New insights into the morphology of the Carboniferous tetrapodCrassigyrinus scoticusfrom computed tomography

Abstract

ABSTRACTThe Carboniferous tetrapodCrassigyrinus scoticusis an enigmatic animal in terms of its morphology and its phylogenetic position.Crassigyrinushad extremely reduced forelimbs, and was aquatic, perhaps secondarily. Recent phylogenetic analyses tentatively placeCrassigyrinusclose to the whatcheeriids. Many Carboniferous tetrapods exhibit several characteristics associated with terrestrial locomotion, and much research has focused on how this novel locomotor mode evolved. However, to estimate the selective pressures and constraints during this important time in vertebrate evolution, it is also important to study early tetrapods likeCrassigyrinusthat either remained aquatic or secondarily became aquatic. We used computed tomographic scanning to search for more data about the skeletal morphology ofCrassigyrinusand discovered several elements previously hidden by the matrix. These elements include more ribs, another neural arch, potential evidence of an ossified pubis and maybe of pleurocentra. We also discovered several additional metatarsals with interesting asymmetrical morphology that may have functional implications. Finally, we reclassify what was previously thought to be a left sacral rib as a left fibula and show previously unknown aspects of the morphology of the radius. These discoveries are examined in functional and phylogenetic contexts.