Integration of tetrapod ichnofaunas and coastal dynamics for paleocommunity reconstruction in the Curtis and Summerville formations (Jurassic), eastern Utah

ABSTRACT

The transition between the aeolian Moab Member of the Curtis Formation and the marginal marine upper tongue of the Summerville Formation, in the area of Arches National Park, Utah, facilitated the registration of the distinctive theropod-dominated Moab Megatracksite complex, which is something of an ichnological cause célèbre. The complex reveals three interpenetrating ichnofaunas each with distinctive ichnofacies characteristics. The present study focuses on the small theropod-dominated assemblages from the coastal dune facies and applies Walther’s Law of Facies to the sequence relationship between onshore ichnoassemblages, the late pulse of transgression by the Summerville sea, and the development of the shallow marine Pteraichnus ichnocoenosis. Together, these spatio-temporal facies relationship between onshore terrestrial, shoreline and shallow marine facies and their contained ichnofaunas shed useful light on the Late Jurassic tetrapod ecology of the region, in an area entirely lacking tetrapod body fossils. The small theropod dune ichnofauna has obvious precursors in other Jurassic dune facies. The shoreline and shallow marine ichnocoenosis also fit certain ichnofacies models, despite the lack of comparable ichnofaunas in the area. Abundant trackway size frequency data yield body mass estimates which allow testing of recently proposed palaeobiological theories on theropod ontogeny and community structure based on the skeletal record.

KEYWORDS:

• ichnofacies
• Moab Member
• eolianites
• theropod ontogeny

Acknowledgments

We particularly thank the late Fran and Terby Barnes for taking us to the sites he discovered and for sharing locality information, also shared with the Bureau of Land Management (Moab Office). Note however, that although Barnes (2003, p. 74, 78, 111 and 120) illustrated small tracks from what he called the Moab - Entrada deposits, he did not give their locations, so the full list of Barnes’ discoveries remains uncertain. Likewise, the late George Odier privately published illustrations of tracksites (Odier 2003, figs 17-19), in what he referred to (p. 36) as the Moab-Entrada Member, Laura Mitchell (formerly at the University of Colorado) and Gerard Gierlinski (Polish Geological Survey) helped with our field studies in the mid-2000s. The University of Colorado Dinosaur Trackers Research Group conducted a formal survey authorized and jointly funded by the BLM and executed under permit number UT-S-04-005. Work undertaken since 2007 has been conducted under UT-S09-006. Finds in recent years are attributed to Gerard Gierlinski and Martin Lockley. Nora Noffke, Old Dominium University, Virginia, has recently worked with the author to provide valuable insights into the factors that have influenced track preservation in the Moab Megatracksite area. Alena Iskanderova is gratefully acknowledged for her help with obtaining the photogrammetric image of the small, low relief track illustrated in Figure 4 B₂.

Disclosure statement

No potential conflict of interest was reported by the author(s).