of the opinion that the Rhyacotritoninae were derived from an early stock, the line lead- ing to Rhyacotriton becoming established prior to the appearance of the genus Ambystoma as such. REFERRED FORMS.- Rhyacotritort olympicus olympicus (Gaige), 1917, Occ. Pap. Univ. Mich. Mus. Zool., no. 40 : 2, pl. 1. Rhyacotriton olympicus variegatus Stebbins and Lowe, 1951, Univ. Calif. Publ. Zool., 50 : 471. 28 BULLETIN FLORIDA STATE MUSEUM Vol. 8 Subfamily AMBYSTOMATINAE Hallowell DEFINITIoN.-Ambystomatids lacking an independent lacrimal bone; prootic and exoceipital fused into a single periotic bone, with which the columella is also usually fused in adults; skull not excep- tionally solid; individual teeth essentially conical, though the tips may be modified in various ways; nasal bones present; premaxillary spines varying in proportions, embracing no apprEciable fontanelle in metamorphosed adults (excepting in Rhyacosiredon); a linear varia- tion in proportions of trunk vertebrae; lungs, eyes, and ypsiloid carti- lage normally developed. DISCUSSION.-MajOr specializations of this subfamily include the loss of the independent lacrimal and the fusion of the exoccipital with the prootic. In most forms the columella is also fused with'the re- sulting periotic. The prefrontal of the Ambystomatinae has almost exactly the form and relationships of the prefrontal plus the lacrimal of the oiher subfamilies, and may well represent a fusion of these two elements. Howdver, no trace of an independent lacrimal is observable macroscopically even in very young larvae. Fusion Of the exoccipital and prootic occurs very early in ontogeny-as soon as these bones . really begin to take form-but fusion of the columella with the periotic does not occur until metamorphosis; it remains independent in neotenic individuals, and occasionally also in metamorphosed adults of some species. The parasphenoid is always clearly distinguishable from that in either of the other subfamilies, though it has been modified somewhat differently in the different groups of Ambystomatinae. Variation in this element will be discussed more fully in connection with these groups. The prevomers are usually completely free of the premaxillae, and narrowly or not at all in contact with the maxillae; there is never a solid sutural connection with either of these elements. Similarly although the pterygoid may in some species approach :the maxilla closely, it never abuts against that bone as it appears to do in Di- camptodon. The cranial sutures generally are moderately firm; the skull lacks the extreme solidity of the Dicamptodon skull, though it is typically more solid than in Rhyacotriton. The premaxillary spines are well separated and embrace a large foatanelle in all larvae including mature neotenic individuals, but in adults of all forms excepting Rhyacosireion the spines are closely ap- proximated in the midline and the foritanelle is virtually or completely obliterated. In general, the spines are of moderate proportions, with 1958 TIHEN: AMBYSTOMATID SALAMANDERS 29, only minor variation betweeil species except ib some members of the subgenus Linguaelapsus. The premaxillae do not possess atrue palatal shelf, such as is found in Dicamptodon. The hyobranchium is variable within the subfamily, both as ta extent of ossification and details of morfhology. The carpils and tar- sus are cartilaginous in larvae, including neotenic larvae, and ossified in metamorphosed adults. There are eight carpal elements. Genus Rhyacosiredon Dunn Rhyacosiredon Dunn, 1928, Proc. New Engl. Zool. Club, 10 : 85. Type species altamirani Dugds, 1895. DEFINITION:-Ambystomatinae in which the metamorphosed adults retain the prevomerine teeth in approximately the larval position and retain a large premaxillary fontanelle; maxiliae much reduced in ex- tent; hyobranchium extensively ossified; tooth tips not strongly bifid, but pointed, often recurved; parasphenoid typically diverging from an- terior end to posterior expanded pdrtion; four phalanges in fourth toe; metamorphosis customary, but neoteny may occur. RANGE.-The high mountains at the southern edge of the main Mexican plateau. DiscussioN.-Several of the characteristics of this genus probably represent a partial neoteny. The position of the prevomerine teeth is almost certainly such a feature, as. is probably the persistence of a premaxillary fontanelle, and possibly the limited extent of the maxillae and the relatively simple form of the individuAl teeth. At the time the genus was proposed, Dunn suggested that it was derived from, or at least had amnities with, the tigrinum group of Ambystoma, with the implication that partial neoteny was an important factor in producing the. generic differentiati6n. This view has been generally assumed to be correct. Partial neoteny has probably played a part in the development of the genus, but there is considerable evidence that it is derived from a primitive ambystomatine stock entirely independently of the deriva- tion of the tigrinum group. The ossified condition of the hybbranehium in Rhyacosiredon is cobsidered primitive; elsewhere in the family it occurs only in Dicamptodon; it certainly cannot be interpreted as re- tention of a larval condition. The shape of the parasphenoid is unique, and quite dissimilar from the rather specialized parasphenoid of the tigrinum group. The pointed teeth might conceivably be a nebtenic feature, but in the A. mexicanum-tigrinum-Bathysiredon complex the 30 BULLETIN FLORIDA STATE MUSEUM Vol. 8 tooth tips of all mature neotenic larvae are strongly bifid and often expanded, exactly as those of adults; it is more likely, then, that the pointed, scarcely bifid teeth of Rhyacosiredon are primitive rather than neotenic. The reduced extent of-the maxilla is somewhat compa- rable with. that of small larvae of tigrinum, but not with that of larvae approaching metamorph6sis nor with that of mature neotenic larvae. The position of the prevomerine teeth is a neotenic feature. The prevomer itself is, however, more extensive than the typical lanfal prevomer, and the teeth are,reduced to a single row. In these respects, the Rhyacosiredon prevomer resembles a metamorphic stage in Am- bystoma rather than a true larval stage. The pterygoid has lost its connection with the palatine and is of relafively limited extent, de- cidedly smaller than is typical for the tigrinum group. In general form it bears a marked resemblance to the pterygoid of Batrachuperus and Hynobius, perhaps another indication of a primitive origin for the genus. The number of mandibular teeth is relatively small. The dentaries bear only a slight ridge on their lingual surface, as is true in the pre- ceding subfamilies and in larval