A new atoposaurid crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 5 2018 © 2018 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. A NEW ATOPOSAURID CROCODYLOMORPH FROM THE MORRISON FORMATION (UPPER JURASSIC) OF WYOMING, USA John R. Foster Theme Issue An Ecosystem We Thought We Knew— The Emerging Complexities of the Morrison Formation SOCIETY OF VERTEBRATE PALEONTOLOGY Annual Meeting, October 26 – 29, 2016 Grand America Hotel Salt Lake City, Utah, USA GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover The Little Houston Quarry (Mammal Pit), in the lower middle right of the photograph, is in the Morrison Formation of the north- western Black Hills, Crook County, Wyo- ming, which yielded the crocodyliform jaw described here. i Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would like to volunteer. 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Sprinkel Utah Geological Survey 801.391.1977 GIW@utahgeology.org Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Society of Vertebrate Paleontology Editors Kelli C. Trujillo — University of Wyoming John Foster — Utah Field House of Natural History State Park Museum Cary Woodruff — University of Toronto Octavio Mateus — Universidade Nova de Lisboa GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 5 2018 287 ABSTRACT A left mandible of a small crocodyliform found in the Upper Jurassic Morrsion Formation of north- eastern Wyoming represents the first occurrence of the atoposaurid Theriosuchus in North America. The specimen demonstrates lower jaw morphology, including heterodonty (as indicated by alveolus shape), similar to Theriosuchus and Knoetschkesuchus, but autapomorphies and a unique combination of charac- ters among these taxa indicate that it is a distinct, new species of Theriosuchus. A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA John R. Foster Utah Field House of Natural History State Park Museum, 496 East Main St., Vernal, UT 84078; eutretauranosuchus@gmail.com Citation for this article. Foster, J.R., 2018, A new atoposaurid crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA: Geology of the Intermountain West, v. 5, p. 287–295. © 2018 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. INTRODUCTION Fossil crocodylomorphs are diverse in the Upper Ju- rassic Morrison Formation of western North America, with seven species of terrestrial and semi-aquatic forms occurring, with collective widespread distribution and high abundance (e.g., Clark, 2011; Pritchard and oth- ers, 2013; Foster and McMullen, 2017). In 2004, the left mandible of a small crocodyliform was collected from the Little Houston Quarry in the Morrison Formation of the Black Hills, northeastern Wyoming (figure 1). This specimen was found along with a diverse assemblage of dinosaurs and microvertebrates, the latter including mammals, fish, amphibians, a lizard, sphenodontians, the choristodere Cteniogenys, and turtles, among oth- ers (Foster, 2001). The dinosaurs included particularly abundant basal neornithischians, theropods, and di- plodocine and camarasaurid sauropods. The crocody- liform specimen was initially described as a juvenile goniopholidid that would have demonstrated strong allometric growth in the relative elongation of the lower I-25 I-90 W YO M IN G LHQ Site 50 km Wyoming SO U TH D A KO TA Jackson Laramie Sheridan Casper SundanceGillette Devils Tower NM Newcastle I-90 Figure 1. Location of Little Houston Quarry (LHQ) in Crook County, northeastern Wyoming, USA. 288 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA Foster, J.R. Geology of the Intermountain West 2018 Volume 5 jaw through ontogeny (Foster, 2006). Re-examination of the specimen (MWC 5625) indicates heterodonty in the lower jaw and strong similarities to Theriosuchus, and a redescription is provided here. INSTITUTIONAL ABBREVIATIONS MWC – Museums of Western Colorado, Dinosaur Journey Museum, Fruita, Colorado; BMNH – The Nat- ural History Museum, London, England. SYSTEMATIC PALEONTOLOGY Crocodylomorpha Crocodyliformes Neosuchia Mesoeucrocoylia Atoposauridae Theriosuchus Owen, 1879 Type species – T. pusillus Theriosuchus morrisonensis sp. nov. Figures 2 and 3 LSID. urn:lsid:zoobank.org:act:DAB8542A-7444-4026- 9460-7735F0143C82 Type Specimen MWC 5625, left mandible, nearly complete but missing teeth. Type Locality Little Houston Quarry (Mammal Pit), Crook Coun- ty, Wyoming (Foster and Martin, 1994; Foster, 2001). Type Horizon Morrison Formation undifferentiated; thin local Morrison section of only ~23 m (Mapel and Pillmore, 1963); exact stratigraphic level and intraformation- al correlation with other localities in Wyoming un- known. Etymology For its occurrence in the Morrison Formation. Diagnosis Theriosuchus species with the