358

DinoTracks.indb 358 1/22/16 11:23 AM Tracks in Eolian Strata: New Insights into Track Formation, Walking Kinetics and Trackmaker Behavior 18

David B. Loope and Jesper Milàn

Dinosaur tracks are abundant in wind-blown hooves of the bison deformed soft, laminated sediment – the Mesozoic deposits, but the nature of loose eolian sand perfect medium to preserve recognizable tracks. The next makes it difficult to determine how they are preserved. This windstorm buried the tracks. Today, the thick cover of grasses also raises the questions: Why would be walking protects the land surface so well that there are no soft, lami- around in dune fields in the first place? And, if they did go nated sediments for cattle to step on. And, if any tracks were, there, why would their tracks not be erased by the next wind somehow, to get formed, no moving sediment would be avail- storm? able to bury them. Mesozoic eolian sediments around the world, which have been the focus of a number of case studies Introduction in recent years, preserve the tracks of dinosaurs that walked on actively migrating sand dunes. This chapter summarizes Most dunes today form only in deserts and along shore- the known occurrences of dinosaur tracks in Mesozoic eo- lines – the only sandy land surfaces that are nearly devoid of lian strata and discusses their unique modes of preservation plants. Normally plants slow the wind at the ground surface and the anatomical and behavioral information about the enough that sand will not move even when the plant cover trackmakers that can be deduced from them. is sparse. However, some dunes, such as the Great Sand Dunes of , form in semiarid areas where deflation Tracks in Deposits of Lower combines with local wind corridors to permit accumulations Dunes, , that cover otherwise vegetated areas. Because are , United States totally dependent on plants as an energy source, it might seem that dunes would be a poor place to look for The Navajo Sandstone is a thick, widespread tracks. A short walk in a modern dune field when the sun is sedimentary layer on the of Utah and Ari- low and shadows are long will demonstrate that this is not zona. The sandstone and its correlative strata, the Nugget the case. The scarcity of life in dune fields is actually a boon Sandstone, have preserved more than 60 sites with dinosaur for generating distinct, recognizable tracks. In the Nebraska tracks and trackways (e.g., Lockley, Hunt, and Meyer, 1994; Sand Hills (a giant dune field in central North America), the Rainforth and Lockley, 1996a, 1996b; Milàn, Loope, and now stabilized, grass-covered dunes are at present traversed Bromley, 2008; Lockley, 2011a, 2011b; Lockley et al., 2011), and by huge numbers of cattle, but none of their tracks will get a sparse but diverse vertebrate fauna comprising tritylodonts, preserved. However, thin, 800-year-old cross-beds inside the crocodylomorphs, and dinosaurs (Irmis, 2005). dunes were deposited while the region was a howling desert The Navajo Sandstone was deposited by large sand dunes and contain large numbers of distinct bison tracks and track- that migrated southward along the subsiding, western coast ways (Loope, 1986). The scarcity of trackmakers prevented of Pangaea. The sloping layers (cross-beds) deposited by the bioturbation (complete mixing of the sediment), thereby migrating dunes contain thousands of tracks of small the- allowing full, three-dimensional preservation of the tracks ropod dinosaurs. Many of these tracks are preserved in dry and the trackways that did get made by the relatively small avalanches (grain flows) that were deposited at the angle of number of animals inhabiting or traversing the dunes. The repose of dry sand (about 32°). In a few places, it is possible to

18.1. Dinosaur tracks in the Navajo Sandstone penetrated several layers of sand. Strata slope adopting a sideways walking gait for the first at Coyote Buttes, Utah. (A) Tracks of small downward away from viewer. (C) Trackway of part of the trackway. The later (upper) part theropod dinosaurs on the upper surface of an a crouching theropod on a dune slope, with shows that the animal then started moving eolian grain flow (layer deposited by avalanching interpretative drawing inset (from Milàn, Loope, directly up the slope. The solid arrow shows the dry sand on the steep, downwind slope of a and Bromley, 2008). (D) trackway direction of progression and the dashed arrow sand dune). (B) Close-up of dinosaur tracks in on a firm, wind-rippled interdune surface. indicates the orientation of the animal’s body. cross-section. Notice how the sharp digits have (E) Trackway of sauropodomorph dinosaur

