Tetrapod Tracks Research Articles

A reappraisal of the Middle Triassic chirotheriid Chirotherium ibericus Navás, 1906 (Iberian Range NE Spain), with comments on the Triassic tetrapod track biochronology of the Iberian Peninsula.

Triassic vertebrate tracks are known from the beginning of the 19th century and have a worldwide distribution. Several Triassic track ichnoassemblages and ichnotaxa have a restricted stratigraphic range and are useful in biochronology and biostratigraphy. The record of Triassic tracks in the Iberian Peninsula has gone almost unnoticed although more than 25 localities have been described since 1897. In one of these localities, the naturalist Longinos Navás described the ichnotaxon Chirotherium ibericus in 1906.The vertebrate tracks are in two sandy slabs from the Anisian (Middle Triassic) of the Moncayo massif (Zaragoza, Spain). In a recent revision, new, previously undescribed vertebrate tracks have been identified. The tracks considered to be C. ibericus as well as other tracks with the same morphology from both slabs have been classified as Chirotherium barthii. The rest of the tracks have been assigned to Chirotheriidae indet., Rhynchosauroides isp. and undetermined material. This new identification of C. barthii at the Navás site adds new data to the Iberian record of this ichnotaxon, which is characterized by the small size of the tracks when compared with the main occurrences of this ichnotaxon elsewhere. As at the Navás tracksite, the Anisian C. barthii-Rhynchosauroides ichnoassemblage has been found in other coeval localities in Iberia and worldwide. This ichnoassemblage belongs to the upper Olenekian-lower Anisian interval according to previous biochronological proposals. Analysis of the Triassic Iberian record of tetrapod tracks is uneven in terms of abundance over time. From the earliest Triassic to the latest Lower Triassic the record is very scarce, with Rhynchosauroides being the only known ichnotaxon. Rhynchosauroides covers a wide temporal range and gives poor information for biochronology. The record from the uppermost Lower Triassic to the Middle Triassic is abundant. The highest ichnodiversity has been reported for the Anisian with an assemblage composed of Dicynodontipus, Procolophonichnium, Rhynchosauroides, Rotodactylus, Chirotherium, Isochirotherium, Coelurosaurichnus and Paratrisauropus. The Iberian track record from the Anisian is coherent with the global biochronology proposed for Triassic tetrapod tracks. Nevertheless, the scarcity of track occurrences during the late Olenekian and Ladinian prevents analysis of the corresponding biochrons. Finally, although the Iberian record for the Upper Triassic is not abundant, the presence of Eubrontes, Anchisauripus and probably Brachychirotherium is coherent with the global track biochronology as well. Thus, the Triassic track record in the Iberian Peninsula matches the expected record for this age on the basis of a global biochronological approach, supporting the idea that vertebrate Triassic tracks are a useful tool in biochronology.

Stratigraphic and temporal context and faunal diversity of Permian-Jurassic continental tetrapod assemblages from the Fundy rift basin, eastern Canada

<p style="margin: 1em 0px; line-height: 200%; text-indent: 0.5in;"><span style="font-family: 'Minion Pro','serif'; mso-bidi-font-family: 'Times New Roman'; mso-ansi-language: EN-CA;"><span style="font-size: medium;">The Fundy basin in Nova Scotia and New Brunswick is the largest exposed rift basin of the Newark Supergroup and also extends beneath the Bay of Fundy. Its strata can be divided into four tectonostratigraphic sequences (TS). TS I is represented by the probably Permian Honeycomb Point Formation and possibly the Lepreau Formation. TS II includes the Wolfville Formation with the probably Middle Triassic Economy Member and the early Late Triassic Evangeline Member. These members have yielded markedly different assemblages of continental tetrapods. TS III comprises most of the Blomidon Formation, which is Norian to Rhaetian in age. The Blomidon Formation has yielded few skeletal remains of tetrapods to date but many tetrapod tracks. TS IV includes the late Rhaetian top of the Blomidon Formation and the McCoy Brook Formation, which overlies the North Mountain Basalt and is latest Rhaetian and earliest Jurassic (Hettangian) in age. The McCoy Brook Formation has yielded a diversity of continental tetrapods and lacks any of the characteristic Late Triassic forms. Recent work has correlated the Global Boundary Stratotype Section and Point (GSSP) for the base of the Jurassic (Hettangian) to a level well above the North Mountain Basalt. Thus most of the tetrapod fossils from the McCoy Brook Formation are latest Rhaetian in age, but the higher horizon with skeletal remains of sauropodomorph dinosaurs may be earliest Hettangian in age. The Fundy basin preserves the only known, stratigraphically tightly constrained record of the profound biotic changes in continental ecosystems across the Triassic-Jurassic transition.</span></span></p>

