Jesper Milàn

The Impact of Sediment Consistency on Track and Undertrack Morphology: Experiments with Emu Tracks in Layered Cement

To demonstrate the influence of sediment consistency on track and undertrack morphology, an emu foot was impressed in four packages of layered, colored cement. The packages contained different ratios of cement to water and were subsequently sliced vertically to reveal the subsurface deformation caused by the foot, and to expose the differences in track and undertrack morphology at different degrees of firmness. A further package of alternating layers of sand and cement layers was prepared, allowing the package to be spilt along horizontal surfaces in order to investigate the undertrack formation and loss of information with depth. The experiments clearly show that increasing water content in the sediment has a strong effect on the morphology of both the true track and the undertracks.

Vertical sections through dinosaur tracks (Late Triassic lake deposits, East Greenland) – undertracks and other subsurface deformation structures revealed

Tracks and trackways of theropod dinosaurs (Grallator footprints) are abundant in the Late Triassic lake sediments of East Greenland. For this study we selected a rather diffuse theropod track preserved on the upper surface of a red heterolithic mudrock, and a better preserved track seen on the upper surface of a greyish mudrock. In order to examine undertracks and other subsurface deformation structures, both slabs were sectioned vertically at closely-spaced intervals, perpendicular to the length of the axis of the impression of digit III. Each section was subsequently polished and internal structures revealed. The digit impressions of both tracks were associated with well-defined undertracks which were cut by deep and narrow claw imprints at the distal end of the digit impressions. Marginal ridges at the tracking surfaces were typically associated with subsurface marginal folds. The marginal ridges were asymmetrically developed suggesting an outward movement of the proximal part of the foot, probably during the kick-off; this is in contrast to what is observed in tracks from Lower Jurassic theropods. The study shows that cross-sections through dinosaur tracks display large structural variation and it is suggested that some disturbed layers in continental deposits could be the result of trampling by vertebrates.

First American Record of the Jurassic Ichnospecies Deltapodus brodricki and a Review of the Fossil Record of Stegosaurian Footprints

We describe the first American stegosaur track of the ichnospecies Deltapodus brodricki, collected in the Upper Jurassic Morrison Formation of San Juan County, southeastern Utah, United States. The track is preserved as a natural cast on the underside of a slab of fluvial sandstone and consists of a well‐preserved pes track and the eroded remains of a manus track. Previously, Deltapodus was known only from the Middle Jurassic Yorkshire coast of England and the Upper Jurassic of Portugal and Spain. The new discovery thus substantially extends the geographic record of this ichnospecies and highlights the similarities between the Late Jurassic dinosaur faunas of North America and those of Western Europe.

Rheotactic Macaronichnus, and Human and Cattle Trackways in Holocene Beachrock, Greece: Reconstruction of Paleoshoreline Orientation

Macaronichnus segregatis occurs in an upper foreshore conglomeratic to sandy beachrock dating from Roman times, on the east coast of Rhodes, Greece. In some laminae, M. segregatis is strongly oriented perpendicularly to the Recent and Holocene seashore. In other laminae, this trace fossil displays a winding to spiral course. These forms are introduced as new ichnosubspecies, M. segregatis lineiformis, M. segregatis maeandriformis and M. segregatis spiriformis, respectively. It is suggested that these middle-tier trace fossils were produced under conditions of different porewater flow. During higher energy periods, predominantly in winter, water movement perpendicular to the coastline caused M. segregatis to be oriented rheotactically. During periods of more or less stationary pore-water, predominantly in summer, M. segregatis assumed a spiral form. The winding form represents intermediate conditions. Closely beneath the Macaronichnus horizons a large burrow fill was found, probably referable to Psilonichnus isp. In the same beachrock are undertracks attributed to artiodactyls, associated with tracks and a trackway of human origin. The anatomy of the tracks indicates the direction of travel of the trackmakers, eastward and westward along-shore in all cases but one. The morphology of the tracks has been influenced by the gravitational effect of beach slope. Orientation, morphology and structure of trace fossils provide clear evidence that the sandstone and gravel unit is a beachrock and reveal the precise orientation of the paleoshoreline.

Dinosaur Tectonics: A Structural Analysis of Theropod Undertracks with a Reconstruction of Theropod Walking Dynamics

A dinosaur trackway in the Middle Jurassic eolian Entrada Sandstone of southern Utah, U.S.A., exposes three undertracks that we have modeled as isolated tectonic regimes showing the development of fold‐thrust ramp systems induced by the dinosaur’s feet. The faulted and folded sequence is comparable to crustal‐scale tectonics associated with plate tectonics and foreland fold‐thrust belts. A structural analysis of the dinosaur tracks shows the timing and direction of the forces exercised on the substrate by the animal’s foot during the stride. Based on the structural analysis, we establish a scenario for foot movements and weight distribution in the feet. During the end of the weight‐bearing phase of the stride, the weight of the animal was transferred to the front of the digits, creating a rotated disk below the foot that was bounded by an extensional fault at the front and a thrust ramp toward the back. As the body accelerated, the foot was forced backward. The rotated disk was forced backward along a detachment fault that was bounded by lateral ramps. The interramp segment matches the width of the dinosaur’s foot, which created an imbricate fan thrust system that extended to the far end of the undertrack. The total length of the tectonic disturbance created by the dinosaur is up to three times that of the original footprint. Early, near‐surface cementation gave the substrate the rheological properties necessary for development of the observed structures.