following unique combination of characters (*denote autapomorphic for species relative to Theriosuchus and Knoetschkesuchus): greatly enlarged D2 and D3 alveoli*; dramatic reduc- tion in mesiodistal diameter of alveoli from D3 to D4*; overall structure of mandible similar to T. pusillus in depth:length ratio, lack of external mandibular fenestra, retroarticular process angle, and dentary dorsal profile in two “waves” in lateral view; however, dentary nutri- ent foramina row and symphysis orientation relative to tooth row both more similar to K. guimarotae. Revised Description (modified from Foster, 2006) Specimen MWC 5625 is 141 mm long, with a pit- ted to rugose lateral surface and a relatively deep dorso- ventral dimension (figure 2). The lingual side of the jaw (particularly along the ventral margin) is not well pre- served in the central area, but the articular region and the rostral third are in good condition. The tooth row is relatively short. There are alveoli for 16 teeth, although all teeth are missing, and the lateromedial widths of the alveoli for the caudal seven of those teeth have been reduced somewhat by postmortem crushing. There is no external mandibular fenestra. In lateral view, the retroarticular process is short, and its dorsal surface is steeply inclined in a caudoventral direction. Although sutures of the bones of the lower jaw are difficult to dis- tinguish in many areas, the splenial is involved in the caudal part of the oval symphysis. A deep foramen in- termandibularis oralis occurs just caudal to the sym- physis. A row of foramina extends along the occlusal surface of the dentary just lingual to the alveoli of D2 to D9 (figure 3). The edges of the alveoli are vertically festooned around D2 through D5 and are flat posterior to that. There is slight damage to the anterior end of the dentary so the nature of the alveolus for D1 is obscured to some degree. Diameters of alveoli D2 and D3 are greatly enlarged relative to surrounding tooth positions; diameters of D4 through D9 alveoli are significantly reduced; and mesial-distal diameters of D10 through D16 are somewhat enlarged, some as large as D2 and D3 (table 1; figure 3). Despite some mediolateral crush- 289 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA Foster, J.R. Geology of the Intermountain West 2018 Volume 5 ing of the jaw posterior (caudal) to D8, the mesial-dis- tal diameters of the alveoli do not appear to have been significantly altered, and the crushing appears to have been relatively minor at the caudal end of the tooth row. Alveoli for D10 through D16 are so significantly elon- gate mesial-distally, compared to their labiolingual di- ameter, that even prior to crushing, the alveoli for these posterior tooth positions appear to have been very dif- ferent in shape compared with more anterior (rostral) ones. Given that the preserved widths of D10 through D16 are about 1 mm each, with a mesiodistal length of ~3 to 4 mm for each, and about 1 mm of labiolingual crushing, these alveoli still appear to have been nearly twice as long mesiodistally as wide labiolingually. Thus, from the moderate-sized D1 alveolus pos- teriorly: D2 and D3 are very large and nearly circular, D4 through D9 are very small and circular, and D10 through D16 are seemingly large and elongate-oval Figure 2. Theriosuchus morrisonensis (n. sp.), specimen MWC 5625, left mandible. (A) Lateral (labial) view. (B) Medial (lin- gual) view. (C) Occlusal view; note enlarged dentary teeth D2 and D3, small circular D4 through D9, and slightly crushed but large and elongate D10 through D16. Scale bars = 5 cm. Abbreviations: D1, dentary alveolus 1; dedw, “waves” in dorsal edge of dentary; nfr, nutrient foramina row; rap, retroarticular process; sd, external sculpturing of dentary; sym, symphysis. 290 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA Foster, J.R. Geology of the Intermountain West 2018 Volume 5 shaped (figure 3; table 1). The interalveolar spaces be- tween positions D10 through D16 appear to have ex- tremely thin septae (at least three positions) or to lack them entirely (confluent), a feature still apparent de- spite some crushing. The preserved septae are oriented diagonally between alveoli due to crushing (originally perpendicular to the tooth row), but they are generally only about 1 mm in length, again suggesting that teeth D10 through D16 were laterally compressed. These great size differences and apparent change in shape sug- gest that the tooth row exhibited significant heterodon- ty and that the dentary possessed at least three tooth types with conical teeth anteriorly and possibly laterally compressed teeth posteriorly. D1? D2-D3 D4-D9 D10-D16 A B Figure 3. Close-up of anterior third of mandible, specimen