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| Plastomenus joycei Lyson, Petermann & Miller, 2021 |
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.
SYSTEMATIC PALEONTOLOGY
TESTUDINES Batsch, 1788
(sensu Joyce, Parham, Anquetin, Claude, Danilov, Iverson, Kear, Lyson, Rabi, and Sterli, 2020)
PAN-TRIONYCHIDAE Joyce, Anquetin, Cadena, Claude, Danilov, Evers, Ferrera, Gentry, Georgalis, Lyson, Perez-Garcia, Rabi, Sterli, Vitek, and Parham, 2021
PLASTOMENIDAE Hay, 1908
(sensu Joyce, Anquetin, Cadena, Claude, Danilov, Evers, Ferrera, Gentry, Georgalis, Lyson, Perez-Garcia, Rabi, Sterli, Vitek, and Parham, 2021)
PLASTOMENUS Cope, 1873
Type Species—Trionyx thomasii Cope, 1872.
PLASTOMENUS THOMASII (Cope, 1873)
PLASTOMENUS VEGETUS, nov. comb. (Gilmore, 1919)
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| Plastomenus joycei, sp. nov., DMNH EPV.95454, holotype, ventral view. |
PLASTOMENUS JOYCEI, sp. nov.
Etymology—The eponym ‘joycei’ is named in honor of W. G. Joyce, prominent turtle paleontologist and friend and colleague to T. R. Lyson.
Tyler R. Lyson, Holger Petermann and Ian M. Miller. 2021. A New Plastomenid Trionychid Turtle, Plastomenus joycei, sp. nov., from the earliest Paleocene (Danian) Denver Formation of south-central Colorado, U.S.A. Journal of Vertebrate Paleontology. e1913600. DOI: 10.1080/02724634.2021.1913600
