Braincase and Inner Ear Anatomy of the Late Carboniferous Tetrapod Limnoscelis dynatis (Diadectomorpha) Revealed by High-Resolution X-ray Microcomputed Tomography

Jozef Klembara,Miroslav Hain,David S Berman,Marcello Ruta

doi:10.3389/fevo.2021.709766

Abstract

The braincase anatomy of the Pennsylvanian diadectomorphLimnoscelis dynatisis described in detail, based upon high-resolution X-ray microcomputed tomography. Both supraoccipitals and most of the prootics and opisthotics are preserved. The known portions of the left prootic, opisthotic, and supraoccipital enclose complete sections of the endosseous labyrinth, including the anterior, posterior, and lateral semicircular canals, the vestibule, the cochlear recess, and the canal for the endolymphatic duct. The fossa subarcuata is visible anteromedial to the anterior semicircular canal. The presumed endolymphatic fossae occur in the dorsal wall of the posteromedial portion of the supraoccipital. Both the fossa subarcuata and the fossa endolymphatica lie in the cerebellar portion of the cranial cavity. In order to investigate the phylogenetic position ofL. dynatiswe used a recently published data matrix, including characters of the braincase, and subjected it to maximum parsimony analyses under a variety of character weighting schemes and to a Bayesian analysis.Limnoscelis dynatisemerges as sister taxon toL. paludis, and both species form the sister group to remaining diadectomorphs. Synapsids and diadectomorphs are resolved as sister clades in ∼90% of all the most parsimonious trees from the unweighted analysis, in the single trees from both the reweighted and the implied weights analyses, as well in the Bayesian tree.

Highlights

The origin and diversification of amniotes are at the center of considerable resurgent interest motivated by novel phylogenetic hypotheses (e.g., Ford and Benson, 2019, 2020), new evidence from reproductive paleoecology (e.g., Maddin et al, 2020) and key findings in comparative anatomy, developmental biology, and paleontology (e.g., Kuratani et al, 2011; Schoch and Sues, 2019)
In which the cochlear recess occurs ventral to the vestibule, the recess in D. absitus and O. pabsti lies posterior to the vestibule, a condition observed in the stem-group amniote Seymouria baylorensis (Klembara et al, 2020b)
These observations led Klembara et al (2020b) to conclude that the inner ears of D. absitus and O. pabsti display a combination of plesiomorphic and apomorphic features, some of which appear to be morphologically transitional between those of amniotes and non-amniote tetrapods

Summary

Introduction

The origin and diversification of amniotes are at the center of considerable resurgent interest motivated by novel phylogenetic hypotheses (e.g., Ford and Benson, 2019, 2020), new evidence from reproductive paleoecology (e.g., Maddin et al, 2020) and key findings in comparative anatomy, developmental biology, and paleontology (e.g., Kuratani et al, 2011; Schoch and Sues, 2019). In which the cochlear recess occurs ventral to the vestibule, the recess in D. absitus and O. pabsti lies posterior to the vestibule, a condition observed in the stem-group amniote Seymouria baylorensis (Klembara et al, 2020b) This anatomical construction is in striking contrast to that of extant amphibians, in which the cochlear recess is not developed, and the sensory hair cells associated with hearing are situated in the posterior portion of the vestibule (lagenar macula). These observations led Klembara et al (2020b) to conclude that the inner ears of D. absitus and O. pabsti display a combination of plesiomorphic (e.g., position of the cochlear recess) and apomorphic features (e.g., presence and morphology of the recess and its size relative to the vestibule), some of which appear to be morphologically transitional between those of amniotes and non-amniote tetrapods

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