A well‐preserved cranium from the Judith River Formation (Montana, USA) reveals the inner ear and neuroanatomy of a Campanian baenid turtle

Abstract

A cranium belonging to a baenid turtle was recently recovered from the lower half of the Judith River Formation, Montana. Badlands Dinosaur Museum (BDM) 004 is a well‐preserved partial cranium that includes the posterior cranial vault, cranial base, and otic capsules. Based on diagnostic characters, the skull can be attributed to Plesiobaena antiqua, which has been previously reported from the Judith River Formation. It also shares with palatobaenines projecting posterior processes of the tubercula basioccipitale and a prominent condylus occipitalis with a deep central pit, demonstrating variation within the Pl. antiqua hypodigm. In a phylogenetic analysis, an operational taxonomic unit of BDM 004 was positioned within Baenodda in an unresolved polytomy with Pl. antiqua, Edowa zuniensis, Palatobaeninae, and Eubaeninae. Microcomputed tomographic (μCT) scans revealed morphology of the middle and inner ear and endocast that are largely unknown in baenids. Semicircular canals of BDM 004 are virtually identical to those of Eubaena cephalica and consistent in dimensions to those of other turtle taxa, including anterior and posterior semicircular canals that are robust and taller than the common crus and diverge from each other at an angle of approximately 90°. The digital endocast reveals a moderately flexed brain with rounded cerebral hemispheres and minimal separation between the metencephalon and myelencephalon. Its well‐preserved columella auris (stapes) is gracile with a posterodorsally flared basis columella. It arcs across the middle ear and flattens near its terminus. This study adds to the understanding of baenid middle and inner ear and neuroanatomical morphology and expands the morphological understanding of Pl. antiqua.

Updated cranial and mandibular description of the Late Cretaceous (Maastrichtian) baenid turtle Saxochelys gilberti based on micro-computed tomography scans and new information on the holotype-shell association

“…The epipterygoid is a small, elongated and rod-like bone (Fig. 4 B) that is morphologically very similar to the epipterygoid described for Plesiobaena antiqua (Smith et al, 2023 ). The epipterygoid of DMNH EPV.96000 is located just anteroventral to the trigeminal foramen without contributing to its margin (Fig.…”

“…6 F, G, H, I; e.g., Foth et al, 2019 ). The stapes is slightly more robust than recently reported for Plesiobaena antiqua (Smith et al, 2023 ), although this is difficult to ascertain due to the incomplete preservation of the stapedial rod in DMNH EPV.96000.…”

“…The epipterygoid of DMNH EPV.96000 is located just anteroventral to the trigeminal foramen without contributing to its margin (Fig. 4 B), again as in Plesiobaena antiqua (Smith et al, 2023 ). The epipterygoid of DMNH EPV.96000 contacts the pterygoid medially on its posterior half and the parietal dorsally and medially.…”

“…This study adds to the increasing number of paracryptodiran cranial re-descriptions based on CT data (e.g., Arundelemys dardeni : Evers et al, 2021 ; Pleurosternon bullockii : Evers et al, 2021 ; Uluops uluops : Rollot et al, 2021a ; Trinitichelys hiatti : Rollot et al, 2022a ; Lakotemys australodakotensis : Rollot et al, 2022b ; Denazinemys nodosa : Spicher et al, 2023 ; Plesiobaena antiqua : Smith et al, 2023 ; Glyptops ornatus : Evers, 2023 ; Saxochelys gilberti : this study). Most of these studies contradict details found in the original descriptions that were based on traditional examination of specimens (e.g., Arundelemys dardeni : Lipka et al, 2006 ; Pleurosternon bullockii: Evans & Kemp, 1975 ; Uluops uluops : Carpenter & Bakker, 1990 ; Trinitichelys hiatti : Gaffney, 1972a ; Lakotemys australodakotensis : Joyce et al, 2020a ; Denazinemys nodosa : Lucas & Sullivan, 2006 ; Lively, 2016 ; Plesiobaena antiqua : Brinkman, 2003 ; Lyson & Joyce, 2009a ; Glyptops ornatus : Gaffney, 1979b ).…”

New findings on the evolution of turtles: A symposium in honor of Marcelo S. de la Fuente

“…New insights into the cranial osteology and neuroanatomy of the bothremydid turtle Galianemys from Morocco (Martín‐Jiménez & Pérez‐García, 2023) reveal important differences between the species of this genus and allowed the recognition of shared characters in Bothremydidae. Neuroanatomical studies were also conducted in Plesiobaena antiqua from the Campanian of the United States (Smith et al, 2023) providing details of its inner ear expanding the knowledge of the endocranial anatomy of baenid turtles. The taxonomic revision of Pleistocene geoemydids from Taiwan (Liaw & Tsai, 2023) confirms the synonymy of Chinemys pani with the extant Mauremys reevesii .…”