Ambystoma, rather than a strong flange of bone completely surrounding Meckel's cartilage such as oc- curs in most adult Ambystoma. The prearticular has only a low, in- distinct coronoid process, again resembling Ambystoma larvae (and adult hynobiids); adult Ambystoma have a rather high, distinct coro- noid process. Many of these features could be the result of partial neoteny; most might equally well or better represent retention of an ance5tral condition. The trunk vertebrae are short and broad, with proportions similar to those of the tigrinum group, but probably even slightly shorter than in that group. In both this genus and Ambystoma there are occa- sional vertebrae witli a pair of protuberances ventrolaterally at the anterior end of the centrum (accessory basapophyseal processes). These do not appear to be limited to, nor consistently characteristic of, any particular species or group of species, nor any specific portion of the vertebral column. A few specimens have been seen in which these processes were found at the p6sterior, instead of the anterior, end of the centrum of some Of the vertebrae. In brief, the presence, absence, - location in the column, extent, and position on the centrum of these processes are all highly variable features throughout the subfamily. This fact, parenthetically, casts some doubt on the validity of the as- signment of the fossil genus Dehmiella from Europe to the family Plethodontidae on the basis of the position of the basapophyseal processes. 1958 TIHEN: AMBYSTOMATID SALAMANDERS 31 REFERRED FORMS.- Rhyacosiredon altamirani (Dugas), 1895, Description d'un axolotl des Montagnes de las Cruces (Amblystoma altamirani, A. Dugas):.1, pl. 1. Rhyacosiredon Zeorae Taylor, 1948, Univ. Kan. Sci. Bull., 29 : 845, pl. 26. Rhyacosiredon rioularis Taylor, 1940, Herpetologica, 1 : 171, pl. 17. Rhyacosiredon zempoalensis Taylor and Smith, 1945, Proc. U.S. Natl. Mus., 95 : 527. Genus Ambystoma Tschudi Siredon Wagler, 1830, Naturl. Syst. Amphib.: 209. Type species axolotl.Cuvier = mexicanus Shaw, 1789. Ambustoma Tschudi, 1888, M6m. Soc. Sci. Nat. Neuchatel, 2 : 92. Type species subviolacea Barton = maculatum Shaw, 1802. Xiphonura Tschudi, 1838, Mdm. Soc. Sci. Nat. Neuchatel, 2 : 95. Type species le#ersonianum Green, 1827. , Salamandroidis Fitzinger, 1848, Syst: Rept. : 88. Type species, subviolacea .Bar- ' ton = maculatum Shaw, 1802. Axolotes Owen, 1844, Ann. Mag. Nat. Nist., 14 : 23. Type species guttata Owen = mexicanus Shaw, 1789. Amblystoma Agassiz, 1846, Nomenclator ZooL -Rept., 8 2 Emendation. Heteromton Gray, 1850, Cat. Batr. Grad. Brit. Mus.: 88. Type species ingens Green = tigrinum Green, 1825. Plagiodon Dum6ril, Bibron and Dumdril, 1824, Erpa. Gan., 9 : 101. Substitute name for Ambustoma Desmiostoma Sager, 1858, Penin. Jour. Med., 5 : 428. Type species maculatum Shaw, 1802. Camarataxis Cope, 1859, Proc. Acad. Nat, Sci. Phila., 11 : 122. Type species maculatum Shaw; 1802. Pectoglossa Mivart, 1867, Proc. Z661, Soc. London, 1867 : 698. Type species persimilis Gray = ie#ersonianum Green, 1827. Sirenodon Desor, 1870, Bull. Soc. Sci. Nlit. Neuchatel, 8 : 269. Type species lichenoides Baird = mauortium Baird. Linguaelapsus Cope, 1887, Amer. Nat., 21: 88. Type species annuatum Cope, 1886. Ptioambustoma Adams and Martin, 1929, Amen Jour. Sci. (Sth ser.), 17 : 504. .Type species kansense Adams and Martin, 1929. Bath{/siredon Dunn, 1989, Not. Nat., no. 86 : 1. Type species dumerili Dugds, 1830. ?Lanebatrachus Taylor, 1941, Kans. Geol. Surv. Bull., 25 (18) : 180: Type species martini Taylor (= kafisense Adams and Martin, 1929 ?). ?Ogalldlabatrachus Taylor, 1941, Kans. Geol. Surv. Bull., 25 (18): 181. Type species horarium Taylor (= kansense Adams and Martin, 1929 ?). 32 BULLETIN FLORIDA STATE MUSEUM Vol. 8 DEFINITION.-Ambystomatinae in which metamorphosed adults have the prevomerine teeth in a typically adult position; premaxillary fontanelle nearly or completely obliterated in adults; maxillae of nor- mal extent; hyobranchium almost entirely cartilaginous; tips of teeth definitely bifid, often very blunt or even expanded, occasionally pointed but very sharply hooked inwards; sides of parasphenoid parallel or concave, not diverging; four phalanges in the fourth toe in most forms; metamorphosis is customary, but neoteny may Occur in some species and is apparently obligatory in a few. RANGE.-The range of the genus is virtually eoextensive with that of the family. DIscussIoN.-I am fully aware of the violence being done to the law of priority by the use of· the name Ambystoma for this genus while retaining in it the species mexicanum. I can, however, find no morpho- logical basis for separating the genera Siredon and Ambystoma, and the physiological basis, if any, is certainly extremely tenuous. As Schmidt (1958) has pointed out, the substitution of the earlier name , of Siredon for Ambystoma has long been avoided by American authors as too fiagrant a disregard of usages although most of them are well aware that a generic distinction between the two is more imaginary, than real. The only logical solution of the problem appears to be an action by the International Commission on Zoological Nomenclature to invalidate the name Siredon and place Ambystoma on the list of nomina conservanda. On the assumption that such action will at some time be taken, I have chosen for the present to disregard priority in ~ preference to disregarding either usage or the biological evidence. The genus Ambystoma is a widespread and rather variable one, containing the great majbrity of the known Species .in the family. Three subgenera and several species groups are discernible within the genus, although the grouping is not always so clearly indicated as might be desired. Comparisons between Ambystoma kind Rh!/acosire- don have been discussed in connection with the latter; details of struc- ture and, more particularly, variation within the genus will be dis- cussed under the various subgroupings. Subgenus Ambystoma Tschudi DEFINITIoN.-Members of the genus Ambystoma in which the premaxillary spines are ndt exceptionally long and narrow, and lack a vertical lamina or thickening; tongue with longitudinal plicae, or plicae radiating from a posterior field; teeth of each dentigerous ele- ment arranged in a single row in adults; otoglossal cartilage annular 1958 TIHEN: AMBYSTOMATID SALAMANDERS 33 in adults; dentary of adults with a promment Range of bone on the lingual surface; costal grooves 10 to 18 (rarely 14). RANGE,-The range of the subgenus is coextensive with that of the genus. DIscussIoN.