MWC 5625, showing diversity of tooth alveoli; dentary tooth alveolus D1 difficult to distinguish; dentary alveoli D2 and D3 greatly enlarged and roughly circular, D4 through D9 roughly circular but greatly reduced in diameter, and D10 through D16 mesiodistally elongate in occlusal view. (A) Photo with some key alveoli labeled; nf = nutrient foramina at anterior and posterior end of row; scale bar (red) = 1 cm. (B) Tracing of jaw and alveoli with color coded zones of similar alveolar shapes below. Note that spaces between positions D13 through D14 and D15 through D16 have very thin septae, indicating minimal labial-lingual crushing; remaining alveoli in positions D10 through D16 may have been confluent. 291 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA Foster, J.R. Geology of the Intermountain West 2018 Volume 5 DISCUSSION AND CONCLUSIONS The specimen was found in an abandoned chan- nel-fill pond deposit in interbedded laminated siltstone and green claystone (Foster and Martin, 1994; Foster, 2001). This is a dense bone bed preserving microver- tebrate bones (e.g., jaws of mammals) and macrover- tebrate bones (e.g., articulated sauropods) in the same layer and just centimeters away from each other. The abandoned channel deposit occurs as an elongate, lat- erally restricted deposit that lies stratigraphically just above a convex-bottomed channel sand, and it consists of two 1- to 10-cm-thick laminated siltstone beds with many aquatic, semi-aquatic, and terrestrial taxa mixed in the same layers. Charophytes, horsetails, bivalves, and other non-vertebrate taxa recovered from the de- posit are also indicative of a relatively wet environment. Teeth of possible goniopholidids as well as unidentified crocodylomorph osteoderms are also found in the de- posit. Specimen MWC 5625 was originally described as a juvenile goniopholidid that would have undergone dramatic allometric growth in its lower jaw through ontogeny (Foster, 2006). I proposed that Morrison go- niopholidids would have greatly increased the relative length of their tooth row and greatly reduced the rela- tive depth of their lower jaw (greatest mandible depth/ overall mandible length) as they grew to adult size. In both these ratios, however, specimen MWC 5625 is well off the trend lines set by a sample of Morrison goniop- holidids (Amphicotylus and Eutretauranosuchus) and a growth series of the modern Alligator (figure 4). The ap- parent growth pattern collectively shown by those three taxa was in fact the reverse of what would have been required to turn the individual represented by specimen MWC 5625 into an adult (Foster, 2006). And in shape of the symphysis of the lower jaws, specimen MWC 5625 in fact plotted among the alligators and nowhere near the region of goniopholidids of having a more antero- posteriorly elongate symphysis (Foster, 2006). My own graphs in that paper (Foster, 2006) suggested, howev- er, that Alligator at least did not undergo such dramatic allometric growth from the same small size up to that of the largest Morrison specimen, and even the trend among the known Morrison goniopholidids (though all are significantly larger than specimen MWC 5625) did not suggest dramatic changes. In re-examining the specimen and data in that pa- per, it is clear that specimen MWC 5625 shares charac- ters with the atoposaurid crocodyliform Theriosuchus, which was first named from the Purbeck (Lower Cre- taceous) of England (Owen, 1879; Salisbury, 2002) and has since had members of its genus or close relatives show up in Asia, Portugal, Spain, Romania, and possi- bly (from teeth) in a few other places, usually in the Late Jurassic to Early Cretaceous (Martin and others, 2010; Young and others, 2016) but also ranging into the Late Cretaceous. Theriosuchus was a small neosuchian with a short and triangular-shaped skull (top view) with large eyes and a short, narrow snout (Salisbury, 2002; Martin and others, 2014). Specimen MWC 5625 is relatively large and differs from the dentary of Knoetschkesuchus from Portugal in lacking an external mandibular fenestra and in having a mandible less elongate relative to its maximum depth (figure 4; Schwarz and Salisbury, 2005; Schwarz and Dentary Alveoli Mesiodistal Diameters D1 2.1 D2 3.2 D3 4.0 D4 1.5 D5 1.4 D6 1.4 D7 1.2 D8 1.5 D9 1.2 D10 2.7 D11 4.2 D12 4.2 D13 2.9 D14 3.3 D15 2.6 D16 3.3 Table 1. Mesiodistal diameters of dentary alveoli in MWC 5625 (in mm). 