Rivista Italiana di Paleontologia e Stratigrafia volume 117 no. 1 2 pls. pp. 161-172 April 2011 MORPHOLOGIC VARIATIONS OF THE TRACE FOSSIL RUTICHNUS IN CM-THICK TURBIDITES FROM THE VERGHERETO FORMATION (NORTHERN APENNINES, ITALY) PAOLO MONACO Received: October 20, 2010; accepted: December 20, 2010 Dipartimento di Scienze della Terra, piazza dell’Università, 06100, Perugia, Italy. E-mail: pmonaco@unipg.it Key words: Ichnology, Rutichnus, taphonomy, trace fossils, turbidites, Miocene, Apennines. Abstract. The analysis of 56 specimens of the branched, walled post-depositional trace fossil Rutichnus, sampled from 31 thin-bedded, overbank turbidites from the Verghereto Formation, Romagna Ap- ennines, indicates high morphologic variability of this ichnotaxon in hypichnial preservation. The variability concerns the arrangement of outer and inner walls, number and shape of annulations, distribution of pustules, the (false) branching arrangement and the general external shape. Highest variability has been observed mainly in the Rutichnus rutis ichnospecies, clearly less in the R. irregularis ichnospecies that is rare. The Dino-Lite microscope camera analysis of oriented thin sec- tions indicates that endichnial preservation exhibits many structures due to burrowing in soft substrate, destroying laminae of sand. Con- sequently, Rutichnus probably was produced by a worm or arthropod, inducing strong deformation in turbidite sand during the feeding activ- ity on phytodetritus and organic matter transported by the turbidity flow. Riassunto. Vengono analizzati 56 campioni della traccia fossile post deposizionale Rutichnus provenienti da 31 livelli centimetrici (da 1 a 5 cm di spessore) di calcareniti torbiditiche di overbank intercalate nelle marne della Formazione di Verghereto, affioranti presso la loca- lità di Ville di Montecoronaro (Verghereto, Appennino Romagnolo). L’esame mostra una grande variabilità morfologica in questo ichnota- xon, specialmente nella preservazione hypichnia. La variabilità nella specie Rutichnus rutis è stata analizzata nella forma esterna, nelle pareti esterna ed interna, nel numero e nella disposizione delle annulazioni, nell’aspetto delle (false) ramificazioni e infine nella distribuzione delle pustule. Lo studio della variabilità morfologica è stato meno agevo- le nell’altra specie R. irregularis, a causa della scarsità degli esemplari raccolti. L’analisi mediante camera microscopica Dino-Lite in sezio- ne sottile ha permesso di evidenziare anche una forte variabilità nella preservazione endichnia; si osservano molte deformazioni avvenute durante la fase post-deposizionale nelle lamine di sabbia alla base del- le calcareniti e nella disposizione dei granuli associati, prodotte dalla ricerca e dall’ingestione di fitodetrito e materiale organico da parte di organismi vagili, forse vermiformi (ma senza escludere artropodi), tra- sportati dai flussi di torbida. Introduction For a branched, walled and meniscate trace fos- sil D’Alessandro et al. (1987) introduced the ichnoge- nus Rutichnus, which typically displays wrinkles that are distributed in the exterior of the wall. These au- thors analyzed many samples from the Permian of East Greenland and from the Miocene Flysch (Gorgoglione) of southern Italy remarking peculiar morphologic characteristics of this walled ichnogenus: external roughness, branching and the occurrence of menisci. Branches and menisci are typical and well preserved mainly in the Permian specimens (D’Alessandro et al. 1987, figs. 6-7). Some similarities concerning the wall ornamentation have been found mainly in epichnial Nereites, although in hypichnial Neonereites the wall is thicker and formed by pustules without short annu- lations, while in Rutichnus annulations and roughness are more regularly arranged and the wall is isopachous. Samples of one of the two ichnospecies, Rutichnus ir- regularis, formerly classified as Radionereites irregula- ris by D’Alessandro (1982), has been collected as hy- poreliefs from soles of turbidites in the Gorgoglione Flysch deposits, Basilicata region, southern Apennines, while the holotype of the other ichnospecies, Rutich- nus rutis, comes from the Permian of East Greenland (Domkirken Mount, northern Scoresby; D’Alessandro et al. 1987). 