Elpistostege, step by step Towards a Tetrapod Skull

Australia: The Land Where Time Began

Elpistostege, step by step Towards a Tetrapod Skull

The fish-to-tetrapod transition has been probed over the last decade in order to understand the transformations in anatomy that are associated with locomotion, breathing, hearing, and feeding associated with the change from a water habitat to life on land. The transition of vertebrates has been primarily a matter of comparing 5 taxa along the tetrapod stem from the Devonian that are relatively well known as a result of their anatomical completeness: a true piscine sarcopterygian, Eusthenopteron foordi from the Middle Frasnian, a piscine elpistostegalian, Panderichthys rhomblepis, from the late Givetian, a near-tetrapod elpistostegalian, Tiktaalik roseae, from the early-middle Frasnian, and 2 true basal tetrapods, Acanthostega gunnari and Ichthyostega sp. from the Famennian. Much attention has been given to the pectoral and pelvic girdles and limbs, though changes to the roof of the skull have remained poorly investigated in piscine tetrapodomorphs because of either the usage of reconstructions instead of observation of specimens, e.g., Eusthenopteron, the limited number of specimens, e.g., Panderichthys, or the partial information that is available on the material, e.g., Tiktaalik. According to Cloutier & Béchard it is essential to compare with a pre-tetrapod taxon in order to understand the cranial disparity among tetrapods from the Devonian, e.g., Ventastega, Acanthostega, Ichthyostega and Ymeria. The opportunity to investigate cranial features at the transition from elpistostegalid to tetrapod was provided by the recent discovery of a complete specimen of Elpistostege watsoni dating to the middle Frasnian in the Escuminac Formation, Miguasha, eastern Canada. Cloutier & Béchard reinterpreted the roofs of the skulls of Elpistostege based on detailed morphological observation, SEM and high energy CT scam imaging and 3D imaging techniques. They compared the cranial ornamentation and sutures, the proportions of dermal bone roofing the skull, and the patterns of teeth among elpistostegalids and basal tetrapods. The close sister-group relationships between Elpistostege and basal tetrapods were corroborated by both quantitative and qualitative characters.