-Most of the species are ·included in this subgenus. There is close general agreement among all the forms in regard to the major osteological features, but variation in details suggest the existence of four species groups. As has been stated, the distincti6ns between the groups are not always sharp and clear-dit, but certain generalizations can be made and apparent evoluti6nary tendencies noted. The proper assignment of some of the species to a species group is open to question, particularly in the case of'some of the rarer Mexican forms. The tigrinum group DEFINITIoN.-Members of the subgenus Ambystoma with the trunk vertebrae relatively short and broad (see the section on vertebrae); premaxillary spines tend to be rather short and broad; parasphenoid typically straight-sided, or with sides only very slightly concave, and without or with only a slight posterior expansion underlying the otic region; no extensive diastema between prevomerine and palatine teeth; a postdentigerous process on the prevomers; choanae without „. a lateral bony border; os triangulare tending to be longitudinal; neoteny occasional to common in many, perhaps all, species. RANGE.-Southeastern British Columbia and the southern half of Alberta to New York, south throughout most of the United. States and Mexico to the southern edge of the Mexican plateau, excepting only the Pacific Northwest, Baja California, part of the Florida peninsula, and the 'Mexican coastal lowlands. Fossil representatives are known . from the upper Pliocene of Kansas and from the Pleistocene of Various localities in the central and western United States and northern Mexico. DISCUSSION.-The characterization of this group is based primari- ly on the species tigrinum. Several of the Mexican forms are poorly known, especially with respect to skeletal characteristics. All have been referred tentatively to this group, largely on a geographic basis, but it is possible that some either should not be so referred or that, when they become better known, it will be necessary to modify the definition of the group. This would appear to be the most primitive group of the genus, and is characterized primarily by the absence of specializations found 34 BULLETIN FLORIDA STATE MUSEUM Vol. 8 in members of the other groups. The form of the parasphenoid is somewhat variable, in some cases even approaching the condition in the maculatum and opacum groups, but there is a strong tendency towards a reduction, even loss, of the posterior expansion, and a lack of concavity of the sides. The great majority of individuals possess a parasphenoid that can be recognized as the tigrinum type without difficulty. This particular feature is considered. to be a development that has occurred within the line leading to this group, and is about the only 6steological feature in which this group is considered to be more highly modified from an ancestral condition than are the other groups. The ds triangulare is frequently about equally triradiate, that is, the three rami are of about equal length. There is a tendency in some forms for an elongation of the anterior ramus, and a reduction of the other two, so that the element becomes A -shaped. This is rather variable even intraspecifically, so is probably not a matter of great significance. The proportions of the premaxillary spines are also subject to some variation, but there is a definite tendency in most forms for these spines to be relatively short and broad, at least as compared with those of the maculatum group (cf. figs. 2D, E). In occasional specimens there may be a slight diastema between the prevomerine and palatine teeth; when such a diastema is present it is of limited extent as compared with the wide diastema usually found in other groups. Palatine teeth are always present. As mentioned in the introductory section, the posterolateral corner of the prevomer proper is not produced poste- riorly to form a lateral choanal border. This, so far as has been ob- served, seems to constitute a constant, th6ugh minor, distinction be- tween the tigrinum group and all others. The retention of a small postdentigerous process on the prevomer also would appear to be a constant characteristic. The majority of species are relatively large forms, with snout-vent lengths of 90 to 100 mm. being common for adults. In some rieotenic larvae the total length exceeds 800 mm., with a snout-vent length of somewhat over half of this t6tal. They are, in general, rather heavy- bodied forms, with well-developed limbs that overlap strongly when adpressed. The tail is typically heavy but fairly long, usually between 40 and 50 percent of the total length. Individual tail length is, even within a species, a highly variable feature, and useful only in a gen- eral way. The number of costal grooves is usually 12 or 18, occasion- ally 11, rarely 14. The color pattern is variable, but yellow and black are the predominating colors in the group. 1958 - TIHEN: AMBYSTOMATID SALAMANDERS 85 Neoteny is a common phenomenon in the tigrinum group, partic- ularly in those subspecies of tigrinum inhabiting the R6cky Mountains and northern Plains region, and some forms of the Mexican plateau. The neotenic 'populations usually produce individuals of decidediy greater size than those which metamorphose normally. Mature ne- otenic larvae can be distinguished from normal immature larvae by the development of ~ vertebral septa interrupting the notochord, the de- velopmeht of strongly bifid and blunt or somewhat expanded tooth tips, thelarger number of maxillary-premaxillary and mandibular teeth and, frequently, simply by size. If the characteristics · of the upper Pliocene A. hibbardi have been correctly interpreted, specificallythe apparent poor ossification of certain cartilage bones, the diagnostic features may be a result of partial neoteny, though there is good evi- dence that the species consistently underwent metamerphosis, and was not ne6tenic in the full sense of the term. REFERRED FoRMS.-As pointed out above, many of the Mexican species are poorly known, and can be only tentatively referred to this group. There is, I believe, special reason to question Such a reference for the Chihuahuan A. #uvinatum. Although no adult individuals are known, it seems likely that this species differs rather markedly from the typical members of the tigrinum group m size, proportions, num- ber of costal grooves, and possibly in number of teeth. It is, however, along with one or two .other forms of questionable affinity, referred to the tigrinum group for the time being on the basis of geographical probability. Amb!/stoma (Ambystoma) amblycephalum Taylor, 1940, Univ. Kans. Sci. Bull., 26 (for 1989) : 420, pl. 45: Ambystoma (Ambystoma) bombypellum Taylor, 1940, Uni*. Kans. Sci. Bull., 26 (for 1989) : 418,· pl. 45. Amb!