Articles

Print Email

orphology and sediment deformation of downslope Brasilichnium trackways on a dune slipface in the Nugget Sandstone of northeastern Utah

George F. Engelmann and Daniel J. Chure

Plain Language Abstract

Deposits of a great sand sea that covered the western United States during the Late Triassic and Early Jurassic preserve a sparse fossil record that largely consists of footprints, trackways, trails, and burrows made by vertebrates and invertebrates.

Brasilichnium is the most common vertebrate trackway found in the dune sediments and was probably made by near mammals or primitive mammals. Brasilichnium tracks are quadrupedal, wide relative to their length as compared to other types of tracks, often less than 2 cm across, and with stubby toes. Although Brasilichnium is a common track fossil, virtually all known records are of trackways walking up slope on the dune face, even in occurrences where hundreds of footprints are preserved on a single surface.

A previously unreported slab of Nugget Sandstone on exhibit at the University of Utah preserves four Brasilichnium trackways walking down slope amongst 13 trackways heading up slope. The downslope trackways are associated with unique sediment deformation which easily distinguishes them from the upslope trackways. A raised, flat-topped, arcuate platform of sand formed in front of each downslope track when the foot was planted, displacing the sand upward and forward. And behind each track, a large depression with curved transverse ridges formed when the foot was withdrawn, allowing the sand to collapse as a slump. A second Brasilichnium occurrence in Dinosaur National Monument corroborates these observations.

Despite the widespread occurrence throughout the geological record of fossil vertebrate tracks produced by taxonomically diverse trackmakers on the downwind slope of sand dunes, downslope trackways are exceedingly rare, completely absent from most locations. Although no consensus on an explanation for this pattern has appeared, downslope tracks should be highly recognizable from the sediment deformation documented here.

Resumen en Español

Morfología y deformación del sedimento de las pistas de bajada de Brasilichnium en una superficie de dunas en la Nugget Sandstone del noreste de Utah

La presunta pista de huellas producida por un sinápsido conocida como Brasilichnium está caracterizada por numerosos ejemplos en muchas localidades a lo largo de los depósitos eólicos, del erg del Triásico Tardío al Jurásico Temprano, que cubrían gran parte del oeste de los Estados Unidos. Estas pistas se producen principalmente en los lechos frontales (foreset beds) de las dunas. La gran mayoría de las pistas de huellas fueron hechas por animales que se desplazaban pendiente arriba. Una losa de Nugget Sandstone que se exhibe en el Departamento de Geología y Geofísica de la Universidad de Utah conserva al menos 17 pistas de Brasilichnium, 13 subiendo la pendiente y cuatro bajándola. Las huellas descendentes tienen una morfología distintiva que consiste en un cojín de presión anterior levantado y una posterior depresión de colapso por detrás, una impresión pequeña y mal conservada del pie. La almohadilla de presión anterior puede explicarse como un empuje de ángulo bajo impulsado hacia adelante por el impacto del pie, mientras que la depresión posterior es un desplome descendente cuando se retira el pie. Esta interpretación es apoyada por las observaciones realizadas, en un afloramiento en el "Monumento Nacional de Dinosaurios", de una pista de Brasilichnium que atraviesa la pendiente con características similares, y también por el estudio de pistas realizadas por otros vertebrados en arenas eólicas. La gran preponderancia de las pistas ascendentes respecto a las descendentes en arenas eólicas sigue siendo una circunstancia que carece de explicación. El carácter distintivo de la morfología de la pista descendente y el hecho de que estén bien conservadas en el material estudiado, sin embargo, sugieren que la respuesta no es la de un sesgo debido a coincidencia o preservación.

Palabras clave: Brasilichnium; Nugget Sandstone; Triásico-Jurásico; eólico; icnología; huellas de locomoción

Traducción: Enrique Peñalver (Sociedad Española de Paleontología)

Résumé en Français

Morphologie et déformation sédimentaire de pistes de Brasilichnium sur des pentes descendantes de la face d’avalanche d’une dune dans le « Nugget Sandstone » du nord-est de l’Utah

Les ichnofossiles Brasilichnium sont des pistes potentiellement laissées par des synapsides, et ils sont représentés par de nombreuses pistes dans de nombreuses localités des dépôts éoliens d’un erg qui couvrait la plus grande partie de l’ouest des États-Unis du Trias récent au Jurassique ancien. Ces pistes sont trouvées principalement sur les faces d’avalanche des dunes. La très grande majorité des pistes ont été laissées par des animaux qui se déplaçaient en remontant la pente. Une dalle du « Nugget Sandstone » exposée au Département de Géologie et de Géophysique de l’Université d’Utah préserve au moins 17 pistes de Brasilichnium, 13 remontant la pente et quatre la descendant. Les pistes descendantes présentent une morphologie distincte comprenant une petite empreinte du pied mal préservée avec à l’avant, une plateforme de pression antérieure surélevée, et à l’arrière, une dépression postérieure d’effondrement. La plateforme de pression antérieure peut s’expliquer par une poussée vers l’avant avec un angle faible provoquée par l’impact du pied, alors que la dépression postérieure est un effondrement vers le bas de la pente créé au moment où le pied s’est retiré. Cette interprétation est soutenue par l’observation d’un affleurement du « Dinosaur National Monument » qui montre des caractéristiques similaires sur une piste de Brasilichnium qui est perpendiculaire à la pente, et par des études de pistes produites par d’autres vertébrés dans des sables éoliens. La forte prépondérance des pistes ascendantes sur les pistes descendantes dans les grès éoliens reste une question ouverte. La particularité de la morphologie des pistes descendantes et le fait qu’elles soient bien préservées dans le matériel étudié suggèrent cependant que la réponse est plus qu’une coïncidence ou qu’un biais de préservation.