Isochirotherium trackways, their possible trackmakers (?Arizonasaurus): intercontinental giant archosaur migrations in the Middle Triassic tsunami-influenced carbonate intertidal mud flats of the European Germanic Basin

A remarkable Middle Triassic (Pelsonian, Anisian) fossil track site at Bernburg in Central Germany permits reconstruction of a tetrapod fauna at the point of recovery when early poposaur quadruped dinosaurs diversificated globally. The site has yielded numerous tetrapod and thousands of arthropod horseshoecrab tracks from several levels in intertidal biolaminites with intercalated storm and seismic-influenced carbonates. The tetrapod tracks, Procolophonichnium, Rhynchosauroides, Chirotherium, and Isochirotherium, are assigned to smaller Archosauromorpha and large Archosauria. The largest chirotheroid tracks Isochirotherium herculis (Egerton in Proc Geol Soc London 3:14–15, 1838) reach up to 350-mm-long pes sizes and 120-mm-long manus imprints. Those were most probably made by a 5-m-long poposauroid archosaur like Arizonasaurus, based on matching skeleton anatomy and trackways including a new three-dimensional model of those giants in locomotion. A first German poposauroid bone record is added herein with a scapula from shallow marine carbonates of similar Pelsonian ages. These crurotarsan archosaurs were at the top of their food chains globally, and fed most probably on smaller tetrapods (Hescheleria, Macrocnemus), the latter also represented well by long behavioural escaping Rhynchosauroides trackways. The smaller reptiles may have fed especially on “Millions” of horseshoecrab eggs in those arthropod reproduction seasons in the intertidal carbonate mud flats. Such tidal flats and sabkhas extended over hundreds of square kilometres surrounding the Germanic Basin in Central Europe in the Middle Triassic, representing a drastic change in the palaeoenvironment of central Pangaea, and a possible trigger for continuing diversification of archosauromorphs and eventually dinosaurs. The horseshoecrab reproduction seasons seem to have caused a food chain reaction along the southern Pangaean (northern Tethys) coast in carbonate mud flats between Northern America, Africa, Europe and China, with possibly long-distance migrations of poposauroids such as Arizonasaurus, whose bones were found in terrestrial to shallow marine carbonate deposits in terrest to coastal context.

Tracking a Legend: An Early Cretaceous Sauropod Trackway from Zhaojue County, Sichuan Province, Southwestern China

Cretaceous tetrapod (dinosaur and pterosaur) tracks from Zhaojue County in Sichuan Province are locally very abundant. Large scale quarrying operations at the Sanbiluoga Copper Mine site have produced extensive exposures, and track material for detailed study. However, natural track-bearing outcrops also occur at a site in Jiefang Township. The traditions of the local Yi people, indigenous to the area, attribute such tracks to Zhigealu, a central creator hero-ancestor, who made the footprints while riding his heavenly steed through the area. Through seeing tracks exposed by quarrying the local people offered these legend-based interpretations, and reported the Jiefang site which was previously unknown to scientists from outside the area. Thus, it is important to pay attention to local legends about track makers since they may lead directly to significant fossil footprint discoveries. Thereby paleontology and ichnological research can benefit largely from archeological sciences as well as from oral narratives from the local people. The recently discovered sauropod trackway from Jiefang is an excellent example. It comprises 16 pes-manus sets arranged in a narrow-gauge pattern. A peculiarity is the combination of this feature with morphological characteristics known from typical wide-gauge Brontopodus trackways suggesting a tentative assignment to cf. Brontopodus. The discovery enlarges the distribution and diversity of Brontopodus-like trackways and their producers in the Cretaceous Sichuan Basin.