/stoma (Ambystoma) fluoinatum Taylor, 1941, Copeia, 1941 (8) 144. Ambystoma (Ambystoma) granulosum Taylor, 1944, Univ. Kans. · Sci. Bull.,30: 57, Dl. 8 An*ystoma (Ambystoma) hibbardi Tihen, 1955, Contrib. Mus. Paleont. Univ. Mich., 12 (11) : 230. Ambystoma (Ambyston:a) lacustris Taylor and Smith, .1945, Proc. U.S. Natl. Mus., 95 : 582, pl. 18. Ambystoma (Ambystoma) ordinarium Taylor, 1940, Univ. Kans. Sci. Bull., 26 (for 1989) : 422,·pl. 46. 86 BULLETIN FLORIDA STATE MUSEUM - Vol. 3 Ambystoma (Ambustoma) rosaceum rosaceum. Taylor, 1941, Copeia, 1941 (3):143. Ambystoma (Ambystoma) rosaceum nigrum Shannon, 1951, Proc, U.S. Natl. Mus., 101 : 466. Ambystoma (Ambystoma) rosaceitm sonoraensis Shannon, 1951, Proc. U.S. Natl. Mui., 101 : 468. Ambystoma (Ambystoma) subsalsum Taylor, 1948, Copeia; 1948 (8) : 151. Ambystoma (Ambystoma) tigrinum tigrinum (Green), 1825, Jour. Acad. - Nat. Sci. Phila. (ser. 1), 5 : 116. Ambystoma (Amb!/stoma) tigrihum californiense (Cray), 1853„ Proc. Zool. Soc. London, 1858 : 11, pl. 7. Ambystoma (Ambystoma) tigrinum diaboli Dunn, 1940, Copeia, 1940 (8) : 160. Ambystoma (Ambustoma) tigrinum maportium Baird, 1850, Jour. Acad. Nat. Sci. Phija. (2), 1 : 284. Ambystoma (Ambystonza) tigrinum ;nelanostictum Baird, 1860, Expl. Surv. R.R. Miss. Pacific, 12, pt. 2 : 806. Ambystoma (Ambystoma) tigrinum nebulosum Hallowell, 1854, Proc. Acad. Nat. Sci. Phila., 6: 209. Ambystoma (Ambystoma) tigrinum stebbinsi Lowe, 1954, Proc. Biol. Soc. Wash., 67: 248. Ambystema (Ambystoma) tigrinum velasci Dugds, 1888, La Naturaleza (Ser. 2), 1 :142. The mexibanum group DEFINITION,-Members of the subgenus Ambystoma virtually in- distinguishable morphologically from the members of the tigrinum - group, but differing in being usually or consistently neotenous under nfitural conditions; parasphenoid sometimes straight-sided and un- expanded as in the tigrinum ·group, but frequently approaching a form more typical of the other groups of the genus; members of this group tend to be larger than those of the tigrinum group, but do not exceed the maximum size of neotenic larvae of that group. RANGE.-The anciedt lakes of the southern Mexican plateau. One species from the middle Pliocene of Kansas is also tentatively referred to this group. DIscussIoN.-There is some question as to whether the forms as- signed here should even be placed in a group separate1from tigrinum, but a few very minor features of morphological difference, in conjunc- 1958 TIHEN: AMBYSTOMATID SALAMANDERS 87 tion with the apparently usually obligatory neotenic mode of life, have induced me to consider them a separate group. I have previously (1955) bointed out the possibility that the middle Pliocene Plioambystoma kansense is not a distinct gend5, but a neot- enic form of Ambystoma. In that paper it was suggested that the name Plioanibystonia be tentatively retained pending some informa- tion concerning the adult condition. Since that time I have examined a larger number and variety of larval skeletons of Recent forms, and am convinced that when distinctions of superspecific taxonomic signifi- cance exist, they can usually be discerned in.the larvae. In the absence of any evidence to the contrary, I am now of the. opinion that Plio- atnbystoma kansense does not differ in any important respect from Recent members of the, tigrinum-mexicanum complex. Since, so far as can be determined from the known fossil populations, this species was consistently heotenic, it is referred to the mexicanum rather than to the tigrinum group. I am also of the opinion that Lantbatrachus and Ogallalabatrachus are most probably based on aberrant specimens of kansense, and have therefore placed these genera in the synonymy of Ambystoma. Ad- mittedly this must be a matter of conjecture, based on'the limited number of specimens available, but I believe it is the most likely interpretation of the eurrently known material. .REFERRED FORMS.- Ambystoma (Ambystoma) kansense (Adams and Martin); 1929, Amer. Jour. Sci. (Sth ser.), 17 : 504. Anibyktonia (Ambystonia) lermaensis (Taylor), 1940, Univ. Kans. Sci. Bull., 26 (for 1989) : 427, Dl. 48. Ambystoma (Ambystoma) mexicanum (Shaw), 1789, Nat. Misc., 9 : pls. 848, 344. The opacum group DEFINITION.-Members of the subgenus Ambystoma with vertebral proportions somewhat intermediate between those of the two preced- ing and those of the fbllowing group; parasphefloid with concave sides and a definite alate expansion posteriorly; a wide diastema be- tween the prev6merine and palatine teeth; no postdentigerous process on 'the prevomers; a partial lateral bony border to the choanae; os triangulare tending to be transverse; neoteny unknown. RANGE.-Most of the United States east of the Great Pfains. DISCUSSION.-This group is very similar to the maculatum gioup in most respects, but the vertebral prop6rtions are somewhat intOr- 88 BULLETIN FLORIDA STATE MUSEUM Vol. 8 ' mediate between that group and the tigrinum group. Also, the mem- bers of the opacuin group have a somewhat mOFe rounded, less oval skull, and the proportions of the premaxillae resemble those of the tigrinum group more closely than they do the maculatum group, al- though in both the vertebrae and the premaxillae there is overlapping variation in both directions. Only two species are included in this group, opacu,n and tal- poideum. Both are small forms with stout bodies, well-developed limbs, wide heads, and moderately short tails. The t6tal length never exceeds 120 to 125 millimeters, and is usually decidedly less. Al- though the color patterns differ ,markfdly, in both forms there is a brown to black ground color with whitish or grayish markings. In correlation with the short bodies, the number of costal grooves is ' low, only ten or eleven in talpoideum. eleven or twelve in opacum. Neoteny is unknown in either form. The skull of talpoideum is particularly short and broad, with these proportions reflected in most of the individual cranial bones. It is · further characterized by a particularly strong development of the vari- ous crests for muscle attachment. A. opacum is apparently devoid of any unique' osteological features, but is distinctive in bping the only member of the family that lays its eggs on land. ' REFERRED FORMS.- Ambystoma (Ambystoma) opacum (Gravenhorst), 1807, Vergl. Ueber- sicht Zool. Syst. : 481. Ambystoma (Ambystoma) talpoideum Holbfook, 1888, N. Amen Herp., ed. 1,8:117, pj. 29. The maculatum grouB DEFINITION.-Members of the subgenus Ambystoma with the ver- tebrae definitely elongated; premaxillary spines relative19 longer and narrower than in the preceding groups, parasphenoid with concave sides and a deSnite alate expansion posteriorly; usually a wide di- astema between the prevomerine and , palatine teeth (occasionally lacking in maculatum); no postdentigerous process on the prevomers; a partial lateral bony choanal border; os triangulare tending to be transverse; neoteny may occur rarely in A. gracile, but is not known > in any of the other species. RANGE.