162 Monaco P. This paper addresses to describe an unusual con- centration of Rutichnus trace fossils that is otherwise rarely known (Uchman & Demircan 1999; Demircan 2008). The highly abundant Rutichnus has been found in several 3-5 cm-thick calcarenitic turbidites in marly deposits of the Verghereto Formation (Miocene) of the Romagna Apennines (Ville di Montecoronaro, Vergh- ereto area). These calcarenitic turbidites probably rep- resent distal fringe (overbank?) sediments deposited in the marginal Marnoso-arenacea Basin at the transition to the Verghereto High (Milighetti et al. 2009; Mona- co et al. 2009). These deposits belong to an area that expresses an excellent preservation and abundance of trace fossils with 50 different ichnotaxa in the ichno- fossil-lagerstätte of Poggio Alto (Monaco & Checconi 2010). This paper is directed to describe features of hypichnial Rutichnus that had not been described be- fore, focusing some taphonomic aspects that can help in the definition of palaeobiological and palaeoenvi- ronmental setting of very thin-bedded turbidites. Trace fossils have been sampled bed by bed and have been observed in variably cut surfaces which were photo- graphed. Analysis of endichnial preservation was per- formed mainly using the microscope camera Dino-Lite at 10x and 20x magnification, in order to observe tex- tural arrangement of grains close to the burrow. Investigated locality and geological setting The Verghereto Formation, Miocene in age, con- sists of thick, rhythmically-arranged gray marl depos- its (up to 600 m thick towards NE at M. Fumaiolo) and sporadic calcarenitic turbidites. The Verghereto For- mation was deposited in the Marnoso-arenacea Basin during a multi-phase tectonic activity that in the lower- middle Miocene (Serravallian?) produced ridges and elongated basins in a NW-SE oriented foredeep system of the northern Apennines. In the Verghereto area, that represents probably one of the most important of these intrabasinal highs, fringe and overbank deposits are represented by thin-bedded and fine-grained turbidites that reached the marginal sectors of the Marnoso-aren- acea Basin (see the extensive geological description in Monaco et al. 2009; Monaco & Checconi 2010). In the Poggio C site (N 43°76’93,63”; E 12°03’55,83”), that is located about 1 km at south of Poggio Alto ichnofossil-lagerstätte (Monaco & Chec- coni 2010), about 8 m of section have been logged (Fig. 1). Thirtyone calcarenitic beds, from 1 to 5 cm thick, have been analyzed bed by bed for the ichnofaunal content (Fig. 2). Seven of them exhibit Rutichnus rutis which is very common in thicker turbiditic beds (see five asterisks in Fig. 2). In the locality of Poggio B (N 43°77’14,24”; E 12°03’06,05”), just 250 meters to the Fig. 1 - Localization of studied sections. (A-D) Marls and cm-thick calcarenite beds in the Poggio C section. Numbers in beds are explained in Figure 2. (E) Epichnial community (mainly Scolicia prisca, Sp). Rutichnus from the Verghereto Formation (N. Apennines, Italy) 163 north of Poggio C, eleven sam- ples of Rutichnus rutis and six samples of Rutichnus irregula- ris have been collected (Fig. 1). The Poggio B section requires further analyses for its very rich assemblage of trace fossils, and therefore its log and ichnologic data will be provided separately. In the locality of Poggio C cal- carenitic beds of 1 to 3 cm thick dominate, but the thickest of them (levels 5, 13, 27), are those with the highest abundance and diversity of ichnofauna (Fig. 2). Calcarenites consist of rhyth- mical sharp-based beds, fining- upward disposed and brown to yellowish in colour. Quartz, mica flakes, feldspar, lithic frag- ments, calcium carbonate and clay suggest immature aren- ites (Milighetti et al. 2009). The basal grain size remains in the range of medium sand while the top is silt or mud; ripple struc- tures are frequent in thicker beds 5, 13 and 27, disposed in- ternally, more rare at the top. Both base and top are strongly bioturbated (Monaco & Chec- coni 2010). The cm-thick cal- carenites correspond to the dis- tal expression of the facies F9b, that is a variety of the facies F9a in the model of Mutti (1992), probably an immature deposit of very small-volume flows from low-density turbidity cur- rents in marginal and very distal environments (see discussion in Monaco 2008; Monaco & Checconi 2010). The mate- rial studied was collected at south of the village Ville di Montecoronaro at the foot of a hill that is located about 2 km to the North of the Canili-Piantrebbio exit in the E45 Orte-Ravenna road (Romagna Apennines). Trace fossil assemblage The trace fossil assemblage of the Poggio C sec- tion consists of rich hypichnia and epichnia in addic- tion to endichnia that are considered separately (Fig. 2). Hypichnial assemblage is dominated by Rutich- Fig. 2 - The Poggio C stratigraphic column with ichnologic as- semblage. Numbers in circle are thin-bedded calcarenites. Asterisks indicate thicker beds where the most abun- dant concentration of trace fossil Rutichnus occurs. 