A manus dominated pterosaur track assemblage from Gansu, China: implications for behavior

The Yangouxia dinosaur tracksites are well known for a diverse assemblage of tetrapod tracks preserved as natural impressions (concave epireliefs) on large bedding planes, representing a locally widespread surface marking the transition from a sand- to a mud-dominated sequence in the Hekou Group. Previous ichnological studies at these large sites have focused on the morphology and ichnotaxonomy of the tracks, including a single trackway representing the first pterosaur tracks reported from China. Here, we report a distinctly different assemblage associated with minor sandstones in the mud-dominated sequence 20m above the main tracksite level. This assemblage consists of at least 20 pterosaur manus track casts attributed to a single ichnotaxon (Pteraichnus). No pes tracks have been identified. These tracks mostly occur in random orientations, although one possible trackway segment is inferred, to represent walking progression. Manus-only pterosaur track assemblages are common and likely reflect differential registration depths of manus and pes and/or sub optimal preservation conditions. The tracks are associated with distinctive invertebrate traces including Cochlichnus, Spongeliomorpha and Paleophycus and suggest the pterosaurs were likely feeding on the invertebrate tracemakers.

MICROBIAL MATS AND ICHNOFAUNA OF A FLUVIAL-TIDAL CHANNEL IN THE LOWER PENNSYLVANIAN JOGGINS FORMATION, CANADA

ABSTRACT A meandering fluvial channel body at Coal Mine Point in the Joggins Fossil Cliffs, Nova Scotia contains an unusual fossil assemblage. During an early stage of channel abandonment, a wrinkled surface attributed to microbial mats was traversed by large arthropleurids (Diplichnites cuithensis trackways). Closely associated are smaller Diplichnites gouldi trackways, probably made by myriapods, as well as tetrapod tracks (Pseudobradypus?, Dromillopus, Hylopus) and invertebrate traces (Cochlichnus, Gordia), collectively representing the Scoyenia ichnofacies. The mats stabilized the sediment surface, allowing excellent trackway preservation, and may have formed a food source, although no feeding traces were identified. Overlying strata yield Protichnites followed by a succession containing paired mud drapes and an impoverished Skolithos ichnofacies encompassing Skolithos, Arenicolites, Cochlichnus, and possible Rhizocorallium, collectively suggesting brackish influence. The channel deposits contain some...

First Occurrence of Tetrapod Footprints from Westphalian Strata of the Sidi Kassem Basin, Central Morocco

The Sidi Kassem Basin is the only limnic basin of Westphalian age in Morocco. It is built up of 1,250 m of alluvio-fluvial to lacustrine deposits that have so far yielded plant fossils and invertebrate remains only. Recent exploration for fossil tetrapod footprints in floodplain-deposits of the basin revealed a moderately diverse vertebrate ichnofauna composed of tracks assigned to cf. Batrachichnus Woodworth, 1900; cf. Hylopus Dawson, 1882; Dimetropus Romer and Price, 1940; and Notalacerta Butts, 1891. The tracks can be referred to temnospondyl, anamniote reptiliomorph, non-therapsid synapsid (pelycosaur), and captorhinomorph trackmakers. The described ichnoassemblage is important in at least three aspects: (1) It suggests an Early to mid-Pennsylvanian age for the footprint-bearing strata of the study area. (2) It is the oldest association of tetrapod footprints from Africa. (3) It is the first evidence of the relatively rare ichnogenera cf. Hylopus and Notalacerta outside of North America and Europe. Judged from the variety of tetrapod tracks and previously collected floral remains, the Sidi Kassem Basin must have represented a well-established continental ecosystem during Pennsylvanian time. Further exploration for trace and body fossils of Palaeozoic vertebrates in this basin may be important for the reconstruction of early tetrapod evolution.