-Divisible into two disjunct areas. AIong the Pacific coast from southern Alaska · to northern California and east into Montana; in the east from southern Labrador to southeastern Manitoba and south throughout most of the United States east of the Great Plains. .1958 TIHEN: AMBYSTOMATID SALAMANDERS 89 DISCUSSION.-This is not a compact group, the five species referred to it being rather diverse, but they appear nevertheless to be more closely related to each other than fo any other member of the genus. The primary distinguishing feature of the group is the elongation of the vertebrae. In gracile and to a lesser extent macrodact!/lum. this elongation is somewhat less marked than in the eastern forms. Both gracile and maculatum are large, heavy-bodied forms, resembling the members of the tigrinum group in general habitus; maculatum also resembles that group in the possession of a yellow and black color pattern. A. le#ersonianum, laterale, and macrodact!/lum are smaller forms, with more' slender bodies and limbs. The tail is moderately long, usually approximating 50 percent of the total length. The parasphenoids frequently po'ssess a characteristic small'process on the anterior end, but this is not universally present even in a given species. The pterygoids tend to be somewhat reduced, with a con- striction shortly behind the tip of the anterior process, so that this process appears to terminate in a knob rather than tapering to a point. The number of costal grooves is typically 11 in gracile, 12 or 18 in macrodactylum, and 12 (occasionally 11 or 18) in the other species. This may be thought of as a northern group, each of the forms ex- cepting laterate, which has a rather limited range, havihg a more northerly distribution than does any member of any other group oc- cupying a similar type of habitat, although tigrinum may range nearly as far northward in the more arid Canadian Plains. REFERRED FORMS.- Ambystonza (Ambystoma) gracile gracile Baird, 1859, Pacific R. R. Surv., Rept., 10, Williamson's Route, pt. 4, no. 4 : 13, pl. 44. Ambystoma (Ambystoma) gracile decorticatum Cope, 1886, Proc. Amer. Philos. Soc., 28 : 522. Ambystoma (Ambystoma) ie#ersonianum (Green), 1827, Contr. Mac- lurean' Lyc., 1: 4, pl. 1. Ambystoma (Ambystoma) laterale Hallowell, 1858, JBur. Acad. Nat. Sci. Phila., ser. 2, 3 : 855. Ambystoma (Ambystoma) macrodactylum macrodactylum Baird, 1849, Jour. Acad. Nat. Sci. Phila., ser. 2,1 : 299. Ambystoma (Ambystoma) macrodactylum croceum Russell and Ander- son, 1956, Herpetologica, 12 : 187. Ambystoma (Ambystoma) maculatum Shaw, 1802, Gen. Zool., 8 (pt. 1) : 804. 40 BULLETIN FLORIDA STATE MUSEUM Vol. 3, Subgenus Linguaelapsus Cope DEFINITION.-Members of the genus Ambystoma in which the premaxillary spines are typically long and narrow, and have a ventral lamina or thickening just above the dentigerous ramus; tongue with plicae branching from a median groove; teeth (except in mabeei) ar- ranged in more than one row on all dentigerous elements; palatine teeth lacking (except in mabeet); no annular otoglossal cartilage; den- tary of adults lacking a prominent Bange of bone on the lingual sur- face in most forms, resembling closely the larval dentary of members of the subgenus Ambystoma, costal grooves 18 to 15. RANGE:-Southeastern United States, west to Texas, Oklahoma and Kansas, north to Illinbis, Indiana, Ohio and North Carolina; most of peninsular Florida excluded. A single form apparently referable to this subgenus is known from the Mexican state of Mexico. DISCUSSION.-The more specialized members of this group are so distinctive that they might well be considered as representing a genus distinct from Ambystoma were it not for the existence of intermediate forms. There is, within the group, a gradation from a nearly typical Ambystoma condition to these· highly modified forms. For this rea- son, subgeneric status seems preferable to full generic status. All of the species· are of small to moderate size, with slender bodies and slight limbs that do not meet when adpressed. The tail tends to be relatively short, frequently less than 40 percent of the total length. The color pattern is'variable, but blacks, grays, and whites predom- inate. The number of costal grooves is consistently rather high, as . compared with the subgenus Ambystoma. There is a strong tendency for the cranial sutures to be extremely irregu}ar. The parasphenoid is consistently strohgly expanded posteriorly. Interspecific variation is considerable, and is discussed in the following paragraphs. A. inabeei is scarcely recognizable as belonging to the subgenus Linguaelapsus, excepting for the condition of the tongue and the lack of an annular otoglossal cartilage. The premaxillary spines are rela- tively short and broad; the vertical lamina is very limited in extent; its development being so slight that probabiy no significance would be attached to it in the absence of other resemblances to Linguaelapstis, There is, however, a definite irregularity to the cranial sutures, and at least some of the individual teeth are of the form occurring in other species of Linguaelapsus-that is, the inner fork of the bifid tip is long, sharp-pointed, and very strongly bent inwards, the outer fork becoming nearly obsolete. In contrast to conditions in the more typi- cal members of the subgenus, mabeei possesses palatine teeth, all 1958 TIHEN: AMBYSTOMATID SALAMANDERS 41 teeth are arranged in single rows, the dentary has a prominent Range of bone on the lingual surface, the premaxillary spines are rela- tively short and broad, the vertebral proportions are very iimilar to those of the nwculatum group, and all skeletal elements are very sim- ilar in form and proportions to those in members of the maculatum group. The prevomer bears a postdentigerous process-a condition found otherwise in the genui only in the tigrinum group (and pre- sumably in metamorphosed individuals of the-mexicanum group). In annulatum the premakillary spines are longer, and the vertical expansion somewhat more extensive. The teeth are multirowed (usu- BIlly three rows) and the palatine teeth lacking, the palatine itself being greatly reduced in extent.' In this and the, succeeding species, the prevorner has a less transverse, more diagonal, orientation, rernin- iscent of the larval position, though by no means identical with it (cf. figs. 4K-L). The dentary is relatively short; there is a Range of bone on the lingual surface, but it is much reduced in extent. The vertebrae are still