359

DinoTracks.indb 359 1/22/16 11:23 AM 18.2. Stratigraphic column showing the distribution of tracks and burrows in the Lower Jurassic Navajo Sandstone. Tracks are restricted to one interval, but the burrows indicate there were three time periods when sufficient moisture was present in the dune field to support abundant life.

see many three-toed tracks on the upper surface of a single sandstone layer (Fig. 18.1A). Many more tracks, however, can be seen only in vertical cross-section (Loope and Rowe, 2003; Loope, 2006) (Fig. 18.1B). When viewed in cross-section, the preserved track has a U or W shape, with the top-most portion cut off by erosion. Apparently, when these animals stepped on the steep dune slopes, they created small, thin avalanches of dry sand, initiating from above the track. As an animal traversed the dune slope, each step was onto the sliding sand that it triggered by its previous step. The end re- sult is that different tracks within the same animal’s trackway are sometimes preserved in different layers of sand, giving the false impression that the tracks are emplaced at differ- ent times and not by the same animal. As each avalanche buried a track, it eroded down into the tracked surface. The tracks of very small animals did not penetrate deeply into the dune slope, so many probably were completely eroded. The theropods, although small by dinosaur standards, were sufficiently large that they deformed the layered sand deeply enough (about 10 cm) so that most of the track (but not all) escaped erosion. Theropod tracks are not the only signs of life in the Juras- sic dune deposits. There are also abundant, small burrows, and surface trails made by insects or other invertebrates (Fig. 18.2). Theropods likely fed on the burrowers, but it is a mys- tery what the burrowers ate. There are no traces of rooted plants in the vicinity of the tracks, and very few in the whole formation. The three intervals containing abundant traces of animal life record relatively wet climatic conditions in the dune field (Fig. 18.2), but the dunes (apparently never stabilized) continued to migrate southward during both wet and dry intervals. In a few other places, the Navajo Sandstone contains thin, isolated limestones that are completely surrounded by sand- stone. These were deposited in lakes that formed between the dunes during the wet climatic intervals that lasted thou- sands of years. Petrified wood and stromatolites are recorded and sometimes abundant at some of these sites, and dinosaur tracks are also found around these ancient oases (Eisenberg, 2003; Parrish and Falcon-Lang, 2007). Among the abundant tracks and trackways in the Navajo Sandstone are rare examples of trackways that have preserved evidence of individual behavior of the trackmakers. At the Coyotes Buttes locality, one trackway has preserved tracks of a small theropod, walking directly up a sloping dune front; crouching down; making full impressions of the metatarsi, the belly, and both hands; and then continuing straight up the

360 David B. Loope and Jesper Milàn

DinoTracks.indb 360 1/22/16 11:23 AM 18.3. Tracks in the Entrada Sandstone at Twentymile Wash, Utah. (A) Trackway of large theropod preserved on a laminated inter- dune surface. (B) Close-up of single track with an extensive zone of disturbed sediment around it. The estimated extent of the orig- inal footprint is indicated by broken line. (C) Track where the dynamic contact between the trackmaker’s foot and the substrate has caused an extensive set of faulting and rotated discs. (D) Interpretation of C (from Gravesen, Milàn, and Loope, 2007).

New Insights into Track Formation 361

DinoTracks.indb 361 1/22/16 11:23 AM 18.4. dune deposits from southern Mongolia. (A) Cross-bedded sandstone on the left contains well- preserved tracks; deposits on the right are bioturbated and have abundant dinosaur bones (man sits at bone site). (B) Cross-section of a typical dinosaur track from one of the cross-bedded parts of the formation.

