Cranial osteology and systematics of the enigmatic early ‘sail-backed’ synapsid Echinerpeton intermedium Reisz, 1972, and a review of the earliest ‘pelycosaurs’

The early synapsid Echinerpeton intermedium from Florence, Nova Scotia, was first described as a basal ophiacodontid ‘pelycosaur’, displaying the oldest record of hyper-elongated neural spines in an amniote. Since its initial discovery, the systematic placement of Echinerpeton intermedium has been less clear than that of its contemporary Archaeothyris florensis. The difficulty in assigning the remains of Echinerpeton firmly to any broader clade is largely the result of the incomplete nature of specimens, particularly the lack of diagnostic cranial material. The most recent large-scale phylogenies of basal synapsid interrelationships do not consistently recover Echinerpeton in a stable position. Despite being one of the earliest known synapsids, the volatility of the phylogenetic position of Echinerpeton has resulted in its exclusion from many large-scale phylogenetic analyses. Here, we describe the first new remains of Echinerpeton since its initial discovery. RM 206866 includes substantial new cranial material, and we provide the first comprehensive description of its skull. Informative remains include a relatively complete parabasisphenoid complex. Additionally, we revisit the phylogenetic position of Echinerpeton using Bayesian inference and parsimony-based methods, confirming the taxon as a member of Ophiacodontidae.