164 Monaco P. nus rutis and R. irregularis, and biramous meander- ing graphoglyptid Desmograpton that in this area is the most abundant among graphoglyptids (Monaco 2010). In the calcarenitic levels 5, 13, 15, 18, 19 and 27 Desmograpton can reach a very high ichnodensity, up to four specimens/20 cm2. The dominant ichnospecies is D. dertonensis while D. ichthyforme and D. alternum are less abundant. Other graphoglyptids include Mega- grapton, while net-shaped Paleodictyon is scarce (P. strozzii and P. minimum). Bergaueria-like structures and many indeterminable small plug-shaped structures dominate in thicker levels 5, 9, 13 and 27, while sporad- ically they occur in thinner ones. These plug-shaped forms are frequently fluted mainly in thicker levels (5, 13 and 27). Among string-shaped forms Helminthop- sis (mainly H. tenuis), and Protovirgularia are the dominant ichnogenera, while Scolicia strozzii, which is abundant in the ichnofossil- lagerstätte of Poggio Alto, in the Poggio C and B sections is very rare or absent (1 dubitative specimen at level 5). The epichnial community of thicker levels 5, 13 and 19 is similar to that described in the ichnofossil- lagerstätte of Ville di Montecoronaro (Poggio Alto) (Monaco & Checconi 2010), while some differences exist in thinner levels (see Fig. 2). In fact, while Scoli- cia prisca and Nereites isp. or N. missouriensis are the dominant forms in thicker and rippled beds (Monaco & Checconi 2010), the epichnial community is domi- nated by delicate forms as Chondrites (C. intricatus, C. targionii), Pilichnus and Phycosiphon that are abundant in the sharp top of thinner calcarenites that not exceed- ing 1-2 cm in thickness (Fig. 2). Fig. 3 - Thin sections of Rutichnus in Dino-Lite microscope camera. (A-F) Endichnial features are irregular deformations in the sand laminae which are curved or irregularly arranged bands forming micro-convolute structures or micro-patchy structures. Further explanations in the text. Rutichnus from the Verghereto Formation (N. Apennines, Italy) 165 Morphologic variations and taphonomic aspects of Ru- tichnus in endichnial and hypichnial preservations In order to characterize the morphologic vari- ation and taphonomic aspects of Rutichnus rutis, 56 samples have been analyzed (Fig. 2). The ichnospe- cies R. irregularis from the Poggio B locality has not been considered in the morphologic analysis (Tab. 1). Since each studied rock sample usually exhibits differ- ent single specimens of R. rutis (from 1 to 8 see column 2 in Tab. 1), a total number of 174 single structures are present at sole of calcarenitic beds (Tab. 1). They are studied in hypichnial preservation, since the endichnial structures show a strong morphologic variability and must be analyzed separately, adopting an approach that consists of the newly available Dino-Lite microscope camera. This camera obtains a continuous spectrum of photographs from 5 to 100x magnifications, and can be very useful in cases of structures that were produced by displacement of sand grains induced by burrowing. Image contrast and other settings may be enhanced by Photoshop. To study endichnial deformations numer- ous thin sections were produced and analyzed, ori- ented transversally and longitudinally (e.g. samples MV339, 345, 349, 389, Fig. 3). All studied samples are in the repository of the Biosedimentary Laboratory of Earth Science Department of the Perugia University. The deformation structures affect the lower part of a calcarenite (usually 2-3 cm in thickness from the base) but do not correspond to those preserved at the base of a bed. Endichnial features consist of irregular defor- mations in the sand laminae that appear as curved or irregularly arranged bands forming micro-convolute structures (sample MV339, Fig. 3A arrow). Above the concavity, which demarcates the exterior wall of a hyp- ichnion, a characteristic cutting edges of sand laminae is present which bend upward