New finds of stegosaur tracks from the Upper Jurassic Lourinhã Formation, Portugal

Eleven new tracks from the Upper Jurassic of Portugal are described and attributed to the stegosaurian ichnogenus Deltapodus. One track exhibits exceptionally well-preserved impressions of skin on the plantar surface, showing the stegosaur foot to be covered by closely spaced skin tubercles of ca. 6 mm in size. The Deltapodus specimens from the Aalenian of England represent the oldest occurrence of stegosaurs and imply an earlier cladogenesis than is recognized in the body fossil record.

Crouching theropod and Navahopus sauropodomorph tracks from the Early Jurassic Navajo Sandstone of USA

Numerous tracks and trackways are preserved in the a cross-strata of the Lower Jurassic Navajo Sandstone of northern Arizona and southern Utah, USA. Tracks and trackways of small theropod dinosaurs are particularly abundant within one 10-m-thick interval. This paper describes a crouching trace from a theropod dinosaur that shows impressions of all four limbs, the ischial callosity, the tail, and tracks leading to and away from the crouching site, and revises the interpretation of a well preserved trackway hitherto referred to the synapsid ichnogenus Brasilichnium and here considered to be from a sauropodomorph dinosaur. It is named Navahopus coyoteensis isp. nov. on the basis of morphological differences from the type ichnospecies N. falcipollex. The ichnofamily Navahopodidae is revised to include Tetrasauropous unguiferus, Navahopus falcipollex, and N coyoteensis.

Preservation and Erosion of Theropod Tracks in Eolian Deposits: Examples from the Middle Jurassic Entrada Sandstone, Utah, U.S.A.

The Middle Jurassic Entrada Sandstone, exposed near the town of Escalante, southern Utah, consists of large‐scale cross‐bedded eolian deposits that are interbedded with horizontally laminated sand sheets and thin sets of eolian cross‐strata, representing periods with a moister climate. The flat‐bedded units contain numerous tracks and trackways from small to large‐sized theropod dinosaurs. These tracks are today exposed in several distinct erosional states, allowing detailed studies of track and undertrack formation in eolian deposits. Tracks that originally were emplaced on sloping surfaces show, in their present‐day erosional state, a morphology distinct from those originally emplaced on horizontal surfaces. Further, the range of eroded track morphologies can help identify badly eroded tracks from nonbiogenic structures in similar deposits.

The vertebrate-bearing Late Triassic Fleming Fjord Formation of central East Greenland revisited: stratigraphy, palaeoclimate and new palaeontological data

Abstract In Late Triassic (Norian–Rhaetian) times, the Jameson Land Basin lay at 40° N on the northern part of the supercontinent Pangaea. This position placed the basin in a transition zone between the relatively dry interior of the supercontinent and its more humid periphery. Sedimentation in the Jameson Land Basin took place in a lake–mudflat system and was controlled by orbitally forced variations in precipitation. Vertebrate fossils have consistently been found in these lake deposits (Fleming Fjord Formation), and include fishes, dinosaurs, amphibians, turtles, aetosaurs and pterosaurs. Furthermore, the fauna includes mammaliaform teeth and skeletal material. New vertebrate fossils were found during a joint vertebrate palaeontological and sedimentological expedition to Jameson Land in 2012. These new finds include phytosaurs, a second stem testudinatan specimen and new material of sauropodomorph dinosaurs, including osteologically immature individuals. Phytosaurs are a group of predators common in the Late Triassic, but previously unreported from Greenland. The finding includes well-preserved partial skeletons that show the occurrence of four individuals of three size classes. The new finds support a late Norian–early Rhaetian age for the Fleming Fjord Formation, and add new information on the palaeogeographical and palaeolatitudinal distribution of Late Triassic faunal provinces.

Archosaur footprints (cf. Brachychirotherium) with unusual morphology from the Upper Triassic Fleming Fjord Formation (Norian-Rhaetian) of East Greenland

Abstract The Ørsted Dal Member of the Upper Triassic Fleming Fjord Formation in East Greenland is well known for its rich vertebrate fauna, represented by numerous specimens of both body and ichnofossils. In particular, the footprints of theropod dinosaurs have been described. Recently, an international expedition discovered several slabs with 100 small chirotheriid pes and manus imprints (pes length 4–4.5 cm) in siliciclastic deposits of this unit. They show strong similarities with Brachychirotherium, a characteristic Upper Triassic ichnogenus with a global distribution. A peculiar feature in the Fleming Fjord specimens is the lack of a fifth digit, even in more deeply impressed imprints. Therefore, the specimens are assigned here tentatively to cf. Brachychirotherium. Possibly, this characteristic is related to the extremely small size and early ontogenetic stage of the trackmaker. The record from Greenland is the first evidence of this morphotype from the Fleming Fjord Formation. Candidate trackmakers are crocodylian stem group archosaurs; however, a distinct correlation with known osteological taxa from this unit is not currently possible. While the occurrence of sauropodomorph plateosaurs in the bone record links the Greenland assemblage more closer to that from the Germanic Basin of central Europe, here the described footprints suggest a Pangaea-wide exchange. Supplementary material: Three-dimensional model of cf. Brachychirotherium pes–manus set (from MGUH 31233b) from the Upper Triassic Fleming Fjord Formation (Norian–Rhaetian) of East Greenland as pdf, ply and jpg files (3D model created by Oliver Wings; photographs taken by Jesper Milàn) is available at https://doi.org/10.6084/m9.figshare.c.2133546