A new specimen of Sauripterus taylori(Sarcopterygii, Osteichthyes) from the Famennian Catskill Formation of North America

Abstract

A new specimen of the rhizodontid sarcopterygian, Sauripterus taylori (Sarcopterygii, Osteichthyes), from the Late Devonian (Famennian) Catskill Formation of Pennsylvania consists of a well-preserved right pectoral fin, girdle, and associated scales. Rhizodontid affinity is supported by the unique pattern of overlap between the clavicle and cleithrum, the robustness of the pectoral girdle, the presence of elongate and unjointed lepidotrichia in the pectoral fin, and the pattern of connectivity between the endochondral bones of the pectoral fin. The new specimen of Sauripterus provides a three-dimensional and articulated view of basal rhizodontid pectoral girdle and fin anatomy and preserves the first known scapulocoracoid for a Devonian rhizodontid. The glenoid fossa faces posteroventrally, in contrast to the more posterolaterally facing glenoids of other tetrapodomorph fish. The humerus is short proximodistally and possesses an expansive ventral flange for the attachment of the adductor, flexor, and rotatory musculature of the fin. Together, these data suggest that the pectoral fins of Sauripterus were optimized for adduction and flexion against the water column or a solid substrate. To maximize the propulsive force of these motions the distal fin was stiffened by the presence of eight radiais, similar in position to the autopodials of tetrapods. However, unlike the digits of tetrapods, the unit that interacts with the environment in rhizodontids is a combined structure consisting of both dermal and endochondral fin skeletons. As recent phylogenetic hypotheses do not support a sister-group relationship between rhizodontids and tetrapods, the similarities between the endoskeletal radiais of Sauripterus and the autopodium of tetrapods are independently derived.