Mots-clés : Brasilichnium ; Nugget Sandstone ; Trias-Jurassique ; éolien ; ichnologie ; pistes

Translator: Antoine Souron

Deutsche Zusammenfassung

Morphologie und Sedimentdeformation der hangabwärts führenden Spuren von Brasilichnium auf einem Dünengleithang im Nugget Sandstein von Nordost Utah

Die mutmaßliche Synapisden-Fährte, Brasilichnium, ist von vielen Fährten aus zahlreichen Fundstellen während der äolischen Ablagerungen des spättriassischen bis frühjurassischen Ergs bekannt, das einen Großteil der westlichen USA bedeckte. Die Fährten kommen hauptsächlich auf den Forset Beds der Dünen vor. Die überwiegende Mehrheit der Spuren stammt von hangaufwärts gehenden Tieren. Eine Platte Nugget Sandstein, die im Department of Geology and Geophysics der Universität von Utah ausgestellt ist, enthält mindestens 17 Brasilichnium Abdrücke, von denen 13 hangaufwärts und vier hangabwärts gehen. Die abwärts führenden Spuren haben eine markante Morphologie, mit einem erhöhten anterioren Druckpolster davor, und einer posterioren Kollaps-Depression danach, ein schmaler, schlecht erhaltener Fußabdruck. Das anteriore Druckpolster kann als flachwinkliger Schub, nach vorne getrieben durch das Aufsetzen des Fußes, erklärt werden, wohingegen die posteriore Depression Hangsackungen sind, die durch das Anheben des Fußes entstanden. Diese Interpretation wird unterstützt durch eine Beobachtung in einem Steinbruch im Dinosaur National Monument wo ähnliche Merkmale bei einer Brasilichnium-Fährte, die den Hang quert, zu sehen sind und durch Untersuchungen von Spuren anderer Wirbeltiere in äolischen Sanden. Das enorme Übergewicht von hangaufwärts zu hangabwärts führenden Fährten bleibt eine ungeklärte Frage. Die Einzigartigkeit der hangabwärts führenden Spurenmorphologie und die Tatsache, dass sie im untersuchten Material gut erhalten sind, legt jedoch nahe, dass die Antwort über Zufall oder Erhaltungs-Bias hinausgeht.

Schlüsselwörter: Brasilichnium; Nugget Sandstein; triassisch-jurassisch; äolisch; Ichnologie; Fährten

Translator: Eva Gebauer

Arabic

Translator: Ashraf M.T. Elewa

 

 

FIGURE 1. UUIC 1873, slab of Nugget Sandstone bearing multiple trackways. 1, the slab as it is exhibited in the halls of the Frederick Albert Sutton Building of the University of Utah in Salt Lake City. 2, image of the slab rotated 150o clockwise to show the trackways in their original orientation. Scale bars equal 10 cm.

figure1

FIGURE 2. High resolution image of UUIC 1873. Scale bar equals 10 cm.

figure2

FIGURE 3. Map of tracks and trackways on UUIC 1873. Scale bar equals 10 cm.

figure3

FIGURE 4. Distant and near views of locality DNM 490. 1, site as approached from the southeast. Arrow points to track bearing surface. 2, the track surface of 490 is the large bedding plane surface on the erosional remnant of Nugget Sandstone.

figure4

FIGURE 5. Composite photograph of the track-bearing surface of DNM 490. The crests of wind ripples are evident, especially near the center of the image, and provide a good indication of the primary dip direction of the slope of the dune. Scale bars equal 10 cm. Center scale bar distorted by photo stitching process.

figure5

FIGURE 6. Map of 490, based on composite photo in Figure 5. Scale bar equals 10 cm in 5 cm increments.

figure6

FIGURE 7. Composite image of the long transverse trackway at DNM 490 presented in 3 parts and the entire trackway. In each case, the trackway is oriented so that left (west) end is to top of page and upslope is to right of page. 1, left (west) end of the trackway. 2, section of the trackway adjacent to and to the right (east) of the section in Figure 7.1. 3, section of the trackway adjacent to and to the right (east) of the section in Figure 7.2. 4, the entire composite image of the trackway. Scale bars equal 10 cm.

figure7

FIGURE 8. Transverse trackway from locality 490, in sections. These are enlargements of sections of the composite photo in Figure 7. Sections do not overlap, but do abut one another. 1, left side of trackway (cf. with Figure 6) is the top section. 2, the next section in the trackway, adjacent to and to the right of Figure 8.1. 3, the next section in the trackway, adjacent to and to the right of Figure 8.2. Scale bars equal 5 cm.

