Hitchcock’s Leptodactyli, Penetrative Tracks, and Dinosaur Footprint Diversity

ABSTRACT

Starting with his first report on fossil footprints from the Connecticut Valley over 180 years ago, Edward Hitchcock described what he interpreted as a burgeoning ancient fauna founded on ever-increasing nominal track diversity. For three decades, Hitchcock made countless contributions to ichnology, but his inference of thin-toed animals (Leptodactyli) from thin-toed tracks is flawed by modern criteria. Leptodactylous tracks are now recognized as variants made by thick-toed feet penetrating into soft, collapsing substrates. Herein, we take a closer look at the creation of such penetrative tracks using computer simulations of particle flow. Classic specimens are used to demonstrate how different modes of surface presentation make penetrative tracks challenging to recognize and interpret. Evaluation of 266 specimens from 43 leptodactylous ichnotaxa reveals that ∼90% are penetrative. We propose that a reliance on a single formation mechanism confounded Hitchcock’s ability to reliably recognize different trackmakers. This is not an old problem applicable only to fossils collected long ago; domination of a transmission-based model continues to bias the field today. Most texts and many publications either omit collapsed penetrative tracks or fail to recognize them as a significant source of variation. Without proper regard for subsurface toe movement and sediment flow, inferences of foot shape from track shape can, as for Hitchcock, be led far astray. The misidentification and misunderstanding of penetrative tracks impact our conception of the diversity of life in the Early Jurassic, as well as in other ichnofaunas worldwide.

ACKNOWLEDGMENTS

We thank M. Turner, P. Olsen, and P. Getty for fruitful discussions about penetrative tracks as well as P. Olsen, J. Scott, and the JVP Editors for their many helpful comments on previous versions of this work. K. Wellspring, H. Singleton, T. Harms, A. Martini, and D. Jones at the Beneski Museum of Natural History and D. Brinkman and J. Gauthier at the Yale Peabody Museum of Natural History kindly provided access to specimens. Research was supported by U.S. National Science Foundation grants EAR-1452119 (S.M.G. and P.L.F.) and IOS-0925077 (S.M.G.), a Brown University Salomon Faculty Research Award (S.M.G), and a Marie Curie International Outgoing Fellowship (P.L.F). Simulations used the ARCHER UK National Supercomputing Service (www.archer.ac.uk) via an ARCHER Leadership allocation (P.L.F). Supplemental data including photogrammetric datasets and simulation input data are available from figshare: https://doi.org/10.6084/m9.figshare.12377816