New light on Devonian tetrapod braincases and middle ears
by Per E. Ahlberg | Grzegorz Niedzwiedzki | Ervins Luksevics | Laura Porro | Martin Qvarnström | Henning Blom | John E. A. Marshall | Uppsala University | Uppsala University | Latvian University | University College London | Uppsala University | Uppsala University | University of Southampton
Abstract ID: 52
Event: The 3rd AsiaEvo Conference
Topic: Early evolution of vertebrates from evo-devo and paleontological perspectives
Presenter Name: Per E. Ahlberg

The otoccipital region of the braincase underwent dramatic changes over the course of the fish-tetrapod transition. In fish members of the tetrapod stem group, such as Eusthenopteron and Gogonasus, the otoccipital region is demarcated anteriorly by an intracranial joint. The basioccipital forms a tunnel for the notochord, opening anteriorly into a basicranial fenestra. The dorsal part of the hyoid arch is a hyomandibula, which articulates against a lateral commissure straddling the jugular vein. The lateral otic fissure ends ventrally in a vestibular fontanelle, but this opening into the inner ear space has no direct connection with the hyomandibula. This architecture persists into the elpistostegalians Panderichthys and Tiktaalik. By contrast, at the tetrapod crown group node, the intracranial joint and basicranial fenestra have disappeared, the basioccipital is no longer notochordal, the lateral commissure has been lost, and the dorsal part of the hyoid arch is now a stapes with its footplate lodged in a fenestra vestibuli, in direct contact with the inner ear.

The transition between these endpoints is documented by braincases of Devonian stem tetrapods. Only two are known in their entirety: those of Acanthostega and Ichthyostega. They both have persistent notochordal tunnels, and lack basicranial fenestrae and lateral commissures, but in other respects they are quite dissimilar. The braincase of Ichthyostega is proportionately longer than that of Acanthostega and narrower relative to the skull roof; its fenestra vestibuli is smaller; and its stapes is a delicate paddle-like bone whereas that of Acanthostega is more robust. Uncertainty is introduced by the fact that Acanthostega has been reconstructed from partially flattened specimens whereas Ichthyostega is known from a relatively poor CT scan.

Here we present two informative new otoccipital braincases of Devonian tetrapods: one from Ventastega (late Famennian, Latvia), the other from an undescribed tetrapod (latest Famennian, East Greenland). The latter, which comes from a new locality yielding a unique fossil assemblage with partial soft-tissue preservation, is preserved with both stapes in position. Ventastega has been imaged from a high-resolution CT scan, the Greenland braincase using synchrotron microtomography. Both braincases are notochordal and lack a basicranial fenestra. The occipital regions are short and relatively tall, similar to that in Acanthostega. The fenestra vestibuli is very large in both braincases, much bigger than would be required just to accommodate the footplate of the stapes. The major differences between them are that the Greenland braincase is narrower dorsally, creating flanking spaces underneath the wide skull roof in a manner similar to Ichthyostega, and that it has incipient paired basioccipital condyles which are absent in Ventastega. The stapes of the Greenland braincase, like that of Ichthyostega, is a thin, subcircular paddle with robust dorsal and ventral feet, but its orientation is more vertical and it is concave anterolaterally rather than posteromesially. We interpret it as part of an auditory but non-tympanic ear. Preserved soft tissues include a ligament or muscle connecting the stapes to the skull roof, and part of the mesial wall of the middle ear cavity.