The shell morphology of the latest Cretaceous (Maastrichtian) trionychid turtleHelopanoplia distincta

Abstract

BackgroundHelopanoplia distincta is an extinct soft-shelled turtle (Pan-Trionychidae) for which the type specimen is a fragmentary costal and the inguinal notch portion of the left hypoplastron from the Late Cretaceous (Maastrichtian) Lance Formation of Wyoming, USA that bear a distinct surface sculpture pattern consisting of raised tubercles. Over the course of the past few decades, a number of additional, fragmentary specimens from the Late Cretaceous (Maastrichtian) Hell Creek Formation of Montana and North Dakota have been referred to this taxon based on the presence of these tubercles, but a more complete understanding of the anatomy and phylogenetic relationships of this distinctive soft-shelled turtle is still outstanding.MethodsWe here figure and describe shell remains of eight fossils referable to Helopanoplia distincta from the Hell Creek Formation of Montana and North Dakota that, in combination, document nearly all aspects of the shell morphology of this taxon. We furthermore explore the relationships of this fossil turtle by inserting it into a modified phylogenetic analysis of pan-trionychid relationships.ResultsThe new fossil material thoroughly supports the validity of Helopanoplia distincta. In addition to its unique surface sculpture pattern, this turtle can be diagnosed relative to all other named pan-trionychids by the presence of a distinct corner along the margin of costals II, the complete covering of costal ribs I–VI by metaplastic bone, midline contact of the main plastral elements, hyoplastral shoulder, presence of a lateral, upturned margin on the hyo/hypoplastron that is covered dorsally and laterally by sculptured metaplastic bone, a single, lateral hyoplastral process, and the apomorphic presence of fine scallops along the margin of costals VIII, formation of a laterally embraced, rounded nuchal, anteriorly rounded costals I, distally expanded costals II, and narrow costals VII. A phylogenetic analysis places Helopanoplia distincta as sister to the clade formed by Plastomenus thomasii and Hutchemys spp., thereby confirming its identity as a plastomenid. The vast majority of Helopanoplia distincta material has been recovered from fine-grained overbank deposits, thereby supporting the hypothesis that this turtle favored ponded waters.

A new species of trionychid turtle from the Upper Cretaceous (Campanian) Fruitland Formation of New Mexico, USA

“…3), highlighting that the six equally parsimonious trees did not reveal any topological conflict within this clade. The topology generally resembles those of previous analyses (e.g., Joyce and Lyson, 2011, 2017; Joyce et al, 2016) with exception of a more derived placement of Atoposemys superstes (Russell, 1930) as the sister to the Helopanoplia / Hutchemys clade and placement of Gilmoremys gettyspherensis n. sp. as sister to Gilmoremys lancensis .…”

“…sp., among others, by being smaller and thicker shelled, by exhibiting a pitted shell sculpturing, and, most significantly, by possessing well-developed hyoplastral shoulders that suturally articulate with well-developed entoplastral callosities. Helopanoplia distincta (as described by Joyce and Lyson, 2017) most readily differs from Gilmoremys gettyspherensis n. sp. by being larger, by lacking free rib ends for costal ribs I–VI, by exhibiting a carapacial and plastral surface sculpturing that ranges from netting to isolated tubercles, by only possessing only a single lateral hyoplastral process and much more broadly developed hyo-, hypo-, and xiphiplastral callosities.…”

“…The majority of plastomenid remains are recovered from siltstone and mudstones and the group is therefore often believed to have preferred ponded environments (e.g., Joyce and Lyson, 2017). The discovery of the type specimen of Gilmoremys gettyspherensis n. sp.…”

“…We explore the phylogenetic relationships of Gilmoremys gettyspherensis n. sp. by incorporating it into the character/taxon matrix of Joyce and Lyson (2017), which in turn is based on the successive phylogenetic analyses of Meylan (1987), Joyce et al (2009, 2016), Joyce and Lyson (2010, 2011), and Brinkman et al (2017). To help resolve relationships, we added a single character (character 95) to the matrix that encodes the absence (0) or presence (1) of well-developed anteroposterior striation in the carapace of adults.…”

A new species of trionychid turtle, Axestemys infernalis sp. nov., from the Late Cretaceous (Maastrichtian) Hell Creek and Lance formations of the Northern Great Plains, USA

“…Axestemys infernalis sp. nov. resembles Helopanoplia distincta (as described by Joyce and Lyson, 2017) by having a preneural, a netted carapacial sculpturing, and a single lateral hyoplastral process, but differs most obviously by being much larger, having all costal ribs end free, and by possessing less-well developed plastral callosities that do not cover the lateral hyo/hypoplastral processes and do not meet each other along the midline. Axestemys infernalis sp.…”