A new plastomenid trionychid turtle, Plastomenus joycei , sp. nov., from the earliest Paleocene (Danian) Denver Formation of south-central Colorado, U.S.A.

ABSTRACT

North American soft-shelled turtles, including trionychines and plastomenids, are incredibly abundant in latest Cretaceous through earliest Paleocene sediments. Here we describe a new species of plastomenid turtle, Plastomenus joycei, based on a nearly complete early Danian skeleton from the Denver Formation in the Denver Basin, Colorado. Plastomenus joycei is differentiated from all other plastomenid turtles based on the presence of large eighth costals that are much longer than wide, sinusoidal raised ridges on the carapace, flat posterior edge of the carapace, spike-like epiplastra that lack callosities, a broad entoplastron that lacks a callosity, hyoplastra with two lateral processes, presence of metaplastically ossified hyoplastral shoulders (i.e., anteriorly protruding lappets), and a broad midline contact between the hypo- and xiphiplastra. Plastomenus joycei broadly resembles the Cretaceous Gilmoremys spp. and the Eocene Plastomenus thomasii and exhibits intermediate morphology between the two, most notably in the degree of ossification of the plastron. The increase in plastral ossification, as well as a decrease in overall size and an increase in the doming of the shell, co-occurs with the Cretaceous–Paleogene mass extinction and these morphological changes may be in response to an increase in mammalian predators during the early Paleogene. Plastomenus joycei is most commonly found in riverine sandstone sediments and is hypothesized to be a riverine turtle.

http://zoobank.org/urn:lsid:zoobank.org:pub:F4FE56A9-3E63-4BF0-A5CB-0CF0F9B6B045

ACKNOWLEDGMENTS

We thank Norwood Properties, City of Colorado Springs, Waste Management, Aztec Family Raceway, J. Hawkins, J. Hilaire, J. Carner, W. Pendleton, the Bishop Family, H. Kunstle for land access; the State of Colorado, Office of the State Archaeologist, for issuing collection permits; S. Bastien, S. Begin, J. P. Cavigelli, J. Englehorn, and N. Toth for preparation of specimens; R. Hess, S. Milito, Y. Rollot, P. Sullivan, L. Taylor, and K. Weissenburger for field assistance; S. Luallin, R. Wicker, and the USGS National Unmanned Aircraft Systems team for photography; A. Atuchin for artwork used in Figure 1; K. MacKenzie and N. Neu-Yagle for collections assistance; R. Lavie for assistance with GIS data; B. Snellgrove for logistics; and B. and W. Stevenson for lodging during fieldwork. I. G. Danilov, A. D. Gentry, W. G. Joyce, and N. S. Vitek made useful suggestions that helped improve the manuscript. Funding was provided by The Lisa Levin Appel Family Foundation, M. Cleworth, Lyda Hill Philanthropies, David B. Jones Foundation, M. L. and S. R. Kneller, T. and K. Ryan, and J. R. Tucker as part of the Denver Museum of Nature & Science No Walls Community Initiative.