and return gradually to flat position at the top (Fig. 3A). The core structure, which resembles a bean placed vertically in the central position, is deformed itself by the biogenic compaction during the burrowing (Fig. 3A). Laminae may show a micro-patchy arrangement (samples MV345, Fig. 3B). Vertical dislocations can be seen in the sample MV349 (Fig. 3C). Analysis by Dino-Lite camera confirms that the endichnial preservation of Rutichnus rutis never shows a well-defined, repetitive structure but can eas- ily vary from case to case. In contrast hypichnial preservation exhibits quite different characteristics such as variations in diam- eter, external and internal shape, wall preservation and wrinkle disposition (Tab. 1). The variability of Rutich- nus rutis is very high, mainly in the marker level 5 (Fig. 2), where the ichnodensity is highest, reaching up to 10 specimens for a square decimetre, and decreases in oth- er beds (see asterisks in the log of Fig. 2). The diameter of this ichnospecies is usually about 10 mm (occasion- ally 12 or 13 mm, Tab. 1); a narrowing of the tube has been commonly observed (7-9 mm), while expansions of tunnels (up to 15-17 mm) represent probably abor- tive branches (MV343, 384). In rare cases the point of expansion can be flattened (MV340, 343). External di- ameter appears much more constant in Rutichnus rutis than in R. irregularis that occurs in bundles and shows a complicated arrangement in hypichnial preservation. Fig. 4 - Graph showing morphologic characteristics of the endichnial Rutichnus rutis in 56 samples. 166 Monaco P. It shows a great variation in morphology with ring- shaped structures (sample MV370, 374, Pl. 2C), which are developed in different part of a single structure. The ring arrangement in R. irregularis has been also shown in the study of D’Alessandro et al. (1987, figs 2-5). The shape variation of tunnels of Rutichnus rutis may changes varying from straight (Pl. 1C-D), display- ing a winding course (Pl. 2B), or curved (Pl. 1B, H; Pl. 2A). Branching in particular is typical of 60-70% of specimens, as indicated by D’Alessandro et al. (1987), although probably may be commonly a false branch- ing (see discussion). Tunnels usually depart at angles of 30° to 40° (Pl. 1B), reaching occasionally higher values (80° to 90° in a few specimens, Pl. 1D). The branching is commonly dichotomous. While a no preferred direc- tion has been noted in curved and straight specimens, in up to 30% branched specimens the disposition of tunnels is roughly radial (Pl. 1H). In 25% of specimens with dichotomy, the disposition of tunnels is side by side, running tangentially with sudden changes in di- rections and also in diameter (e.g. sample MV324, Pl. 1L). In this case narrowing and enlargement of tube segments are frequent. Overcrossing is another interesting aspect that has been noted in 23 samples (Tab. 1). This feature con- sists of superimposition of many knobby parts over smoother ones (e.g. samples MV365, 332, Pl. 1A). Usu- ally, the lower trace fossil in the hypichnial assemblage is bent downward, overlapping to a previous one; the geometry of the tube and the shape of tunnels seem in- dicate original overcrossing of burrows without com- pactional features (e.g. sample MV365). The ending of the tube is another very typical fea- ture of Rutichnus rutis; it consists of a short and thin bulge, usually 4-5 mm wide, that expands sharply in two rings 10 mm wide, made of elongated pellets (samples MV324, 333, 362, Pl. 1E-F). The end has been found in almost 40% of samples and this feature has figured also in the holotype of the East Greenland (D’Alessandro et al. 1987). The ending structure in Rutichnus irregula- ris is similar, although much more gradual and without sharp enlargements of rings of pustules (Pl. 2E, arrow). The outer wall surface of Rutichnus rutis is an- other very interesting aspect, with changes in their preservation (Fig. 4). Three types have been observed: (1) annulated with oriented and elongated pustules (type 1), (2) poorly annulated with casually oriented pustules (type 2) and (3) smooth to lumpy with few and scattered pustules (type 3) (Tab. 1; Fig. 4). (1) In the type 1, annulations are well developed (Pl. 1A, B, H) and form somewhat imbricated folds 2-4 mm thick (sample MV326, Pl. 1H), with usually equi- distant rings of pustules (2 - 3 rings, rarely more rings/ cm2). In some samples, annulations are