292 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA Foster, J.R. Geology of the Intermountain West 2018 Volume 5 others, 2017). Based on alveolar diameters, specimen MWC 5625 also appears to have had more pronounced heterodont dentition anteriorly than Knoetschkesuchus. It differs from Atoposaurus, Alligatorium, and Alliga- torellus in lacking homodont dentition and from Allig- atorium and Alligatorellus specifically in lacking the ex- ternal mandibular fenestra and smooth external surface of the mandible, respectively (Tennant and Mannion, 2014; Tennant and others, 2016). Specimen MWC 5625 also differs from species of Sabresuchus in lacking a lat- eral dentary concavity for reception of an enlarged 5th maxillary tooth, in lacking a diastema between D7 and D8, in lacking as short a symphysis, in not having all dentary teeth in a continuous groove, and in not hav- ing the occlusal surface of the dentary entirely lacking nutrient foramina (Tennant and others, 2016). Turner (2015) found species of Theriosuchus and Alligatorium to form an atoposaurid clade, although another recent analysis has suggested that Atoposauridae may be re- stricted to Atoposaurus, Alligatorium, and Alligatorel- lus, and that traditional species of Theriosuchus formed a polyphyletic group (Tennent and others, 2016). The same latter analysis found, however, that Theriosuchus pusillus and Knoetschkesuchus at least were sister taxa. Specimen MWC 5625 is most similar overall to a referred lower jaw of Theriosuchus pusillus (specimen BMNH 48328; figure 5) illustrated by Salisbury (2002). Although the anterior tip of that specimen is missing, an estimation of the full jaw length suggests that the depth:length ratio of the mandible is very similar to MWC 5625 (figures 4 and 5). Additionally, the appar- ent tooth row length, external sculpturing, and retroar- ticular process of MWC 5625 are all most similar to T. pusillus; the symphysis length and orientation and the dentary dorsal profile with two “waves” also are similar to Theriosuchus (figure 5). Specimen MWC 5625 possesses the following char- acters from the generic diagnosis of Theriosuchus in Young and others (2016). The folloing numbered list refers to the corresponding character numbers in Young and others (2016), and the missing character numbers relate to characters not preserved in the mandible: (1) heterodont dentition with pseudocaniniform and likely labiolingually compressed teeth (judging from alveoli); (5) some of the dentary alveoli form a confluent chain from dentary alveolus D4 through D8 (D10 through D16 in MWC 5625); (7) dentary alveolar size strong- ly heterogeneous; and (8) external surface of dentary is ornamented with heterogeneously spaced pits, ventro- laterally rugose. Specimen MWC 5625 appears to differ in lacking a notch in the dentary for the enlarged 5th maxillary tooth (character 6 in Young and others, 2016) and in seeming to lack a progressive reduction in al- veolus size from D4 through D6 (character 3; instead Figure 4. Comparison of man- dible greatest depth to total length, by size, for: the atopo- saurids Theriosuchus pusillus, T. morrisonensis (specimen MWC 5625, labeled), and Knoetschkesuchus (stars), sev- eral goniopholidids (squares), and a growth series of Alliga- tor (circles), showing great- er mandible depth:length of Theriosuchus. Green trendline is for Alligator series. Length of T. pusillus mandible esti- mated due to missing tip in BMNH 48328. Modified and updated from Foster (2006). 293 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA Foster, J.R. Geology of the Intermountain West 2018 Volume 5 MWC 5625 demonstrates dramatic reduction in diam- eter from D3 through D4). The external sculpting of the mandible and hetero- donty were also characters listed in the revised diagnosis of Theriosuchus in Tennant and others (2016). The re- vised diagnosis of T. pusillus includes three mandibular characters that MWC 5625 matches: heterodont denti- tion (apparent indirectly in MWC 5625), absence of a mandibular fenestra, and dorsal edge of dentary with two dorsally projecting “waves” in lateral view. Speci- men MWC 5625 lacks all mandibular characters listed by Tennant and others (2016) for a single speciment assigned to Theriosuchus sp. (Young and others, 2016) from the Middle Jurassic of the Isle of Skye, Scotland. The mandibular configuration and combination of characters of MWC 5625 distinctly separate the speci- men from goniopholidids, shartegosuchids, and proto- suchids/sphenosuchians and suggest that MWC 5625 is within the genus Theriosuchus, closest to T. pusillus (also illustrated by Salisbury, 2002, and Schwarz and others, 2017); however, it differs from that genotype species in having greatly enlarged D2 and D3 alveoli and having the splenial more evenly distributed dorso- ventrally along the symphysis (not restricted to dorsal part). Specimen MWC 5625 also differs from T. pusillus, and is similar to Knoetschkesuchus guimarotae, in hav- Figure 5. Theriosuchus mandibles compared. (A) Referred left