The age of the oldest tetrapod tracks from Zachełmie, Poland

The age of the oldest tetrapod tracks from Zachełmie, Poland

Middle Devonian invertebrate trace fossils from the marginal marine carbonates of the Zachełmie tetrapod tracksite, Holy Cross Mountains, Poland

The dolomitic deposits of the Middle Devonian Wojciechowice Formation exposed at the tetrapod tracksite in the Zachelmie Quarry in the Holy Cross Mountains (Poland) are characterised by a low diversity of invertebrate trace fossilassociation. Four ichnoassemblages can be identified in the track-bearing, lower part of the succession. The most conspicuous are trace fossils produced by arthropods (probably crustaceans), which can form distinctive and large horizontal burrows. The described ichnotaxa (cf. Skolithos isp., cf. Balanoglossites isp., Alcyonidiopsis isp., Spongeliomorpha isp., Gordia isp., and Rhizocorallium isp.) are well known from typical marginal-marine and shallow-marine deposits. Nevertheless, the studied assemblages were found in sparsely distributed horizons and are dominated by a single or a few ichnotaxa with locally high trace-densities. Distribution and composition of the trace fossil assemblages probably reflects occurrence of the impoverished, stressed Cruziana ichnofacies. It was affected by changes in water depth with intermittent periods of subaerial exposure connected with salinity fluctuations. The invertebrate trace fossil assemblage, tetrapod tracks and associated sedimentological features point to deposition in a marginal-marine, mostly peritidal and lagoonal environment with minor terrestrial influences.

Deutschlands älteste Wirbeltierfährte wurde im Nationalen GeoPark Ruhrgebiet entdeckt

Deutschlands älteste Wirbeltierfährte wurde im Nationalen GeoPark Ruhrgebiet entdeckt

Tetrapod and invertebrate trace fossils from aeolian deposits of the lower Permian of central-western Argentina

Abundant tetrapod footprints are described from the Early Permian Yacimiento Los Reyunos Formation including both collected and in situ specimens. The slabs come from several quarries at the Sierra Pintada and Sierra de las Peñas area, south-west of Mendoza, Argentina. The trace fossil assemblage, which constitutes one of the oldest known from Gondwana, comprises excellent-preserved tetrapod tracks (Chelichnusduncani, Chelichnusgigas and ‘pear-like’ footprints) and invertebrate simple sub-horizontal (Palaeophycustubularis) and vertical (Skolithos isp.) burrows formed in a aeolian dune field. The analysis of the tetrapod track producers indicates the presence of at least three different taxa of sprawling to semi-erect therapsids, thus suggesting the presence of members of this clade, or closest relatives, in the Early Permian of southern Gondwana. Moreover, a series of measurements and simple indexes were developed to estimate body proportions and locomotion styles of the putative trackmakers. The new assemblage, analysed in the context of other known Permian assemblages from Pangea, is one the few known in Gondwana to be present in an aeolian environment. The evaluation of the assemblage, in the light of aeolian ichnofacies (Chelichnus, Octopodichnus and Entradichnus), shows that it has common elements with the Chelichnus and Entradichnus ichnofacies.

A Review of the Tetrapod Track Record in China, with Special Reference to Type Ichnospecies: Implications for Ichnotaxonomy and Paleobiology

Abstract“Splitting” and “lumping” are perpetual problems in vertebrate, especially dinosaur, ichnotaxonomy. Chinese dinosaur ichnotaxonomy, which began in 1940, provides a series of interesting case studies, highlighting the dual problems of historical and dubious ichnotaxonomy. Chinese Mesozoic tetrapod track types have been placed into 63 ichnospecies (one Triassic, 28 Jurassic, and 34 Cretaceous), exclusive of other, non‐type ichnospecies or ichnotaxa identified from China. Fifty‐two (∼83%) of these 63 tetrapod ichnospecies were placed in monospecific ichnogenera. At the ichnogenus level, we prune—either by recognizing nomina dubia or by synonymy—17 from the list of 53 dinosaurian ichnogenera (a 32% reduction), leaving 36 ichnotaxa that we consider valid. Most of the cuts affect Jurassic theropod ichnotaxa, which are reduced from 23 to only nine because most ichnogenera are subjective junior synonyms of Grallator and Eubrontes. Fewer Chinese Cretaceous ichnotaxa (only six of 21 ichnogenera) are obvious nomina dubia or subjective synonyms, suggesting greater east Asian endemism during this time. Because ichnospecies differences are subtle, we provisionally retain ichnospecies as valid pending detailed comparative analyses of congeneric ichnospecies. This synthesis is long overdue and is necessary to address problems of historical and provincial ichnotaxonomy, which severely hamper comparisons of tetrapod ichnofaunas in space and time.

