Tetrapod Tracks Research Articles

The impact of microbial mats in promoting record of Maastrichtian crocodylomorph tracks from Yacoraite Formation, Argentina

Here we report tetrapod tracks from a stratigraphic level of Maastrichtian age in the Yacoraite Formation (Maimará ichnosite, Jujuy Province, Argentina). The ichnological material consists of convex hyporeliefs whose formation was induced by growth of microbial mats in an upper subtidal-lower intertidal setting. Ichnological features that are recognized in tracks and traces allowed to include them into the categories of ‘punting tracks’ and ‘buoyancy tracks’. These categories suggest swim behaviours of the producers, which are tentatively interpreted as crocodylomorphs. Reconstruction of the filling process leading to tracks preservation and inclusion into the geological record enabled reconstructing at least two different phases of mats development, one dominated by chemical precipitation and the other by trapping and binding of coated grains and other particles. These two phases of mat growth are related to slightly changing energetic conditions of the palaeoenvironment and further support a marine origin for the unit. Finally, it is suggested that biologically induced, in-situ mineral precipitation promotes fast formation of overtracks through early lithification achieving preservation of tetrapod tracks well before their complete filling.

Jurassic–Cretaceous boundary theropod tracks from the slope of the Fallen Feng, Chicheng, China: review and new observations

ABSTRACT The Tuchengzi Formation represents a Jurassic – Cretaceous boundary regional lithological unit that bridges the temporal gap between the Yanliao biota and the Jehol biota. Tetrapod tracks represent the most significant vertebrate fossil records in the formation, and can be employed to supplement the Late Jurassic fauna and palaeoecological data from northern China. Here, we present a re-analysis of the theropod tracks from the Luofenggou site in northern Hebei Province, utilising an additional 3D model and a locomotory analysis to provide further insights into the ecological context. A morphological analysis indicates that these tracks are more similar to Grallator than to the previously proposed Therangospodus. The latter is diagnosed by coalesced, elongate, oval digital pads lacking a separation in distinct phalangeal pads, whereas these are clearly defined in the presented Tuchengzi material. They can therefore be assigned to the widespread G. ssatoi ichnospecies from the Tuchengzi Formation, although they are longer and more variable in size than other G. ssatoi. A general consideration of speed-related locomotive parameters, trackway morphology, and superposition allows for the speculation of the existence of cooperative or predatory relationships between theropod trackmakers that were differentiated by size or stages of ontogeny, but had similar foot structures.

Tetrapod tracks from the Middle/Late Triassic of the Ourika and Yagour basins (Morocco): new ichnotaxonomical data based on the Biron collection

Palaeontological prospecting organized in the Ourika and Yagour basins (Marrakech High Atlas, Morocco) in the 1980's, have yielded abundant and exquisitely preserved tetrapod tracks, some of them preserving thinly detailed skin traces (Collection Biron, Cadi Ayyad University, Marrakech, Morocco). Based on this material, diverse morphologies of tracks were described and six new ichnotaxa were erected. However, these trackways have never been reinvestigated in detail from an ichnotaxonomic point of view. Here, the Biron collection is reevaluated using modern photogrammetric imaging techniques. The fine-grained sandstone slabs bear trackways ascribed to Apatopus lineatus, Isochirotherium isp. and Rhynchosauroides isp. whose trackmakers are possible phytosaurs, crocodylian-stem archosaurs or non-crown-group archosauriforms and lepidosauromorphs/archosauromorphs, respectively. “Chirotherium atlensis”, “Enigmatopus atlensis”, “Hyloidichnus triasicus”, “Prochirotherium triasicum”, “Pseudochirotherium oukaimedensis” and “Rectilinetopus ourikensis” are considered as invalid. The palaeoichnological assemblage includes a mixture of Early to the Middle Triassic (Isochirotherium) and Late Triassic (Apatopus lineatus) tracks. This result corroborates the biostratigraphic data recently obtain from the base of the Oukaïmeden Sandstone Formation, suggesting a Middle-Late Triassic age. The living environment was a semi-arid alluvial plain with ponds and ephemeral rivers.