similar to those of the maculatum group; the dorsal surface of the atlas is slightly flattened. The cranial sutures are defi- nitely irregular. Minor differences in form and proportions of a num- ber of skeletal elements (as compared with the maculatum group) can be observed. In most respects cing#latum and texanum are very similar to each other. In both the premaxillary spines are long and narrow, the region between the nares frequently being so narrow that, because of - the vertical thickening, the bone in this region appears to have a pri- marily vertical orientation. The ends of the vertebral centra are - widened as compared with the two preceding species (see the section , on vertebral variation), and the dorsal surface of the atlas is definitely flattened. There is only a -slight ridge on the lingual surface of the dentary, which thus resembles, in this. respect, the larval dentary of other members of the genus. The palatine is edentulous, of variable extent in cingulatum, always greatly reduced or apparently lacking in texanum. The cranial sutures are irregular, perhaps not so markedly so in cingulatum as in texanum. Nearly all cranial elements are dis- tinguishable in details of form from those of the merribers of the nwcu- Zatum group. The teeth, in both species, are in three or four rows on -all dentigerous elements; the individual teeth of the two species differ markedly in form, as described in the preliminary discussion. The osteology of A. schmidti is virtually unknown, except for the fact that the te6th are in multiple rows. Radiographs (fig. 11) of the type are not clear as to details, but the apparent vertebral propor- tiens,-the apparently long and slender premaxillary spines, and the ar- 42 BULLETIN FLORIDA STATE MUSEUM Vol. 3 rangement of the prevomerine teeth support the idea that this species is properly referable to the subgenus Linguaelapsits, Figure 11.-Radiographs of the type specimen of Ambystoma schmidti. If only the forms cingulatum and texanum were considered, it wouId be tempting to postulate a primitive origin for this group. The absence of a lingual flange on the dentary, the strongly expanded parasphenoids, the Iack of an annular otoglossaI cartilage, and sev- eral minor details, are all points of resemblance with the presumed condition in primitive members of the family. The affinities of mabeei with Linguaelapsus are, however, strongly indicated by the condition of the tongue and by the apparent rudiments of other specializations characterizing the more modified forms, and mabeei is unquestion- ably a member of the genus Ambystoma. Most of the features men- tioned above, instead of representing a retention of primitive condi- tions, could be the result of partial neoteny; this must be the case if Linguaelapsus is to be derived from a stock that could already be con- sidered as belonging to the genus Ambysto,na. The often somewhat diagonal orientation of the prevomers might also be considered a ne- otenic feature. In general, mabeei resembles very strongly the nzaculatum group. It does, however, share with the tigrinum group the apparently primi- i958 TIHEN: AMBYSTOMATID SALAMANDERS 43 tive feature of a postdentigerous process of the prevorher'. The as- sumption is' that the subgenus Linguaelapsus was derived from the . maculatu» group (or the line leading thereto) after most of the macu- " latum group characteristics had been established, but before the post- dehtigerouk process had been lost, at least in all members of the group. REFERRED FORMS.- - Ambystoma (Linguaelapsus) annulatum Cope, 1886, Proc. Amen , Philos. Soc., 28 : 525. Ambystoma (Linguaelapsus) cingulatum cingulatum 'Cope, 1867, Proc. Acad. Nat. Sci. Phila., 19 : 205. Ambystoma (Linguaelapsus) cingulatum bishopi Coin, 1950, Ann. Carnegie Mus., 31 : 800, pl. 1. Ambystoma (Linguaelapsus) mabeei Bishop, 1928, Jour. Elisha· Mitch- ell Sbi. Soc.,· 43 : 157, pl. 28. Ambystoma (Linguaelapsus) schmidti Taylor,.1989, Univ. Kans. Sci. Bull., 25 (for 1938) r 268, pl. 26. Amb!/stoma (Linguaelapsus) texanum (Matthes), 1855, Allg. deutsche naturh. Seitschr. (new ser.), 1 : 266. Subgenus Bathysiredon Dunn , DEFINITION.-A neotenic ambystomid, similar in general to the mexicanum group of the subgenus Ambystoma, but differing in the following respeetsi Only three phalanges .in the fourth toe (four in nearly all other Ambystoniinae); digits short, webbed (uhwebbed in larvae of other forms)i gill branches with filaments to near base (none arising near base in other forms); 8 td 10 gill rakers on the anterior face of the third ardh (twelve or more in most members of the mexi- canum and tigrinum groups); head strongly depressed. RANGE.-Known only from Lake Ptitzcuaro, Michoacdn, Mexico. DISCUSSION.-This form is stili rather imperfectly knowd. .As a whole, the skeleton offers little with which t6 distinguish this group from the mexicanum group of Ambystoma, though some minor differ- ences in proportions of individual cranial elements can be found. These are perhaps associated with the depression of the skull, though this depression is by no means as obvious in the skull itself as it is in the· intact fleshed individual. I have seen 'only a single skeleton of Bathysiredon, so have no idea as to the extdnt of variation in the minor proportional differences noted. The parasphenoid, at last in this one individual, is somewhat more greatly expanded posteriorly than is typical for the mexicanum and 4 - 44 BULLETIN FLORIDA STATE MUSEUM Vol. 8 tigrinum groups, though not at all beyond- the range of variation in ' these groups. Its affinities with. the mexicanum-tigrinum complex can scarcely be questioned. Generic differentiation, then, seems 'to be based on tho following ' features: (a) Only three phalanges in the f6urth toe. This varies infraspe- cifically in some members of the tigrinum group--for example, it is. a subspecific characteristic for A. rosaceum nigrum (Shannon, 195I), and may well be subject to individual variation iii other forms. (b) Digits short and webbed. This apparently is a valid distinction between dumerili and other species, but not one that I can consider to be of.generic importance. (c) Gill branches with filaments to near base. Again this distinc- tion, even if valid, seems'scareely to be one of generic importance. (d) Eight to tep gill rakers on the anterior fade of the third arch. If all members of the inexicanum group and Bathysiredon were put in a Single gfoup, the total variation within that group, in respect to the numbet of gill rakers on the anterior face of the third arch, would 6e less than the corresponding variation among forms all assigned to the single species tigrinum (Dunn, 1940). (e) Head strongly depressed. Proportions of the bead are some- what variable within the genus Ambystoma, and some of these varia- tions are much more clearly reflected in the skull (e.g., comparing . A. talpoideum with ie#ersonianum) than - is any difference, between dumerili and members of the mexicanum group. It does not appear to me that, at least on the basis of present knowl- - edge, generic status for this form is fully warranted. On the other hand, some diffefences do exist between dumerili and the members of the mexicanum group. I suggest, therefore, that the category be con: sidered a subgenus of the genus Amb!