# The cranial and postcranial morphology of Hutchemys rememdium and its impact on the phylogenetic relationships of Plastomenidae (Testudinata, Trionychidae)

## TL;DR

This paper describes new fossil material of a softshell turtle from the Paleocene, revealing its cranial and shell features and re-evaluating its evolutionary relationships.

## Contribution

The study provides new 3D micro-CT data and phylogenetic analyses that clarify the placement of Hutchemys rememdium within Plastomenidae.

## Key findings

- New cranial and shell material shows Hutchemys rememdium is distinct from Hutchemys arctochelys.
- Phylogenetic analyses confirm Hutchemys rememdium belongs to Plastomenidae.
- The study supports parallel evolution of the ossified plastron in two plastomenid lineages.

## Abstract

Hutchemys rememdium is a poorly understood softshell turtle (Trionychidae) from the mid Paleocene of the Williston Basin of North America previously known only from postcranial remains. A particularly rich collection of previously undescribed material from the Tiffanian 4 North American Land Mammal Age (NALMA) of North Dakota is here presented consisting of numerous shells that document new variation, some non-shell postcrania, and cranial remains, which are described based on 3D models extracted from micro-CT data. Although the observed shell variation weakens previously noted differences with the younger species Hutchemys arctochelys from the Clarkforkian NALMA, the two taxa are still recognized as distinct. Parsimony and Bayesian phylogenetic analyses reaffirm the previously challenged placement of Hutchemys rememdium within the clade Plastomenidae, mostly based on novel observations of cranial characters made possible by the new material and the micro-CT data. The new topology supports the notion that the well-ossified plastron of plastomenids originated twice in parallel near the Cretaceous/Paleogene boundary, once in the Hutchemys lineage and once in the Gilmoremys/Plastomenus lineage. Hutchemys rememdium is notable for being the only documented species of trionychid in the mid Paleocene of the Williston Basin. The presence of multiple individuals in a carbonaceous claystone indicates this taxon lived in swamps and lakes and its expanded triturating surface suggests it had a durophagous diet.

The online version contains supplementary material available at 10.1186/s13358-024-00315-8.