TAPHONOMY OF TINY TETRAPOD TRACKS IN AN EXAMPLE FROM THE LOWER PERMIAN (CISURALIAN) SŁUPIEC FORMATION (SW POLAND)

Abstract The continental Lower Permian Słupiec Formation (the Sudetes Mountains, southwestern Poland) is well known for the occurrence of fossilized tracks of Early Permian tetrapods. Previous reports have focused on the taxonomy of tracks and their producers. The track assemblage was apparently missing the footprints of amphibians and thus such tetrapods were thought to be truly absent in the Słupiec Early Permian paleohabitat. The present paper utilizes the results of experiments and field surveys in order to test a possibility that tracks of tiny tetrapods (e.g., with body mass < 9 g), including amphibians, could be missing due to taphonomic filtration. The experimental tests were performed to investigate the following: (1) the interplay between the sizes of the penetrator (representation of trackmaker’s pes and manus) and the particles within the substrate, with emphasis on how the size interplay affects the lamina-modification mechanism, e.g., lamina-piercing by sand versus lamina compaction/displacement by sand or clay; (2) undertrack production in laminated sandy substrates by tiny tetrapods; and (3) the taphonomic impact that biofilm may have on the occurrence of tiny tetrapod tracks in clay. The results of experimental tests and the field survey suggest that tiny tetrapod tracks (e.g., amphibians) may be underrepresented in the Słupiec Formation track assemblage due to negative taphonomic filtration.

A New Specimen of Sand Pseudomorph Dinosaur Bones with Tetrapod Tracks from the Early Jurassic, Hartford Basin, USA

ABSTRACT Tetrapod tracks from the Lower Jurassic strata of the Hartford Basin (Newark Supergroup) abound in areas of Massachusetts and Connecticut, USA. However, tetrapod skeletal fossils are uncommon, and the co-occurrence of tracks with skeletal material in the same bedding plane is exceedingly rare. Due to this paucity of skeletal material, spatiotemporal gaps and taxonomic incompleteness continue to hamper research on bones preserved as natural casts. Here we report on the discovery of new bone cast specimens and three grallatoroid (Anchisauripus) tracks on a single sandstone slab originating from Portland or Middletown, Connecticut. The Nash Specimen – BRCM 2021.10 – holds important implications for Early Jurassic dinosaur taphonomic history and the paleoenvironment in which dinosaur footprints and bones as natural casts were preserved. Results from this study describe three Anchisauripus tracks and five natural bone casts from an early theropod dinosaur. Judicious comparisons to the only comparable specimen of dinosaur bone casts (MOS.2001. 248) are made in describing the morphological structures and the taphonomic pathways leading to cast formation. The unique taphonomic circumstances of bone casts preserved alongside dinosaur tracks on the same substrate layer provide an intriguing case of the non-preservation of body fossils that merits further research in the Hartford Basin.

Captorhinid trackways from mid- to late Permian red beds in Morocco: Implications for locomotion and the palaeobiogeography of northwest Gondwana

In this paper, we report a new vertebrate ichnoassemblage from the mid- to late Permian red beds of the Ikakern Formation (Tourbihine Member, T2) from the Argana Basin of Morocco, to improve knowledge of paleobiology and palaeobiogeography. The ichnofossils comprise than 50 footprints organized as four trackways and assigned to the ichnogenus Hyloidichnus. The tetrapod tracks are preserved in convex hyporelief and concave epirelief on upper and lower mud-rich surfaces near the top of fine-grained sandstone units. The trackway pattern and the alternating arrangement of the pes-manus sets suggest a sprawling and symmetrical gait, with the movements of the front and hind limbs being of similar size, and it is likely that the animal moved at slow trot. We infer that the potential trackmaker was a captorhinomorph within the Moradisaurinae. This new fossil discovery helps reconstruct the palaeoenvironments and the palaeogeographic distribution of these Permian organisms, with Morocco acting as a faunal exchange gateway between western Gondwana and western Laurasia. Besides the tetrapod tracks, the red beds contain ubiquitous rain-drop marks, current ripples, plant fossil impressions, invertebrate burrows, tetrapod skin imprints, and a single insect wing that suggest that the strata were deposited in an episodically inundated alluvial plain.