Late Triassic Tetrapod-Dominated Ichnoassemblages from the Argana Basin (Western High Atlas, Morocco)

Diverse tetrapod track assemblages with Scoyenia invertebrate traces were discovered in the Triassic Timezgadiouine and Bigoudine formations of the Argana Basin (Western High Atlas, Morocco). The ichnofossils occur in alluvial plain sandstones and mudstones of the Irohalène Member (T5) and Tadart Ouadou Member (T6) considered Carnian-Norian in age by vertebrate remains and palynomorphs. Tetrapod footprints are assigned to Apatopus, Atreipus-Grallator, Eubrontes isp., Parachirotherium, cf. Parachirotherium postchirotherioides, Rhynchosauroides ispp., and Synaptichnium isp. They can be referred to lepidosauromorph/ archosauromorph, basal archosaur, and dinosauromorph trackmakers. Apatopus, represented by 11 tracks of a more than 4 m long trackway, is recorded for the first time outside of North America and Europe. The assemblage concurs with the proposed Late Triassic age of the track-bearing beds by the occurrence of Apatopus, Atreipus-Grallator, and Eubrontes. If this is accepted, the stratigraphic range of Synaptichnium and Parachirotherium, hitherto known only from Early or Middle Triassic deposits, has to be extended to the Carnian-Norian. The occurrence of Eubrontes in the Irohalene Member (T5) provides further evidence for large theropods in pre-Jurassic strata. All assemblages are referred to the Scoyenia ichnofacies indicating continental environments with alternating wet and dry conditions.

COMPLEX TETRAPOD BURROWS FROM MIDDLE TRIASSIC RED BEDS OF THE ARGANA BASIN (WESTERN HIGH ATLAS, MOROCCO)

Although burrowing ability has been widespread in tetrapods for more than 300 million years, subsurface dwelling structures that indicate communal behavior are poorly evidenced from pre-Cenozoic strata. Here we present recently discovered tetrapod burrows from Middle Triassic red beds of the Argana Basin in central Morocco, whose complexity suggests an origin by gregarious animals. The well-preserved burrows occur in interbedded mudstones and sandstones interpreted as channel and overbank deposits of ephemeral, braided streams. All burrows originate from the top of thick-bedded sandstones and descend as moderately inclined (10°–30°), partially spiral tunnels to laterally extended, branched chambers in underlying mudstones. Tunnel segments are biconvex to planoconvex in cross section, up to 20 cm wide and 12 cm in maximum height and exhibit transverse scratch marks along the ceilings and sidewalls. Distinctive burrow characteristics include a laterally sinuous geometry (wavelength λ = 38–45 cm; amplitude A = 5–10 cm) of the tubelike passages and the presence of grouped alcoves in terminal chambers. We attribute the burrows to procolophonids or therapsids based on closely associated tetrapod tracks and the limited diameter of the excavations. Our findings represent the second oldest record of communal fossorial behavior by tetrapods and the oldest example from low-latitude areas. Beyond providing refuge from predators, these elaborate underground structures probably functioned as a buffer against diurnal or seasonal variations of air temperature and humidity in a semiarid habitat that was situated just north of the paleoequator.

Late Early Permian continental ichnofauna from Lake Kemp, north-central Texas, USA