dune front (Milàn, Loope, and Bromley, 2008) (Fig. 18.1C). horizontal trackways (Milàn and Loope, 2007) (Fig. 18.3A). One sauropodomorph, trackway, Otozoum, shows normal The animals walked over a flat desert surface that was man- bipedal progression (Fig. 18.1D), whereas another sauropodo- tled by small dunes. Salts lightly cemented the sandy surface, morph trackway, Navahopus, shows a sauropodomorph in so the weight of the large theropods not only depressed the quadrupedal stance walking up the sloping dune front. The material directly under their feet (the “true track”), but it also first part of the trackway shows the animal walking at an an- disturbed a large area around each track (Foster, Hamblin, gle upward, all the time keeping the axis of the body directed and Lockley, 2000; Breithaupt, Matthews, and Noble, 2004; upward, before changing its mode of progression to directly Milàn and Loope, 2007) (Fig. 18.3B). As animals walk, they up the slope (Fig. 18.1E). Trackways showing a similar mode first compress the sediment under their feet, then push it of progression are known from Pleistocene coastal eolianites backward, and, as they remove the foot, stretch the sediment. from the Mediterranean island Mallorca, where Pleistocene The damp, thinly laminated sands of the Entrada Sandstone goats adopted a similar sideways gait when they progressed at the Twentymile Wash locality in Utah has captured this up the steep dune faces (Fornos et al., 2002), and reptile track- interaction between the trackmaker and the substrate. ways in the eolian Coconino Sandstone of Grand Close inspection reveals that the disturbed area around Canyon also show a similar sideways mode of progression each track contains several types of small faults that make (Brand and Tang, 1991; Loope, 1992). it possible to interpret the walking dynamics of the animal (Gravesen, Milàn, and Loope, 2007; Milàn, Gravesen, and Dinosaur Tracks in Interdune Loope, 2014). The sediment layers under the dinosaurs’ feet Deposits, Entrada were pushed down and then radially outward, moving as Sandstone, Utah, United States sheet-like slabs, not as loose sand grains. During the kickoff phase of the dinosaur’s step, a plate of sand is first rotated Although small theropod tracks are found in cross-bedded below the foot, before being pushed backward, creating a dune facies within the Entrada Sandstone (Lockley, Mitch- set of parallel faults in the sediment (Fig. 18.3C). The well- ell, and Odier, 2007), tracks are also found preserved in flat- preserved evidence of “dinosaur-induced tectonics” has been bedded eolian sandstones. In many dune fields, the areas successfully described using terminology from structural between migrating dunes are covered with wind-ripples. geology, which allowed precise reconstruction of the dino- When wind-ripples migrate, they commonly climb over one saurs’ walking dynamics (Gravesen, Milàn, and Loope, 2007) another, producing firm, flat deposits of thin-bedded sand. (Fig. 18.3D). Animals do not sink deeply into these deposits, so tracks are not deep (Fig. 18.1D). In some interdune areas, the water Scattered Tracks in Cretaceous table is near the surface, and the sand can get cemented Dunes, Southern Mongolia by salts when moisture evaporates. In the wind-deposited, Middle Jurassic Entrada Sandstone of south-central Utah, In the dune deposits of the Cretaceous Djadochta For- hundreds of large dinosaur tracks are arranged in long, mation of the Gobi Desert, scattered tracks are visible in