Synapsid tracks with skin impressions illuminate the terrestrial tetrapod diversity in the earliest Permian of equatorial Pangea

Lower Permian deposits of the Boskovice Basin in the Czech Republic have long been renowned for extraordinarily abundant specimens of discosauriscid seymouriamorphs, some of which showing exceptional preservation, including widespread soft tissues. The only other tetrapods from the strata are represented by rare temnospondyls. However, recent fieldwork in the Asselian (lowermost Permian) of the Boskovice Basin has yielded a diverse assemblage of tetrapod tracks, illuminating a hidden terrestrial tetrapod diversity. Here, we describe well-preserved isolated tracks, manus-pes couples, and a slab with trackways composed of approximately 20 tracks in at least four different directions belonging to early-diverging, or ‘pelycosaur-grade’, synapsids. The material originates from three localities situated within the Letovice and Padochov formations and is assignable to the ichnotaxon Dimetropus. The best-preserved specimen further shows rare skin impressions, which have not been observed from the hands or feet of early-diverging mammal-line amniotes before. The new material adds to the scarce record of synapsids from the Carboniferous/Permian transitional interval of equatorial Pangea. At the same time, it highlights the significance of the ichnological record of the Boskovice Basin which has long been neglected despite offering evidence for the presence of diverse faunal components that have not been reported from these basinal deposits before.

Revisiting the southernmost occurrence of Brasilichnium elusivum Leonardi, 1981 with comments on the tetrapod track record of eolian environments

Mammalian lineage is deep rooted in Mesozoic, with several taxa and ichnotaxa described worldwide. One of the most prolific mammaliaform ichnospecies is Brasilichnium elusivum, which is extremely abundant in the Lower Cretaceous Botucatu Formation of the Brazilian Paraná Basin, as well as in other Mesozoic ichnofaunas of North America and Africa. In this contribution, we revisited the proposed southernmost occurrence of B. elusivum using both classical and photogrammetric (3D digitization) approaches. The flagstone containing the studied material was found ex situ, in a sidewalk at Santa Cruz do Sul Municipality (the Rio Grande do Sul State, Brazil), and the results of the performed analysis showed that the tracks cannot be assigned to any ichnotaxon. Lastly, some significant issues related to ichnological research of ex situ track-bearing flagstones and large eolian deposits are discussed. Keywords: Early Cretaceous, Botucatu Formation, mesozoic mammals, South America, vertebrate ichnology.

Ichnofossils and body fossils from the Permian of the Sorgue Valley (Saint-Affrique Basin, southern France): palaeoenvironmental implications

ABSTRACT We report a new palaeontological assemblage from the Sakmarian (Early Permian) of Moulin de Latour in the Saint-Affrique Basin (southern France). Pelitic beds show the co-occurrence of arthropod (Diplopodichnus biformis) and tetrapod (Batrachichnus salamandroides and Dromopus lacertoides) tracks with jellyfishes (Medusina atava). Ichnofossils also include some swimming tracks of fishes. They show several morphologies, including Undichna quadrisulcata and three morphotypes of Undichna isp. The content of the palaeontological assemblage as well as the lithological features of the fossiliferous levels suggest that the depositional environment was a lake/playa system periodically flooded. Terrestrial plants (Autunia conferta) show that this environment was bordered by a pteridosperm-dominated flora. The dense accumulation of jellyfishes observed in some surfaces is interpreted as mass mortality events in lakes during dry periods. The significance of this discovery is threefold: (1) The swimming trails of fishes are the first ones to be reported from the Saint-Affrique Basin, and they are extremely rare in Permian deposits from France; (2) The co-occurrence of Undichna with tetrapod tracks is not common in Permian deposits worldwide; and (3) The recurrent occurrence of jellyfishes in Permian freshwater environments suggests a peculiar ‘taphonomic window’ for that time interval.

Paleosols and Tetrapod Tracks from the Lower Cretaceous of the Northern Caucasus

Paleosols and Tetrapod Tracks from the Lower Cretaceous of the Northern Caucasus

Paleocene (65–63 and 58.5 ma) marine flooding and 62–60 ma sediment bypass in southern Wyoming, U.S.A.: Implications for Laramide sediment flux to the Gulf of Mexico

ABSTRACTSedimentary structures unique to tidally influenced environments and unambiguously salinity-stressed marine ichnofossil assemblages in the lower Paleocene Ferris and upper Paleocene Hanna formations of Wyoming's Hanna Basin (HB) necessitate major revision of local and regional reconstructions of the Paleocene Western Interior Seaway (WIS). Preserved in sandy estuarine bars, sandy tidal flats, heterolithic distributary channels, bayhead delta, and flood-tide-delta deposits similar those in the modern Trinity River, its bayhead delta, and the San Luis Pass flood-tidal delta on the Texas coast, these these assemblages include Arenicolites, Bergaueria, Fuersichnus, Gyrochorte, Ophiomorpha, Palaeophycus, Planolites, Psilonichnus, Rhizocorallium, Rosselia, Siphonichnus, Skolithos, Spongeliomorpha, Taenidium, Thalassinoides, and tetrapod tracks. Mapping an ∼ 325-m-thick succession of lower Paleocene strata (65 to 63 Ma) around the western HB reveals a series of marine flooding events, each followed by coal accumulation. A similar, 170-m-thick succession of interfingering coastal-plain and restricted-marine strata occurs in the upper Paleocene (58.5 Ma) Hanna Formation, following accumulation of lacustrine and floodplain deposits and an episode of major gravel and cobble progradation from 62 to 60 Ma. These younger ichnofossils record the final major transgression of the WIS and have major implications for the depositional environment of the time-equivalent Waltman Shale in the Wind River Basin to the north and for sediment routing to the Gulf Coast Wilcox sands. Ichnofossils are an underutilized source of physicochemical proxy data that are lifting the veil from the cryptic Paleocene transgressions of the WIS that have for so long remained undetected because of the absence of open-marine body fossils.