/stoma rather than a distinct genus. REFERRED FORMS.- Ambystoma (Bathysiredon) dumerili du;nerili (Dugds), 1870, La Nat- uraleza, 1 : 241, pl. 5a. Ambystoma (Bathysiredon) dumerili queretarensts Maldonado-Koer- dell, 1948, Mem. Acad. Nat. Cim Mexico, 56 : 196. DISTRIBUTIONAL CoNSIDERATIONS A large part of the present range of many species occupies areas that were glaciated during Wisconsin time. Even many of the forms 1958 TIHEN: AMBYSTOMATID SALAMANDERS 45 whose range is outside of the glacial limits probably have had their range greatly altered by climatic and ecologic changes associated with the various glacial advances and retreats. It is, in such a case, rather presumptive to speculate on past distributions, particularly in a greup that has apparently been in existence throughout the Tertiary, but for which -fossil records are extremely limited. Nevertheless, a possible arrangement can be postulated. The, ancestral ambystomatids or proto-ambystomatids probably en- ' tered North America from Asia by a northern route in the very early Tertiary or perhaps in the Iate Mesozoic. It seems likely that very early in the history of the family there was a split into two ,groups. One such group became isolated west of the Rockies (or an equiva- lent western barrier, depending -partially on the adtual period of entry into North America). It is assumed that thi,s.group retained a number of characteristics of the ancestral stock, specifically, the independent frootic and exoccipital, the strongly expanded parasphenoid, the rel- atively simple tooth structure, the independent lacrimal, and perhaps to a limited extent, a disposition of prevomerine teeth re'sembling that of hynobiids. This ancestraL stock, must at this time, have already developed ·the essential family characteristics that distinguish the ambystomatids from the hynobiids. It was from this western group that the Dicamptodontinae' and.the Rhyapotritoninae are thought to have been derived. The primary center of dispersal for the other section of the an- cestral stock, giving rise to the Ambystomatinae, seems to have been in the eastern half of the continent, possibly in the general vicinity of what is noiv the Great Lakes region. There is no direct evidence as to the route by· which this area was reached, but it was likely through a northern forested region in what is now Canada, dufing a period of relatively mild climate, rather than across the less humid, generally less wooded, present Plains area further to the south. Par- allel instances of distribution or migration are known in a large num- ber of animal and plant groups-the 'salamandrids, to, cite another example from the Amphibia. The time of establishment of this group in eastern North America is also a matter of speculation but, if the further postulations to be made are correct, it must hai,e been no' later than the Miocene, and probably earlier. This eastern seetibn, either during the course of migration or after establishment, lost the independent laerimal; the exoccipital and prootic were' fused, and the other developments characterizing the subfamily Ambystomatinae appeared. We may assume an outward spread from this center. N6rthward di5persal, though perhaps oc- 46 BULLETIN FLORIDA STATE MUSEUM Vol. 3 curring to some extent, would be limited much of the time by temper- ature. The relatively arid, unforested condition of the Plains region through much of the Tertiary effectively prevented any extensive spread to the west. The major movement from this center must, then, have been southward. Rhyacosiredon, or its precursor, was probably an early entrant into Mexico that has survived only in the southern' part of the plateau, near the periphery of the range of the family. Approximately contemporaneously with the southward mov,ement of the Rhyacosiredon stock, certain modifications, such as,Be chondri- fication of the adult hyobranchium, the development of more highly modified teeth, the devdlopment. of a. lingual Range on the dentary, and so on, were occurring in the central, stock.·A second wave of migration is represented by the A. tigrinum - mexicanum - Bathysire- don complex. · This group, at some point, became adapted to relatively arid, unforested conditions add underwent a secondary dispersal northward from the Mexican region. The appearance of kansense in the middie Pliocene of Kansas, of hibbardi in the upper Pliocene of the same area, of tigrinum there and elsewhere in the early Pleisto- cene as well as throughout the entire northern and western portion of its present range, are postulated to represent northward movement from this secoddary centef, rather thdn direct. dispersal from the original northeastern center. . The mexicanum group and .the related subgenus Bath!/sire~lon are probably early derivatives of this line; they are, like Rhyacosiredon, now limited to the southern edge of the Mexican plateau. It might be mentioned that the development of neoteny, or in some species, aquatic habits even for metamorphosed individuals, might be a factor in the adaptation of this group to arid conditions. These aquatic habits allow full utilization of such permanent watef as is available. ' Food can be obtained in the aquatic habitat, and it is not necessary for the individual animals to be subjected-to the rig- orous terrestrial conditions of the arid areas in which they live. Taylor and Smith (1945) hava suggested that the varying chemical compo- sition of the different permanent bodies of water in the region may have been one of the factors involved in speciation in that region. Simultaneously with the development of the tigrinuni-mexicanum complex in Mexico, further differentiation was occurring in the region of the present eastern United States. Linguaelapsus may well have developed about this period, probably in the southeastern United States or the Gulf region. The retention of a postdentigerous process on,the prevomer suggests that it was derived at a relatively early date from the maculatum-opacum group precursor stock, but not be- 1958 TIHEN: AMBYSTOMATID SALAMANDERS 47 fore most of the characteristics distinguishing