362 David B. Loope and Jesper Milàn

DinoTracks.indb 362 1/22/16 11:23 AM cross-sections of sloping dune deposits (Loope et al., 1998) synapsid trackways Laoporus, the lizard-like ichnogenus Dol- (Fig. 18.4B). Ukhaa Tolgod, a site recently discovered by pa- ichopodus together with abundant invertebrate traces includ- leontologists from the Mongolian Academy of Sciences and ing Octopodichnus and Paleohelcura (Lockley, Hunt, and the American Museum of Natural History, is famous for its Meyer, 1994). As Laoporus is a junior synonym of Chelichnus, dinosaur bones. The bones, however, are never found in the the Laoporus ichnofacies is now referred to as the Chelichnus cross-beds with the tracks. The bones are in crudely bedded ichnofacies (Hunt and Lucas, 2007; Lockley, 2007). or unbedded sandstone in between the cross-bedded deposits The Mesozoic Brasilichnium ichnofacies are confined to (Fig. 18.4A). The unbedded sediment was bioturbated by erg settings and found in the Lower Jurassic Navajo and Nug- abundant burrowing insects (?) and plant roots that are now get sandstones of the Colorado Plateau region, the equivalent replaced by calcite (rhizoliths). of California, and the Botucatu Forma- The distribution of the and the tracks suggests that tion of South America. The trackmaker for Brasilichnium is the animals whose bones were fossilized lived in a dune field considered a synapsid with mammal-like reptilian affinities that was stabilized by plants and had abundant life (somewhat (Lockley, 2011a). Apart from abundant Brasilichnium tracks, similar to the modern Nebraska Sand Hills, a grass-stabilized also the invertebrate tracks Octopodichnus and Paleohelcura dune field on the North American Great Plains). When the are abundant, as well as theropod, prosauropod, and small Cretaceous dunes were active (desert conditions) only a few mammal tracks (e.g., Lockley, Hunt, and Meyer, 1994; Rain- animals walked across the actively migrating dunes, so only forth and Lockley, 1996a, 1996b; Milàn, Loope, and Bromley, a few tracks and no bones are preserved during those time 2008; Lockley, 2011a, 2011b; Lockley et al., 2011). Because of intervals. the morphological similarities between Jurassic and Late Pa- leozoic Octopodichnus, Paleohelcura and mammaloid tracks Eolian Ichnofacies (Chelichnus in the Permian and Brasilichnium in the Juras- sic) the dune ichnofacies from eolian deposits spanning this Based on characteristic assemblages of vertebrate tracks oc- long time interval are almost indistinguishable. curring together in eolian deposits, multiple eolian vertebrate ichnofacies have been erected (Lockley, Hunt, and Meyer, Conclusions 1994). The term “ichnofacies” was introduced by Seilacher (1964, 1967) to cover recurring associations of trace fossils Recent research into tracks registered and preserved in eo- related to sedimentary facies and depositional environments. lian strata and the usage of the concept of eolian vertebrate The term has later been used in many different scales from ichnofacies has provided important information about ani- global associations to individual rock units (Bromley, 1996). mal behavior and ancient environments. Because skeletal Dealing with vertebrate trace fossils, Lockley, Hunt, and remains are uncommon in eolian strata, tracks provide ad- Meyer (1994:242) suggest defining vertebrate ichnofacies as ditional, and often the only, information about the fauna that “multiple ichnocoenosis that are similar in ichnotaxonomic inhabited the area. Tracks are easily recognized in vertical composition and show recurrent association in particular cross-sections as disturbances of the uniformly bedded eolian definite environments.” strata. Close examination of dinosaur tracks in finely lami- So far, two distinct eolian vertebrate ichnofacies have nated interdune strata, provides insight into the second for been recognized. The Paleozoic Laoporus ichnofacies is second interaction between an extinct animal and the sedi- a recurrent association of vertebrate tracks from the Perm- ment it walked on, allowing a very detailed reconstruction ian Coconino Sandstone of Arizona and contemporary Ly- of the walking kinetics of the trackmaker. ons Sandstone of Colorado. The ichnofacies comprise the

References

Brand, L. R., and T. Tang. 1991. vertebrate Bromley, R. G. 1996. Trace Fossils: Biology, Western Mediterranean). Palaeogeography footprints in the Coconino sandstone (Permian) Taphonomy and Applications. 2nd edition. Palaeoclimatology Palaeoecology 180(4): of northern Arizona: evidence for underwater Chapman and Hall, London, U.K., 384 pp. 277–313. origin. Geology 19: 1201–1204. Eisenberg, L., 2003. Giant stromatolites and a Foster, J. R., A. H. Hamblin, and M. G. Lockley. Breithaupt, B. H., N. A. Matthews, and T. A. supersurface in the Navajo sandstone, Capitol 2000. The oldest evidence of a sauropod Noble. 2004. An integrated approach to Reef National Park, Utah. Geology 31: 111–114. dinosaur in the western United States and other three-dimensional data collection at dinosaur Fornos, J. J., R. G. Bromley, L. B. Clemmensen, important vertebrate trackways from Grand tracksites in the Rocky Mountain west. Ichnos and A. Rodriguez-Perea. 2002. Tracks and Staircase-Escalante National Monument, Utah. 11: 11–26. trackways of Myotragus balearicus Bate Ichnos 7: 169–181. (Artiodactyla, Caprinae) in Pleistocene Gravesen, O., J. Milàn, D. B. Loope. 2007. aeolianites from Mallorca (Balearic Islands, Dinosaur tectonics: a structural analysis of