Tetrapod-track taphonomic modes (TTTM) applied to Triassic lacustrine systems from southern South America

Tetrapod-track taphonomic modes (TTTM) have been used recently as sensitive indicators of paleoenvironmental and paleoclimatic conditions during deposition and imprinting of the track-bearing beds in an underfilled lake basin from NW Argentina. Three tetrapod track preservation modes were defined based on the fidelity of preservation of anatomical features: a High Fidelity Mode shows sharp and detailed impressions of the digits and palms, with the clear impression of the print walls; a Moderate Fidelity Mode shows impressions of digits and palms that have little detail of the anatomy of the trackmaker's autopodia; and finally, a Low Fidelity Mode characterized by oval or irregular depressions with outlines surrounded by deep and distinct rims of displaced sediment. In this contribution, we expand our initial TTTM approach to all three lake-basin types: underfilled, balanced-fill, and overfilled. We review the Triassic tetrapod track record preserved in several lacustrine deposits of three continental basins from central-western Argentina and Patagonia. Our results show that our TTTM are consistently expressed in the different lake basins analyzed, and that substrate rheology it is not only a function of water content, but also of grain-size distribution (sand:clay ratio), the type of clay minerals, and stratal stacking at the bed scale. Linking tetrapod-track taphonomic modes to the lake-basin-type framework places observations of vertebrate traces into a broader context of depositional systems, landscape evolution, and climate and tectonic changes.

Unusual sauropod slipping tracks preserved on a biostabilized tidal flat from the Lower Cretaceous of northern Patagonia, Argentina

Abstract Several elongated and other crescent-shaped impressions interpreted as dinosaur tracks and preserved on Lower Cretaceous tidal flat deposits from the Agrio Formation (northern Patagonia, Argentina) are studied in detail. These tracks were documented on a palaeosurface showing palaeotopographic differences over a short distance, related to the lateral migration of a tidal channel. A genetic order between biostabilization and bioturbation of the palaeosurface is recognized. Ripples generated within the channel during the flood tide were first biostabilized; there, horseshoe crab trace fossils were produced. Then, the area bearing the tracks studied here was subaerially exposed, with the biostabilized substrate generating a slippery surface. Scanning electron microscope analysis of the rims indicates that the microbial mat was disturbed by the trampling. Finally, microbial mats recovered, and some tracks preserved wrinkle marks inside the tracks. Taking into account the substrate properties and track features analysed (morphology, size, depth and orientation), it is concluded that the tracks were produced by dinosaurs, probably small sauropods, trampling on a slippery surface and moving parallel to the channel margin. This study is an unusual case of slipping tetrapod tracks preserved on a biostabilized tidal flat developed in a mixed carbonate–siliciclastic environment.

How to verify fossil tracks: the first record of dinosaurs from Palestine

ABSTRACT The identification of presumed tetrapod tracks is not always unequivocal. Other sedimentary structures have been repeatedly mistaken for tracks, including other trace fossils such as arthropod tracks, burrows and fish feeding traces; erosional features; and human-made traces. We here review instances of difficult, ambiguous, or controversial cases that have been discussed in the literature. We then discuss four main criteria for the verification of tetrapod tracks: (1) preservation of regular trackway morphology, (2) preservation of track morphology, (3) deformation structures (best seen in cross-section) and (4) the temporal or environmental context. Of these criteria, criterion 1 is the most unambiguous and has rarely been challenged. We apply these criteria to a new site located within the city of Al-Bireh, Palestine, which belongs to the Lower Cretaceous (Albian) Soreq Formation. The site preserves a surface with many indistinct depressions that lack anatomical detail. Two unequivocal trackways are identified per criterion 1, demonstrating the first known occurrence of dinosaur fossils in Palestine. The tracksite is part of the late Lower Cretaceous carbonate platform of the eastern Levant, demonstrating temporal emergence of the platform above sea level and a connection to the mainland.