rhythmically Samples Rutichnus individuals TF arrangement lenght cm (mean) diameter (mean) N° annulations/ cm2 distribution of pustules N° pustoles/ cm2 longitud. striation type of relief concave relief inner wall branching MV324 7 radiated 2 to 5 1.10 3.00 fractal 32.00 Y convex Y smooth 60° MV325 3 radiated 2 to 4 1.20 2.00 fractal 38.00 Y convex - - 80° MV326 6 radiated 4 to 7 1.40 2.00 oriented 36.00 Y convex - - 90° MV327 1 straight 1 to 2 1.00 2.00 fractal 34.00 - convex - - - MV328 1 straight 5 1.00 5.00 oriented 28.00 Y convex Y smooth - MV331 5 curved 2 to 6 1.30 2.00 fractal 39.00 - convex Y smooth 40° MV332 4 branched 3 to 7 1.20 2.00 oriented 26.00 Y convex Y smooth 40°-90° MV333 3 straight 2 to 6 1.30 3.00 oriented 30.00 Y convex Y irregular 40° MV334 2 irregular 3 to 5 1.00 3.00 oriented 32.00 Y convex Y smooth - MV335 4 irregular 2 to 3 1.10 2.00 fractal 33.00 - convex - - - MV336 2 irregular 2 to 3 1.50 2.00 fractal 27.00 - convex Y irregular - MV337 2 curved 8 1.10 2.00 oriented 32.00 Y convex Y smooth 30° MV338a 2 side by side 5 to 7 1.00 2.00 oriented 33.00 - both Y smooth turn MV338b 2 straight 1 to 4 1.20 2.00 fractal 34.00 - convex - - - MV339 6 straight 2 to 7 1.10 3.00 oriented 35.00 Y convex Y irregular - MV340 5 branched 1 to 5 1.20 3.00 oriented 35.00 Y convex Y - 90° MV341 2 spoon-shaped 4 1.10 2.00 - 28.00 - concave Y ridges - MV342 1 curved with ending 9 1.20 3.00 oriented 29.00 Y convex - - turn MV343a 1 curved, enlarged 6 1.20 3.00 oriented 26.00 Y convex - - - MV343b 2 ending, irregular 1 to 2 1.10 2.00 oriented, elongated 28.00 - convex - - flat MV344 2 straight 2 to 8 1.20 2.00 oriented, elongated 27.00 - both Y smooth - MV345 5 curved, aligned 3 to 4 1.10 2.00 oriented, elongated 32.00 Y convex - - turn MV346 5 irregular 2 to 5 1.00 2.00 oriented, elongated 31.00 Y convex Y irregular 40° MV347 1 enlarged 8 1.30 2.00 oriented 32.00 Y both Y irregular - MV348 3 curved, side by side 3 to 5 1.00 2.00 oriented, elongated 30.00 Y both Y irregular - MV349 4 curved, side by side 3 to 8 1.40 2.00 fractal 28.00 - both Y irregular - MV350 2 curved 1 to 2 1.10 2.00 oriented 38.00 Y both Y smooth 40° Tab. 1a, b - Main morphologic variations analyzed in 56 specimens (174 single structures) from Poggio C and B sections, Verghereto Marls Formation (Romagna Apennines). Rutichnus from the Verghereto Formation (N. Apennines, Italy) 167 arranged showing thickest rings alternated with thin- ner ones (see sample MV324, Pl. 1B). In some speci- mens rings are obliquely arranged (e.g. sample MV326, Pl. 1H). The distribution and shape of pustules in type 1 are variable: in the majority of analyzed forms (70%), pustules are elongated and oriented orthogonally to annulations, while in few cases (30%) they are short even if oriented (Pl. 1A, F, arrows). (2) In the type 2, annulations are very poorly de- veloped and pustules are never oriented, usually knob- shaped and larger than type 1 (up to 1 mm thick, sam- ples MV333, 334, 345, 365, Pl. 1E, G, L); in many cases they appear with different sizes producing a fractal dis- position (up to 80%). (3) In the type 3, the outer wall surface is usually fairly smooth and suddenly becomes lumpy with few and scattered knobs (Pl. 2A, B). The outer wall surface of type 3 resembles the exterior surface of Spirophycus bicornis, although the spiral whorl forming a convo- lute ring has never been found in the studied specimens from the Verghereto area (Monaco & Checconi 2008). The three types may occur in the same sample. Occasionally a transition from type 1 to type 3 has been observed in the same sample (e.g. MV332, Pl. 1A), as gradual transitions from type 1 to type 2 and vice versa can exist (sample MV324, Pl. 1B). In all three types the number of pustules for square centimetre has been measured, even if the measure is easier in type 1 and type 2 than in the other type (Tab. 1). The analy- sis indicates values of 21 to 38 pustules/cm2, with 5 to 7 pustules of type 1 that occur in the same line; fre- quently, the arrangement of pustules in type 1 produc- es elongated and oriented lines of pustules that are con- centrated in 3 rings/cm2 (Tab. 1). In other specimens they are arranged in two rings/cm2. In all these cases longitudinal alignments have been observed showing a high degree of regularity. An alignment has never been observed in the type 3. The distribution of relief (full or semi-relief) has also been