mandible of Theriosuchus pusillus (specimen BMNH 48328) from the Purbeck Formation, England, showing depth:length ratio, short tooth row, and steep dorsal edge of retroarticular process. Also note heterodont dentition and "waves" of dorsal edge of dentary. Scale numbered in cm. Photo courtesy of D. Schwarz (Museum für Naturkunde, Berlin, Germany). (B) Left mandible holotype of T. morrisonensis (MWC 5625), Morri- son Formation, Wyoming, showing similar morphology. Scale bar = 5 cm. 294 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA Foster, J.R. Geology of the Intermountain West 2018 Volume 5 ing a line of foramina lingual to the tooth row on the occlusal surface of the dentary from D2 through D9 and in having a symphysis parallel in line to the tooth row. Characters that make MWC 5625 unique among Theriosuchus and Knoetschkesuchus specimens include: (1) very large D2 and D3, relative to D4 through D9, and (2) dramatic reduction in diameter from alveoli D3 through D4. Specimen MWC 5625 is too incomplete to run a meaningful phylogenetic analysis (only 35 of 329 char- acters known from Tennant and others [2016]; table 2), and preliminary assessments in TNT software and using the datasets of Turner (2015) and Tennant and others (2016) show it to be rather unstable within Neo- suchia. However, its unique features and combination of numerous characters shared with the closely related Theriosuchus pusillus and Knoetschkesuchus guimarotae (Turner, 2015; Tennant and others, 2016) suggest that, if more complete, Theriosuchus morrisonensis (MWC 5625) would likely be found to lie within this clade, pos- sibly as the sister taxon to T. pusillus. This is the first occurrence of Theriosuchus in the Late Jurassic of North America and is a new crocody- liform species for the Morrison Formation. Although isolated teeth from the Early Cretaceous Cedar Moun- tain Formation have been referred to atoposaurids (Cifelli and others, 1999), until now no confirmation of Theriosuchus or close relatives has been found on this continent. The discovery thus strengthens biotic ties once again between the Morrison Formation and the Late Jurassic–Early Cretaceous of Europe (e.g., Mateus, 2006). This new occurrence also increases the diversi- ty of crocodylomorphs in the Morrison Formation to eight taxa, with now at least two goniopholidids (Am- phicotylus, Eutretauranosuchus), two shartegosuchids (Fruitachampsa and an unnamed form), two hallopo- did sphenosuchians (Hallopus, Macelognathus), this new species of Theriosuchus, and a possible protosu- chian (Hoplosuchus). The diversity of crocodylomorphs in the Late Jurassic–Early Cretaceous of Europe may have been even higher. Atoposaurids appear to have been terrestrial or semiaquatic, depending on the species (Tennant and Mannion, 2014). With little of the postcranial skeleton known, the specific ecology of Theriosuchus is not clear, but the environmental setting of Knoetschkesuchus gui- marotae in estuarine lagoons and swamps (Schwarz and others, 2017) suggests that that species at least was semi-aquatic. The occurrence of T. morrisonensis in an abandoned channel pond deposit in the apparently wet- ter northern region of the Morrison Formation (Turner and Peterson, 2004; Foster and McMullen, 2017) aligns with the wet paleoenvironmental settings of K. guima- rotae and T. pusillus and may indicate that T. morrison- ensis too was semi-aquatic in its habits. ACKNOWLEDGMENTS Thanks to the field crews of 2004 that helped on the excavations when we collected this specimen. Thanks to Kay Fredette (Museums of Western Colorado), who prepared MWC 5625, and to Julia McHugh (Museums of Western Colorado) for loan of the specimen. Sug- gestions for improvement of the manuscript are greatly appreciated and were offered by Daniela Schwarz (Mu- seum für Naturkunde), Thomas Adams (Witte Muse- um), and Kelli Trujillo (Laramie County Community College), though none of them necessarily endorses my interpretations. REFERENCES Cifelli, R.L., Nydam, R.L., Gardner, J.D., Weil, A., Eaton, J.G., Kirk- land, J.I., and Madsen, S.K., 1999, Medial Cretaceous verte- brates from the Cedar Mountain Formation, Emery County, Utah—the Mussentuchit local fauna, in Gillette, D.D., editor, Vertebrate paleontology in Utah: Utah Geological Survey Mis- cellaneous Publication 99, p. 219–242. Theriosuchus morrisonensis (scored here) 201 211 221 231 1012210??? ?00?011111 [0,1][0,1]12111000 11100100?0 ?1 Theriosuchus pusillus (from Tennant and others, 2016) 201 211 221 231 1112110??? ?12?101111 2111010 11110200?? 01 Knoetschkesuchus guimarotae (from Tennant and others, 2016) 201 211 221 231 1111111101 120101111 2111010 101002000? 01 Table 2. 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