Dinotracks.Indb 358 1/22/16 11:23 AM Dinosaur Tracks in Eolian Strata: New Insights Into Track Formation, Walking Kinetics and Trackmaker Behavior 18
Total Page：16
File Type：pdf, Size：1020Kb
358 DinoTracks.indb 358 1/22/16 11:23 AM Dinosaur 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 dinosaurs 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 Colorado, form in semiarid areas where deflation Tracks in Deposits of Lower combines with local wind corridors to permit accumulations Jurassic Dunes, Navajo Sandstone, that cover otherwise vegetated areas. Because animals are Utah, United States totally dependent on plants as an energy source, it might seem that dunes would be a poor place to look for animal The Early Jurassic Navajo Sandstone is a thick, widespread tracks. A short walk in a modern dune field when the sun is sedimentary layer on the Colorado Plateau 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) Otozoum 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. Cretaceous 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.