Late Triassic dinosaur tracks from Penarth, south Wales

AbstractEvidence of Late Triassic large tetrapods from the UK is rare. Here, we describe a track-bearing surface located on the shoreline near Penarth, south Wales, United Kingdom. The total exposed surface is c. 50 m long and c. 2 m wide, and is split into northern and southern sections by a small fault. We interpret these impressions as tracks, rather than abiogenic sedimentary structures, because of the possession of marked displacement rims and their relationship to each other with regularly spaced impressions forming putative trackways. The impressions are large (up to c. 50 cm in length), but poorly preserved, and retain little information about track-maker anatomy. We discuss alternative, plausible, abiotic mechanisms that might have been responsible for the formation of these features, but reject them in favour of these impressions being tetrapod tracks. We propose that the site is an additional occurrence of the ichnotaxon Eosauropus, representing a sauropodomorph trackmaker, thereby adding a useful new datum to their sparse Late Triassic record in the UK. We also used historical photogrammetry to digitally map the extent of site erosion during 2009–2020. More than 1 m of the surface exposure has been lost over this 11-year period, and the few tracks present in both models show significant smoothing, breakage and loss of detail. These tracks are an important datapoint for Late Triassic palaeontology in the UK, even if they cannot be confidently assigned to a specific trackmaker. The documented loss of the bedding surface highlights the transient and vulnerable nature of our fossil resources, particularly in coastal settings, and the need to gather data as quickly and effectively as possible.

Fish swim traces from the Jindong Formation (Cretaceous) Korea: Implications for lake basin ichnofacies and paleoecology

The track rich Cretaceous Jindong Formation is famous for abundant tetrapod track assemblages, which occur at multiple, closely-spaced stratigraphic levels in the Gyeongsang Basin lake deposits of South Korea. However, until now no fish traces or fish body fossils had been reported. Thus, the discovery of three Undichna ichnospecies (U. simplicitas, U. bina, and U. britannica) provide the first direct evidence of fish in the basin. Paired in-phase traces vary in width and indicate a range of fish sizes among the tracemakers. Undichna assemblages represent a shallow lacustrine Undichna ichnocoenosis. Bird tracks (Ignotornis) registered on the same surface represent the superposition of a shorebird ichnocoenosis. Dinosaur tracks (Brontopodus and Caririchnium) associated with surfaces only ∼1 m below the bird and fish traces, represent additional large tetrapod ichnocoenoses, and demonstrate multiple, superimposed ichnocoenoses, interpenetrating localized sedimentary successions which are little separated in space or time. The Undichna and shorebird, ichnocoenoses are considered, in schema followed here, as representative of higher-level ‘archetypal’ Charachichnos and Grallator ichnofacies respectively. However, it is not clear whether a third ichnocoenosis and ichnofacies is represented by the Brontopodus and Caririchnium assemblages or the extent to which the overprinting of different ichnofaunas in localized areas always allows for the unambiguous application of ichnocoenois and ichnofacies schema.

Diminutive pterosaur tracks and trackways (Pteraichnus gracilis ichnosp. nov.) from the Lower Cretaceous Jinju Formation, Gyeongsang Basin, Korea

Small pterosaur tracks were discovered from the Cretaceous Jinju Formation of Jinju City, South Korea, now famous as an ichnological Konservat-Lagerstätte and source of large numbers of well-preserved tetrapod track assemblages. These small tracks are among ∼2500 reported from the vicinity of the Jinju Pterosaur Tracks Museum which represented the world's largest concentration of pterosaur tracks and multiple pterosaur track-bearing strata. The small tracks, which are the focus of this study are preserved as natural impressions (concave epirelief) on the surface of a dark grey shale slab. Manus tracks have average values of 27.2 mm length, 9.3 mm width and 3.0 length/width ratio. Pes tracks have average values of 27.8 mm length, 12.3 mm width and 2.2 length/width ratio. The sizes of the tracks are about as small as other small pterosaur tracks previously reported from Korea (P. koreanensis), Japan (P. nipponensis) and Spain (P. longipodus and P. parvus). Nevertheless, morphological characteristics of these tracks are clearly distinguished from those of ichnospecies mentioned above in divarication angles between manus digits, shape of pes imprints and length/width ratio of manus. Especially, the high elongation obtained from the narrow width of the manus and the length of elongated digit Ⅲ and the shape of isosceles triangular pes are the most prominent and diagnostic features. Thus, the small pterosaur tracks described herein, Pteraichnus gracilis ichnosp. nov., are considered a different type among established pterosaurian ichnotaxa.