Continental trace fossils of Early Permian age are well known in the western United States from Wolfcampian (~ Asselian to Artinskian) strata, but few examples are known from Leonardian (~ Kungurian) deposits. A substantial ichnofauna from strata of the lower part of the Clear Fork Formation at Lake Kemp, Baylor County, Texas, augments the meager North American record of Leonardian continental trace fossil assemblages. Ichnofossils at Lake Kemp occur in the informally-named Craddock dolomite member of the Clear Fork Formation, which is 12–15 m above the local base of the Clear Fork. The trace-bearing stratum is an up-to-0.3 m thick, laminated to flaser-bedded, dolomitic siltstone that also contains mud cracks, raindrop impressions, microbially induced mat structures, and some land-plant impressions. We interpret the Craddock dolomite member as the feather-edge of a marine transgressive carbonate deposit of an irregular coastline marked by shallow bays or estuaries on the eastern shelf of the Midland basin, and the trace-fossil-bearing stratum at Lake Kemp is an unchannelized flow deposit on a muddy coastal plain. The fossil site at Lake Kemp yields a low to moderately diverse fauna of invertebrate and vertebrate traces. A sparse invertebrate ichnofauna consists of arthropod feeding and locomotion traces assigned to Walpia cf. W. hermitensis White, 1929 and Diplichnites gouldi Gevers in Gevers et al., 1971. Tetrapod footprints are most common and assigned to Batrachichnus salamandroides (Geinitz, 1861), cf. Amphisauropus kablikae (Geinitz and Deichmüller, 1882), and Dromopus lacertoides (Geinitz, 1861), which represent small temnospondyl, seymouriamorph, and basal sauropsid trackmakers. Both the traces and sedimentary features of the fossil horizon indicate a freshwater setting at the time of track formation, and the trace assemblage represents the Scoyenia ichnofacies and the Batrachichnus ichnofacies in an overbank environment with sheet flooding and shallow ephemeral pools on an extensive coastal plain. The Lake Kemp tetrapod track assemblage is characteristic of the global Early Permian tetrapod ichnofauna found in red beds, which is dominated by a handful of ichnogenera that include Batrachichnus, Limnopus, Amphisauropus, Dromopus, Varanopus, Hyloidichnus, Ichniotherium and Dimetropus, which are the tracks of temnospondyls, seymouriamorphs, diadectomorphs, “pelycosaurs”, “captorhinomorphs”, and araeoscelids. The Lake Kemp tracks also further document the continuity of the ichnogenera Batrachichnus, Amphisauropus and Dromopus from Wolfcampian into Leonardian time and thus support the concept that Wolfcampian and Leonardian red-bed tetrapod footprints represent a single biostratigraphic assemblage.

Sand pseudomorphs of dinosaur bones: Implications for (non-) preservation of tetrapod skeletal material in the Hartford Basin, USA

Early Jurassic tetrapod tracks are common in the Hartford and adjacent Deerfield Basins (Newark Supergroup) of Connecticut and Massachusetts, USA, but skeletal material is rare. Among the few examples is a set of several bones preserved as natural casts on the base of a slab of arkosic sandstone that probably derived from the Portland Formation of Middletown, Connecticut. This mode of preservation is otherwise unknown in the Hartford Basin, and examination of this specimen suggests a complex taphonomic history. The bones were fluvially transported and arranged in a v-shaped pattern as they were deposited on a muddy substrate. Longitudinal cracks, gouging, breakage, rounding, and invertebrate boring indicate significant alteration prior to preservation as casts. Furthermore, the boring represents the first record of osteophagy in the Hartford Basin. Details of the cast preservation, such as sediment rims that wrapped up and over some of the bones, suggest that the bones were dissolved in the subsurface prior to lithification of the overlying sand bed. The decrease in pH that led to bone dissolution probably resulted from the infiltration of naturally acidic rainwater. Root respiration and the decomposition of plants under oxidative conditions may have contributed as well. The depauperate skeletal fossil record of the Hartford Basin is attributable to dissolution under acidic and oxidative sediment conditions. These natural casts offer a unique view of the actions of bone dissolution as a taphonomic process. The lack of body fossils in similar aged deposits elsewhere may also be due to groundwater acidity.

A new dicynodont–archosaur assemblage from the Late Triassic (Carnian) of Poland

ABSTRACTThis paper reports a new assemblage from the Late Triassic (mid–late Carnian) at Woźniki near Częstochowa (Poland). The Woźniki vertebrate assemblage is similar to that of Lisowice–Lipie Śląskie, a new locality bearing vertebrates from latest Triassic (latest Norian–early Rhaetian) strata of southern Poland, in the presence of dicynodonts, shark spines, plagiosaurs and a cyclotosaur, but conchostracans and bivalves are similar to those from the Krasiejów site (late Carnian). The most complete specimen from Woźniki belongs to a dicynodont, and consists of cranial and postcranial bones of a single individual. It demonstrates that large dicynodonts were part of the Late Triassic vertebrate assemblage in Central Europe. Numerous tetrapod tracks and traces are associated with skeletal fossils at Woźniki.