analyzed: in the 66% of studied samples Ru- tichnus rutis occurs in convex full relief, while in 33% of samples it appears in concave semi-relief. Both types of preservation can be present in the same sample and usually they are randomly distributed (50%). In the MV379 2 straight 2 to 7 1.20 2.00 smooth prevalent 25.00 - convex - - - MV380 2 curved 8 to 10 1.40 2.00 smooth prevalent 22.00 - both Y smooth 50° MV381 3 straight 4 to 6 1.50 3.00 fractal 28.00 - both Y smooth - MV382 3 curved, branched 4 to 8 1.40 2.00 oriented, elongated 35.00 Y convex Y smooth 70° MV383 3 curved, ending 3 to 5 1.10 2.00 irregular 22.00 - both Y irregular - MV384 2 branched, ending 4 to 7 1.20 2.00 irregular 20.00 - both Y smooth 30° MV385 1 curved 7 1.10 2.00 irregular 21.00 - convex - - - Total: 174 Samples Rutichnus individuals TF arrangement lenght cm (mean) diameter (mean) N° annulations/ cm2 distribution of pustules N° pustoles/ cm2 longitud. striation type of relief concave relief inner wall branching MV351 3 spoon-shaped 2 to 4 0.90 2.00 - 28.00 - concave Y smooth - MV352 1 ending, curved 6 1.20 3.00 oriented 32.00 Y convex - - - MV353 3 side by side, tangential 4 to 6 1.30 2.00 fractal 27.00 - convex Y irregular - MV354 1 spoon-shaped 4 1.10 2.00 oriented, elongated 26.00 - convex Y smooth - MV355 4 spoon-shaped 1 to 2 1.10 2.00 - 28.00 - concave Y smooth - MV356 3 spoon-shaped 2 to 8 1.00 2.00 oriented, elongated 27.00 Y both Y smooth - MV357 3 side by side, tangential 2 to 4 0.90 3.00 oriented 33.00 - convex - - - MV358 1 straight 5 0.80 2.00 - 28.00 - concave Y irregular - MV359 4 curved, branched 4 to 6 1.10 3.00 fractal 35.00 - convex - - 50° MV360 1 straight 8 1.00 3.00 oriented, elongated 29.00 Y both Y irregular side MV361 1 enlarged 5 1.50 2.00 oriented, flattened 25.00 - convex - - - MV362 3 spoon-shaped, ending 3 to 5 1.60 2.00 oriented 33.00 Y convex Y smooth - MV363 7 ending, irregular 2 to 6 1.30 2.00 oriented, elongated 35.00 Y convex Y smooth 40* MV364 1 irregular 4 1.00 2.00 oriented 29.00 - concave Y smooth - MV365 6 spoon-shaped, ending 1 to 4 1.30 3.00 oriented, elongated 31.00 Y convex Y smooth, ridges 20° MV366 3 spoon-shaped, ending 2 to 5 1.50 2.00 oriented, elongated 26.00 Y convex Y smooth turn MV367 3 smooth, irregular 5 to 7 1.00 2.00 - 28.00 - concave Y smooth 30° MV368 5 side by side, tangential 3 to 10 1.00 2.00 oriented, elongated 35.00 - convex Y smooth 30° MA73 2 curved 3 to 5 1.20 3.00 fractal 36.00 Y convex Y smooth 30° MV376 8 irregular 3 to 10 1.00 3.00 oriented, elongated 35.00 - both Y irregular 30° MV377 3 curved, ending 6 to 9 1.20 2.00 oriented 33.00 Y convex - - 40° MV378 9 curved, ending 3 to 10 1.10 2.00 oriented, elongated 30.00 Y both Y irregular - 168 Monaco P. hypichnial concave semi-relief a spoon-like shape (samples MV334, 341, 351, 354, 355, 362) frequently occurs with the interior wall surface usually smooth (about 60% of samples; Pl. 1I). Longitudinal ridges in the hypichnial concave semi-relief are other very peculiar features. They are 3 to 5 short ridges, 0.2-0.3 mm thick, parallel oriented into direction of the tracemaker locomotion (see sam- ple MV341, 365, Pl. 1I, arrow). These parallel ridges are enigmatic structures, probably produced during the contractions of the body of the animal; however, it can- not be excluded that they were formed by a wormlike organism during the movement by anchoring or elon- gation (D’Alessandro et al. 1987). When these ridges are present, an irregular development of interior wall surface can also occur (samples MV349, 368, 376). As the rule an unstructured mud infilling is present that rarely shows longitudinal striations (samples MV332, 333, 339, 345, 354, 378, Pl. 1M). The sectioned menis- cate interior is very poorly preserved in the studied specimens of the Verghereto Formation (only three doubtful cases, samples MV356, 367, 381), while it has been commonly found in holotype specimen from East Greenland (D’Alessandro et al. 1987, fig. 7). The poor preservation of menisci in the Apennine specimens resulted probably from diagenetic processes that de- stroyed the meniscate structures or by the infilling of the gray mud that hides this structure. Discussion and conclusions A model for the mode of construction of Rutich- nus structures has been inferred by D’Alessandro et al. (1987). These authors classify Rutichnus as a postdepo- sitional feeding structure produced by a worm-like