Insights on the evolution of synapsid locomotion based on tetrapod tracks from the lower Permian of Mallorca (Balearic Islands, western Mediterranean)

“Pelycosaur”-grade synapsids were a successful group of terrestrial tetrapods that lived during the Carboniferous and Permian, utilising a wide diversity of ecological niches. They are considered the trackmakers of the ichnogenera Dimetropus and possibly also Dromopus and Tambachichnium, found in upper Palaeozoic deposits of North America, North Africa and Europe. Here we describe a new morphotype from the lower Permian of Mallorca (Balearic Islands, western Mediterranean), identified as cf. Dimetropus isp., which was purportedly produced by caseid synapsids. The trackmaker identity is inferred based on the digit proportions, as caseids are the only pelycosaurs with species showing mesaxonic autopodia, and a relative depth pattern (corresponding to the functional prevalence of the autopodia of the trackmaker) showing an overall similarity to that of Dimetropus osageorum, which is also attributed to caseids. A detailed study of the expulsion rims and drag traces makes it possible to infer the mode of locomotion of the trackmaker, with a gait alternating the movement of manus and pedes of the two sides of the body and lateral undulation of the spine. Our results provide new information concerning the locomotion of early synapsids, which would undergo important functional modifications later in their evolutionary history such as a shift from abducted to adducted posture and lateral to sagittal bending of the axial skeleton.

Multiproxy analyses of the Triassic Agua de la Zorra Formation, Cuyana Basin, Argentina: Palynofacies, geochemistry, and biotic record

The Triassic Agua de la Zorra Formation crops out in the Paramillos de Uspallata area (NW of Mendoza) and represents a deltaic-lacustrine system with volcanic activity as well as episodic incursions of peperitic basalts. The succession is characterized by distal lacustrine facies at base and top, with proximal shallow facies in the middle section. The paleontological record includes invertebrates (spinicaudatans, insects), vertebrates (mainly fishs), tetrapod tracks and invertebrates traces, and plant remains (mainly referred to Umkomasiales). Kerogen in the distal facies is mostly terrigenous, with scarce amorphous and palynomorphs (disacates and microalgae). It shows a variable percentage of the total organic carbon content (between 0.2 and 4.6) with hydrogen and oxygen indices indicating a type III kerogen. In the shallow facies, kerogen is practically absent and the trace fossils reflect sporadic subaerial exposure. The palynological evidence indicates an oligotrophic lake and the low productivity recorded was likely caused by dilution of the lake water by strong fluvial influence in an overfilled lake basin type. The organic geochemistry data also suggests an overfilled lake basin and the inorganic geochemistry data support the development of anoxic conditions at the bottom of the paleolake. The spinicaudata valves occur in densely packed beds that could have been driven by changes in the water physicochemical parameters as a result of volcanic activity in the area. The volcanic activity in the area is a good explanation for variability in the biota diversity. However, the continuous record of plant remains through out the section seems to indicates that the paleoenvironmental conditions in the surrounding terrestrial areas were stables.

Small bird and mammal tracks from a mid-Cenozoic volcanic province in Southern Colorado: implications for palaeobiology

ABSTRACT The surprise discovery of two small assemblages of well-preserved, small bird and small mammal tracks in a thin unit of reworked tuff in the mid-Cenozoic volcanic field of southwestern Colorado represents the first significant report of tetrapod tracks in the region. The bird tracks are represented by a new ichnospecies of Avipeda (A. circumontis ichnosp. nov.), which differs from other Avipeda ichnospecies in representing a trackmaker with a narrower foot and comparatively robust digit traces and digital pads. The mammal tracks which are also well preserved are small (< 1.5 cm long and wide) tetradactyl-pentadactyl and rodent like. They are arguably unique and among the best-preserved small mammal tracks presently known from the Cenozoic, and are here assigned to the new ichnotaxon Musvestigium minutus ichnogen et ichnosp nov. The track-bearing surface represents a very thin unit of reworked tuff sandwiched between massive ignimbrite flows, and therefore represents a localised window into a brief period of biotic activity in local phase of quiescence in a large, active volcanic province.