First Report of Dinosaur Footprints from Madagascar: Two Tracksites from the Middle Jurassic Bemaraha Formation

Here we report the first record of tetrapod tracks from Madagascar. We document two localities yielding dinosaur footprints, both within the Middle Jurassic Bemaraha Formation (Morondava Basin) in western Madagascar. The Sahalaly River tracksite yielded a single trackway belonging to a quadrupedal dinosaur; probably a sauropod, but a closer determination is hindered by bad preservation. In contrast, the Tsiandro tracksite yielded numerous well-preserved theropod tracks that allow some inference about the behavior of the trackmakers.

Tetrapod tracks in a marginal lacustrine setting (Middle Triassic, Argentina): Taphonomy and significance

Fossil tetrapod footprints not only provide valuable information about trackmaker paleobiology but also to give insight into details of the depositional conditions of the substrate at the time of imprinting. Therefore, in the present study the mode of formation and taphonomy of footprints in different substrates was used to investigate the gait and walking dynamics of the trackmakers as well as a source of additional information on the environmental conditions of the track-bearing beds during imprinting. The analyzed section corresponds to thick Middle Triassic lacustrine/deltaic deposits of the Ischichuca/Los Rastros Formation (Ischigualasto–Villa Unión Basin) that crops out at the Quebrada de Ischichuca in northwestern Argentina. Part of the track-bearing surfaces correspond to the top of sandy distributary channel mouth bars in a distal delta front setting that were exposed along the lake margin during a lake level fall. Cross-cutting relationships observed among ripple-marks, the footprints, and invertebrate traces of a softground suite of the Scoyenia ichnofacies suggest an omission surface. Measured trackway orientations in the sandstones are perpendicular to the paleo-shoreline, with the animals coming and going along the exposed top of the bars, probably for drinking. Laterally, the distal delta front deposits interfinger with track-bearing wackestone beds of palustrine origin deposited in a restricted local embayment lateral to the delta influenced environment. Trackway orientations in the wackestones are, in contrast, consistent with the animals moving nearly parallel to the lake border, probably along a preferred route. Evidences of a relative high groundwater table at the time of imprinting in the track-bearing surfaces are revealed by the well developed rims of extruded sediment and collapsed digits in the studied tracks and the nearly absence of associated desiccation cracks on the same surfaces. Nevertheless, temporary emergence cannot be ruled out when paleosoil formation was probably promoted as can be observed in the microstructure of both sandstones and wackestones. Moreover, footprint preservation in the wackestones might have been enhanced by partial hardening of the trampled surface during subaerial exposure. Combining ichnofossil content and taphonomy with facies analysis we identified in the lower part of the Ischichuca/Los Rastros succession a relatively rapid withdrawal of the water basinward that was probably due to a forced regression during early rifting of basin evolution. Footprints can also provide valuable information about locomotion dynamics and trackmaker behavior. Thus, the sideways deformation observed in the studied footprints, attributed to basal archosaurs and putative basal dinosaurs, can be related to an outward rotation of the foot during the step cycle, a condition that might allied to the development of the parasagittal posture in Archosauria. Besides, the densely trampled surface described herein constitutes the first documented evidence of putative social behavior among therapsid dicynodonts, the most important group of herviborous animals in the early Mesozoic terrestrial ecosystems throughout Gondwana.

Tetrapod trackways from the early Middle Devonian period of Poland

The fossil record of the earliest tetrapods (vertebrates with limbs rather than paired fins) consists of body fossils and trackways. The earliest body fossils of tetrapods date to the Late Devonian period (late Frasnian stage) and are preceded by transitional elpistostegids such as Panderichthys and Tiktaalik that still have paired fins. Claims of tetrapod trackways predating these body fossils have remained controversial with regard to both age and the identity of the track makers. Here we present well-preserved and securely dated tetrapod tracks from Polish marine tidal flat sediments of early Middle Devonian (Eifelian stage) age that are approximately 18 million years older than the earliest tetrapod body fossils and 10 million years earlier than the oldest elpistostegids. They force a radical reassessment of the timing, ecology and environmental setting of the fish-tetrapod transition, as well as the completeness of the body fossil record.