de- posit feeder organism, that constructed and reinforced tunnels during their feeding activity in many steps of repeated forward and backward body movements. The meniscate structures represent different phases of an- choring and elongation of the body; menisci in studied specimens of Verghereto are very poorly preserved, so it is difficult to understand their origin. On the contrary, the branching is a more clear feature than menisci. In the sketch with a sectional view of Rutichnus rutis D’Alessandro et al. (1987, fig. 8) show the structure of branching, indicating that it is not typical as the case of many other trace fossils (e.g. Thalassinoides, crustaceans), but develops as a false branching with overcrossing of a different tun- nel. Usually, the new tunnel departs at the branching point forming a different direction and their external wall cuts the previous burrow. This fact is evident in the figure 9 of the same authors where a false branch- ing is clearly visible in the centre of the photograph. The false branching may be really a normal structure in Rutichnus rutis when a changing in direction occur, as observed in many of 22 specimens from the Verghereto study (see Pl. 1B, Tab. 1). The mode of construction of external ornamen- tation is another important element; it was not con- sidered in detail by cited authors, although the anchor pressure of the soft body may produce irregularities of the outer surface and ornamentation, mostly during the backward movement. These cited authors in their historical treatment compared Rutichnus to other trace fossils mostly in respect to branching, meniscate and walled aspects, but without showing the meaning of morphologic variations in Rutichnus and the mode of the wall exterior fabrication. This latter aspect is fun- damental for understanding the ethology of wall con- struction, as in the case of some hypichnial nereitids (Dreginozoum preservation, Seilacher 2007), where the burrow exterior shows remarkable similarities with that of Rutichnus. Concerning the mode of construc- PLATE 1 Hypichnial preservation and morphologic variations of the trace fossil Rutichnus rutis, Poggio C (Verghereto Marls Formation). (A) Overcrossing of Type 1 (annulated with oriented and elongated pustules) over type 3 (and lumpy with scattered pustules), sample MV332; (B) false branching in walled specimens of type 1; note rhyth- mically arranged rings of pustules showing thickest rings alternated with thinner ones, sample MV324; (C) poorly annulated specimen with poorly oriented pustules (type 2), sample MV345; (D) some specimens with different preservation of rings of pustules; note im- brication (arrow), sample MV340; (E) different specimens with small pustules that show fractal arrangement (lower arrow) and end of trace (upper arrow), sample MV333; (F) detail of a end with sharp change in diameter (arrow), sample MV362; (G) specimens showing inner concave (black arrows) and outer convex (white arrow) wall surface with widening and irregular pustules; note elongated, parallel ridges in inner wall surface (lower arrow), sample MV334; (H) radiated, an- nulated specimens with imbricated folds of oriented pustules, sample MV326; (I) inner wall surface of concave specimens with smooth sur- face and parallel ridges (arrow), sample MV341; (L) tangentially ori- ented specimens with short ends, sample MV365; (M) specimens with mud fill and longitudinal striae (arrow), sample MV378. PLATE 2 Hypichnial preservation and morphologic variations of Rutichnus rutis and R. irregularis trace fossils, Poggio C (Verghereto Marls Formation). (A-B) The outer wall of R. rutis type 3 is usually fairly smooth and suddenly becomes lumpy with scattered knobs; note poorly devel- oped annulations in B, samples MV342 and 359; (C-D) specimens of Rutichnus irregularis with characteristic central ring (r), sample MV370; (E) irregular arrangement of a dubious specimen of Rutich- nus irregularis?, sample MV373; (F) massive, irregular occurrence of pustules (patchwork) of Rutichnus cf. irregularis, sample MV372. Rutichnus from the Verghereto Formation (N. Apennines, Italy) 169 PLATE 1 170 Monaco P. PLATE 2 Rutichnus from the Verghereto Formation (N. Apennines, Italy) 171 tion of nereitids, Seilacher (2007) suggested that the annelid worm can remove the sediment in front but not by radial emersion of a voluminous proboscis, but rather probing and backfilling the