those two groups from tigrinum had appeared. The opacum group may have arisen from a small section of the maculatum group in the southeast.' We can postulate · relatively mild, humid climate conditions at the time the maculatum group was developing, allowing an apparently early rep- resentative (gracile) to reach the Pacific region along some northward path as was postulated previously for the eastward movement of the ancestral Ambystomatinae. The appearance of macrodactylum in the west is considered the result of a second such movement, rdther than representing a development within' the western region. Agaih, 'the time of these occurrences is conjectural. - I think it probable that this· major.pattern of distribution had been established before the close of the Pliocene, so-that at the beginning of ' the Pleistocene the basic distribution Qf the major divisions of the family was much as it is today. That individual ranges were greatly altered by the advance ind retreat of the glaciers during the Pleisto- cene is unquestioned, but this would not require any alteration in the baSic pattern. At times, corresponding with glacial advance, the elements of this pattern were probably compressed and perhaps shifted southward; at other times they expanded and were shifted northward, but: the relationships between, the elements remained essentially con- stant: The presence of A. schmidti in the southern Mexic'an plateau is ' explainable on the basis of one of these southward shifts,,presumably coincident with a major glacial advance. The relatively primitive 'mabeei may have survived the glaciations in a coastal pocket corre- kponding closely toits present range. If it should be found that some of the now poorly known Mexican species are not actidly referable to the tigrinum group, but are more properly placed with one of the other groups, their presence, in Mexico is readily explainable on the same basis as that of A. schmidti. In summary, the appearance of the family in North America, and the establishment of two major stocks, must have occurred in the late Mesozoic or very ear12 Tertiary. The various species groups of Amby- stoma were established before the Pliocene, and their distribution, in major aspects, established at.least before the end of the Pliocene. Some, possibly most, of the extant species were in existence by the earliest - Pleistocene; individual ranges may have been greatly altered from time to time, but the major relationships of distributional pattern have probably been maintained much the sarne since the close of the Plio- cene. 48 BULLETIN FLORIDA. STATE MUSEUM Vol. 8 LITERATURE CITED, Cope, E. D. 1887, Tlie hyoid structure in the amblystomid salamanders. Amer. Nat., vol. 21, pp. 87-88. DeMarco, Michael N. 1952. Neoteny and the urogenital system in the salamander Dicamptodon ensatus (EschBcholtz). Copeia, 1952, no. 8, pp. 192-198. Dunn, Emmett Reid 1920. Notes on two Pacific Coast Ambystomidae. Proc. New England Zool. Club, vol. 7, pp. 53«59. 1,923. The salamanders of the family Hynobiidae. Proc., Amer. Acad.,Arts Eind , Sci., vol. 58, no. 18, pp. 445-528. , 1940. The races of Ambystoma tigrinum. Copeia, 1940, no. 3, pp 154-162. 1941. The "opercularis" muscle of salamanders. Jour. Morphol., vol. 69, no. 2, pp. 207-215. ' Eaton, Theodore H.; Jr. 1933. The occurrence of streptostyly in the Ambystomidae. Univ. California Publ. Zo61., vol. 37, no. 17, pp 521-526. 1984. The affinities of Dicamptodon and Rhyacotriton. Cofeia, 1984, no. 4, p. 182. Geyer, H., arid' Gunther E. Freytag 1949: Ueber Kreuzungen zwiscKen Tigersalamander (Amhystoma tigrinum) - und Axolotl (Ambystoma mexicanum) und ihre FrGeneration. Mitteilun- gen aus dem Museum fur -Naturkunde und Vorgeschichtd und dem naturwissenschaftlichen Arbeitkreis, Magdeburg, v61 2, no. 2, pp. 9-28. Graf, William 1949. Obser.vations on the salamander Dicamptodon. Copeia, 1949, no. 1, ' pp. 79.80. Highton; Richard 1957. Correlating costal grooves with trunk vertebrae in,salamanders. Copeia, 1957, no. 2, pp..107«109. Hilton, William A. 1946. A preliminary study of the 5keletons of Amblystomidae. Jour. Entomol. and Zo61.,vol. 38, no. 2, pp. 29-36. 1948.' The carpus and tarsus of salanyanders. Ibid., vol. 40, no. 1, pp. 1-13. Noble, G. Kingsley, and L. B. Richards 1932. Experiments on the egg-laying of salanianders: Amer. Mus. Novitates, no. 513, pp. 1-25. '* 0 r 1958 TIHEN: AMBYSTOMATID SALAMANDERS 49 Peabody, Frank E. 1954. Trackways of an ambystomid salamander from the Paleocene of Mdn- tana. jour. Paleontol., vol. 28, no. 1, pp. 79-88, pl. 16. Schm,idt, Karl P. 1958. A cheek list of North American amphibians and reptiles. Sixth Edition. Chicago: Amer. Soc, of Ichthyol. and Herpetol., viii and 280 pages. Shannon, Frederick A. 1951. Notes on' a herpetological collecti6n from Oaxaca and other localities in Mexico. Proc. U.S.·Nat!· Mus., vol. 101, pp 465-484. Slater, James R. - 1936. Notes on Ambystoma gracile ' Baird and Ambystoma macrodactulum Baird. Copeia, 1986, no. 4, pp. 284-286. Smith, Hbbart M., and Edward H. Taylor 1948. An annotated checklist and key to the amphibia of Mexico. Bull. U.S. Ndtl. Mus., no. 194, pp. i-iv, 1-118. Stebbins, Robert C., and Chailes H. Lowe, Jr. 1951. Subspecific differentiation in the Olympic salamander Rhyacotriton 04,mpicus. Univ. California Publ. Zoul., vol. 50, ho. 4, PIi· 465-484, pl. 81. Stok6ly, Paul S., and Paul A. Holle 1958. Variation in 'the vertebral axis of the Ambystomidae. Herpetologica, vol. 9, pt. 3, pp. 183-188. 1954. Appendicular skelethn of the Ambystomidae. Ibid., vol. 10, pt. 1, pp. 57-61. Taylor, Edward H.,and Hobart M. Smith ' 1945. Siimmary of the collections of amphibians made in Mexico under the Walter Rathbone Bacon Traveling Scholarship, Proc. U:S. Natl. Mus., vol. 95, pp. 521-618, pls. 18-82. Tihen, Joe A. 1955. A new Pliocene species of Ambystoma, with remarks on other fossil ambystomids. Contrib. Mus. Paleontol. Univ. Michigan, vol. 12, no. 11, pp. 229-244. Twitty, Victor C. 1941. Data on the life · history of Ambystoma tigrinum californiense Gray. Copeia, 1941, no. 1, Dp. 1-4. Villiers, C. G. S. de 1986, Some aspects of the amphibian suspensorium, with special reference 40 the paraquadrate and quadratomaxillary. Anatomischer Anzeiger, vol. 81, pp. 225-304. 50 BULLETIN FLORIDA STATE MUSEUM Vol. 8 1988a. Ueber die angebliche Streptost>die des Amb!/stoma macrodactulum. Ibid., 'vol. 86, pp. 295-299. 1988b. Ueber angebliche Streptostylie bei der amerikanischen Urodelengattung Rhyacotriton. Viertellahrsschrift der naturforschenden Gesellschaft in Zurich, vol. 88, pp. 1-16. Contributions to the BULLETIN OF THE FLORIDA STATE MUSEUM may be in any field of biology. 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