figure8

FIGURE 9. Transverse trackway from locality 490, in sections. These are enlargements of sections of the composite photo in Figure 7. Sections do not overlap, but do abut one another. 1, the next section in the trackway, adjacent to and to the right of Figure 8.3. 2, the next section in the trackway, adjacent to and to the right of Figure 9.1. 3, the next section in the trackway, adjacent to and to the right of Figure 9.2. Scale bars equal 5 cm.

figure9

FIGURE 10. Transverse trackway from locality 490, in sections. These are enlargements of sections of the composite photo in Figure 7. Sections do not overlap, but do abut one another. 1, the next section in the trackway, adjacent to, and to the right of Figure 9.3. 2, the next section in the trackway, adjacent to and to the right of Figure 10.1. Scale bar equals 5 cm.

figure10

FIGURE 11. Detail of a track from the downslope trackway along the right side of UUIC 1873. The location of this figure is indicated by the small rectangle in Figure 3. Note the anterior platform, the posterior, slumped depression, and the small space for the foot filled with lighter colored sand.

figure11

FIGURE 12. Segment of the trackway along the right side of UUIC 1873 that shows closely spaced tracks. The location of this figure is indicated by the large rectangle in Figure 3. Note also the impressions of digits on two of the tracks and the smaller anterior platforms of the manus tracks immediately anterior to the pes tracks.

figure12

FIGURE 13. Reproduction of part of the photograph from Buss (1921). The original figure has been cropped to show only the Brasilichnium trackway and rotated 180o to show the tracks in what we interpret to be their original vertical orientation. No scale in photo, but based on Buss’ description, the block is slightly less than 1 m.

figure13 

 

APPENDIX 1.

The full sized Gigapan image of UUIC 1873 (656 megapixels), presented at reduced size as Figure 1 and Figure 2, can be viewed at the Gigapan website (http://www.gigapan.com/gigapans/174739) where the viewer can zoom to the maximum resolution at any part of the entire slab. The viewer should be aware that when viewed at this site the slab is oriented as displayed at the University of Utah.

APPENDIX 2.

The full size image reduced to create Figure 2. UUIC 1873 (824.8MB) rotated from as exhibited at the University of Utah to the orientation at the time of track formation. This file, append2.tif, can be downloaded from a folder in Box that can be reached with the link (https://unomaha.box.com/s/yqehi4vgasbsdvgpaqo72ef3m8s20ay7).

APPENDIX 3.

The full sized Gigapan image of the track bearing surface at locality 490 (284 megapixels) can be viewed at the Gigapan website (http://www.gigapan.com/gigapans/174736) where the viewer can zoom to the maximum resolution at any part of the entire slab. Discoloration of transverse trackway is artifact of molding the trackway.

APPENDIX 4.

The full size image reduced to create Figure 5 viewable with the map in Figure 6 as an overlay. Composite photograph of the track-bearing surface of DNM 490 and outlines of tracks on an overlying layer (123.7MB). This file, append4.ai, can be downloaded from a folder in Box that can be reached with the link (https://unomaha.box.com/s/yqehi4vgasbsdvgpaqo72ef3m8s20ay7).

APPENDIX 5.

The full size image reduced to create Figure 7. Composite image of the long transverse trackway at DNM 490 (89.4 MB). This file, append5.tif, can be downloaded from a folder in Box that can be reached with the link (https://unomaha.box.com/s/yqehi4vgasbsdvgpaqo72ef3m8s20ay7).

 

 

 

author1George F. Engelmann. Department of Geography and Geology, University of Nebraska at Omaha, 60th & Dodge Street, Omaha, NE 68182, USA. This email address is being protected from spambots. You need JavaScript enabled to view it.

George Engelmann is a professor of Geology at the University of Nebraska at Omaha where he has been teaching and doing research in Vertebrate Paleontology since 1980. Research interests include the phylogeny of vertebrates, especially early mammals, paleoecology of vertebrate faunas and the taphonomy of vertebrate fossil occurrences. Since 1984, his research has focused on Mesozoic vertebrates and the paleontology and paleoecology of formations exposed around Dinosaur National Monument, most recently the Nugget Sandstone.

divider

author2Daniel J. Chure. Dinosaur National Monument, Box 128, Jensen, UT 84035, USA. This email address is being protected from spambots. You need JavaScript enabled to view it.

Dan Chure has been a dinosaur fanatic since seeing his first dinosaur movie at the age of three and the dinosaur halls at the American Museum of Natural History when he was five. Serving as the paleontologist at Dinosaur National Monument since 1979 his research interests include the phylogeny and paleoecology of Mesozoic tetrapods and terrestrial ecosystems, preservation and conservation of in-situ fossil exhibits, and fossil resource protection. Current research efforts are focusing on the paleontology and paleoenvironments of the Nugget Sandstone and the Digital Quarry Project, an interactive website covering the immense history, excavations, and research at the great Carnegie Quarry at Dinosaur.