New Insights into Track Formation 363

DinoTracks.indb 363 1/22/16 11:23 AM theropod undertracks with a reconstruction of Jurassic eolianites of eastern Utah: paleoecolog- Milàn, J., and D. B. Loope. 2007 Preservation and theropod walking dynamics. Journal of Geology ical insights from dune facies in a transgressive erosion of theropod tracks in eolian deposits: 115: 375 –386. sequence. Ichnos 14: 132–143. examples from the Middle Jurassic Entrada Hunt, A. P., and S. G. Lucas. 2007 Tetrapod ichno- Lockley, M. G., A. R. Tedrow, K. C. Chamberlain, sandstone, Utah, USA. Journal of Geology 115: facies: a new paradigm. Ichnos 14: 59–68. N. J. Minter, and J.-D. Lim. 2011. Footprints 375–386. Irmis, R. B. 2005. A review of the vertebrate fauna and invertebrate traces from a new site in the Milàn, J., D. B. Loope, and R. G. Bromley. of the Lower Jurassic Navajo sandstone in (Lower Jurassic) of Idaho: 2008. Crouching theropod and Navahopus Arizona. Mesa Southwest Museum Bulletin 11: implications for life in the northwestern reaches sauropodomorph tracks from the Early Jurassic 55–71. of the great Navajo-Nugget erg system in the Navajo sandstone of USA. Acta Palaeontologia Lockley, M. G. 2007. A tale of two ichnologies: the western USA. New Mexico Museum of Natural Polonica 53: 197–205. different goals and missions of vertebrate and History and Science Bulletin 53: 344–356. Parrish, J. T., and H. J. Falcon-Lang. 2007. invertebrate ichnology and how they relate in Loope, D. B. 1986, Recognizing and utilizing Coniferous trees associated with interdune ichnofacies analysis. Ichnos 14: 39–57. vertebrate tracks in cross section: Cenozoic deposits in the Jurassic Navajo Sandstone Lockley, M. G. 2011a. The ichnotaxonomix status hoofprints from Nebraska. Palaios 1: 141–151. Formation, Utah, USA. Palaeontology 50: of Brasilichnium with special reference to Loope, D. B. 1992. Comment on ‘Fossil vertebrate 829–843. occurrences in the Navajo sandstone (Lower footprints in the Coconino sandstone (Permian) Rainforth, E. C., and M. G. Lockley. 1996a. Jurassic) in the western USA. New Mexico of Northern Arizona: evidence for underwater Tracks of diminutive dinosaurs and hopping Museum of Natural History and Science Bulletin origin.’ Geology 20: 667–668. mammals from the Jurassic of North and South 53: 306–315. Loope, D. B. 2006, Dry-season tracks in dinosaur- America; pp. 265–269 in M. Morales (ed.), The Lockley, M. G. 2011b. Theropod- and prosauropod triggered grainflows. Palaios 21: 132–142. Continental Jurassic. Bulletin 60. Museum of dominated ichnofaunas from the Navajo- Loope, D. B., and C. M. Rowe. 2003. Long-lived Northern Arizona, Flagstaff, Arizona. Nugget sandstone (Lower Jurassic) at Dinosaur pluvial episodes during deposition of the Navajo Rainforth, E. C., and M. G. Lockley. 1996b. National Monument: implications for prosau- Sandstone. Journal of Geology 111: 223–232. Tracking life in a Lower Jurassic desert: verte- ropod behavior and ecology. New Mexico Loope, D. B., L. Dingus, C. C. Swisher III, and brate tracks and other traces from the Navajo Museum of Natural History and Science Bulletin C. Minjin. 1998. Life and death in a Late sandstone; pp. 285–289 in M. Morales (ed.), 53: 316–320. Cretaceous dunefield, Nemegt Basin, Mongolia. The Continental Jurassic. Bulletin 60. Museum Lockley, M. G., A. P. Hunt, C. A. Meyer. 1994. Geology 26: 27–30. of Northern Arizona, Flagstaff, Arizona. Vertebrate tracks and the ichnofacies concept; Milàn, J., O. Gravesen, and D. B. Loope. 2014. Seilacher, A. 1964. Biogenic sedimentary struc- pp. 241–268 in S. K. Donovan (ed.), The Dinosaur tectonics: when biomechanics meet tures; pp. 296–316 in J. Imbrie and N. Newell Palaeobiology of Trace Fossils. Wiley, New York, structural geology. Journal of Vertebrate (eds.), Approaches to Palaeoecology. Wiley, New York. , Program and Abstracts New York, New York. Lockley, M. G., L. Mitchell, and G. Odier. 2007. 2014: 188. Seilacher, A. 1967. Bathymetry of trace fossils. Small theropod track assemblages from Middle Marine Geology 5: 413–428.

364 David B. Loope and Jesper Milàn

DinoTracks.indb 364 1/22/16 11:23 AM DinoTracks.indb 365 1/22/16 11:23 AM