Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high‐resolution x‐ray microcomputed tomography, and the origin of the amniote crown group

Abstract

The origin of amniotes was a key event in vertebrate evolution, enabling tetrapods to break their ties with water and invade terrestrial environments. Two pivotal clades of early tetrapods, the diadectomorphs and the seymouriamorphs, have played an unsurpassed role in debates about the ancestry of amniotes for over a century, but their skeletal morphology has provided conflicting evidence for their affinities. Using high-resolution X-ray microcomputed tomography, we reveal the three-dimensional architecture of the well preserved endosseous labyrinth of the inner ear in representative species belonging to both groups. Data from the inner ear are coded in a new cladistic matrix of stem and primitive crown amniotes. Both maximum parsimony and Bayesian inference analyses retrieve seymouriamorphs as derived non-crown amniotes and diadectomorphs as sister group to synapsids. If confirmed, this sister group relationship invites re-examination of character polarity near the roots of the crown amniote radiation. Major changes in the endosseous labyrinth and adjacent braincase regions are mapped across the transition from non-amniote to amniote tetrapods and include: a ventral shift of the cochlear recess relative to the vestibule and the semicircular canals; cochlear recess (primitively housed exclusively within the opisthotic) accommodated within both the prootic and the opisthotic; development of a distinct fossa subarcuata. The inner ear of seymouriamorphs foreshadows conditions of more derived groups, whereas that of diadectomorphs shows a mosaic of plesiomorphic and apomorphic traits, some of which are unambiguously amniotelike, including a distinct and pyramid-like cochlear recess.

Anatomy of the neural endocranium, parasphenoid and stapes of Diadectes absitus (Diadectomorpha) from the early Permian of Germany based on the high‐resolution X‐ray microcomputed tomography

“…The posterior portion of the ventral crest borders a medially expanded groove that is exposed on the inner ear cavity represented by the medial extension of the vestibule (Figure 9). The foramen of the common crus of the anterior and posterior semicircular canals opens into this medial portion of the vestibule (Figure 10d; and see Klembara, Hain, Čerňanský, et al, 2020;Klembara, Hain, Ruta, et al, 2020). The dorsomedial margin of the anteromedial portion of the opisthotic abuts the ventral margin of the ventral crest, forming the posteromedial wall of the inner ear cavity (Figure 9).…”

“…The endolymphatic canal extends dorsomedially and opens into the cranial cavity. Immediately medial to the anterior section of the ventral crest is a shallow pit, the fossa subarcuata ( Figure 9a; Klembara, Hain, Čerňanský, et al, 2020;Klembara, Hain, Ruta, et al, 2020).…”

“…Farther posteriorly the vestibule mediolaterally narrows and deepens ventrally ( Figure 13b) and extends to the cochlear recess (Figure 13c-f). The cochlear recess forms an anteroposteriorly oriented cavity of triangular shape (Figure 10a-c; Klembara, Hain, Čerňanský, et al, 2020;Klembara, Hain, Ruta, et al, 2020). It gradually narrows dorsoventrally as it extends posteriorly ( Figures 10a-c and 13c,d).…”

Can We Reliably Calibrate Deep Nodes in the Tetrapod Tree? Case Studies in Deep Tetrapod Divergences

“…Although recent work finding the recumbirostran diversification within the Amniota remains controversial (compare Pardo et al, 2017b with Marjanović and, recent studies continue to identify reptile-like anatomy of the recumbirostran temporal fenestra (Gee et al, 2019) and dentition (Gee et al, 2020). Additionally, the Diadectamorpha, traditionally conceptualized as the sister clade of amniotes, has been found within the mammalian total group on the basis of occipital morphology (Berman, 2000), a result that has recently received new support based on anatomy of the inner ear (Klembara et al, 2020). This latter result currently has no major implications for node calibrations, however, as the oldest diadectamorph fossils are substantially younger than most putative early crown amniotes.…”

Limb-Bone Development of Seymouriamorphs: Implications for the Evolution of Growth Strategy in Stem Amniotes

“…Nevertheless, nothing rules out that seymouriamorphs could display a large diversity in their reproductive strategy with Seymouria exhibiting a shorter prereproductive period. For this reason, life histories should be more broadly investigated within seymouriamorphs and other stem-or early-amniotes such as diadectids (Benson, 2012;Berman, 2013;Klembara et al, 2020).…”