rejected sediment laterally around the body. The cited author indicated that it is possible for the worm by bending the head with small proboscis in different directions and at reg- ular intervals producing oriented pellets. Arthropods are excluded by cited author because faecal pellets are nearly the size of the body of the worm. In the case of Rutichnus from the Verghereto area it is not clear if the material is effectively faecal because it is highly variable in size and distribution, frequently fractal (see three types indicated above), and pellets are smaller than the size of the body. The parallel ridges in the concave wall interior, which are impressed also in the mud fill, are similar to scratch marks produced by arthropods (Cur- ran 2007; Giannetti et al. 2007). The absence of menis- cate structures and the high variability in shape and regularity of pellets distribution suggest that arthro- pods are not excluded, although a worm-like deposit feeder could be most probably the tracemaker. Another aspect that should be discussed con- cerns the causes of unusual abundance of Rutichnus at the lower sand/mud interface (hypichnial position) of thicker beds when also strong endichnial activity is common. The most concentration of specimens occur in such beds (3 to 5 cm thick) that exhibit most abun- dant endichnial structures, indicating an intense bur- rowing perhaps at different tiers in the sand (intense postdepositional activity); on the contrary, in thin- ner beds (of 1 to 2 cm) the concentration of Rutich- nus decreases abruptly (few and scattered specimens) and endichnial activity is shallow and rare; in this case other trace fossils prevail such as graphoglyptids (e.g. Desmograpton) and plug-shaped forms. In any case, may be excluded the role of erosion induced by turbid- ity currents that probably was irrelevant and did not involve Rutichnus tracemakers. It is worthy to point out that this ichnogenus was never found in any other parts of the Marnoso-arenacea Basin, while locally be- comes extremely abundant, as at the transition to the Verghereto High, when marginal, cm-thick calcarenites were deposited (Milighetti et al. 2009; Monaco et al. 2009). The unusual abundance can be due to the avail- ability of a large amount of organic matter and bio- or phytodetritus that could be accumulated rapidly by thin gravity flow inducing an unusual proliferation of deposit feeders (Lutz et al. 2007; Uchman 2007; Wetzel 2010). Sudden fluctuations in the organic matter on the sea floor and changes in detritus abundance induced ef- fects on the benthic habitats as pointed out by Wetzel (2010), reflecting differences in the trace fossil distri- bution. In several beds we can observe up- and down- ward shifting of tunnels with overcrossing of many parts of Rutichnus, with sharp changes in direction, in thickness and in morphology at lower surface of beds. This has been observed also in endichnia where dif- ferent types of sand laminae deformations occur. This trace fossil arrangement seems reflect a frenzied behav- iour of opportunistic animals during a very short time, perhaps arthropods or worms, probably transported directly by low-density gravity flows; in this case, the survival of such animals during transport and deposi- tion from nearby areas could not be excluded. In this case calcarenites may not represent a typical distal flow deposit but could be material coming directly from proximal areas. Unfortunately, there are no physical evidences (e.g. tool marks) to validate this hypothesis and remains the open question how to estimate the ef- fects of currents on the distribution of organic matter that surely had to be present along the slopes of the Verghereto High. Ripples at top of calcarenitic beds with widely developed epichnial Nereites missouriensis and Scolicia prisca (or S. vertebralis) suggest that con- centrations in the organic matter persisted even at the top of beds. Acknowledgements. The field work was carried out with the help of T. Trecci and M. Milighetti. Many thanks to reviewers A. Uch- man and A. Wetzel for many important suggestions and improve- ments in the manuscript. Thanks also to M. Gaetani and C. Lombardo for help and advices. This research was supported by research project RB 2008-2010 of the Earth Science Dept. of the University of Perugia (P. Monaco). 172 Monaco P. Curran H.A. 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