Meeting Abstract

56.3  Sunday, Jan. 5 14:00  Gills, structure, and feeding mode: specialization in a remarkably early gnathostome CRISWELL, KE*; FINARELLI, JA; COATES, MI; University of Chicago; University College Dublin; University of Chicago kcriswell@uchicago.edu

Articulated gill skeletons are rarely preserved in early fossil fishes, leaving gaps in the knowledge of the early evolution of this major structural and functional component of vertebrate crania. The Middle Devonian putative shark Gladbachus adentatus preserves the oldest known complete gill basket of any gnathostome, and provides new information on primitive visceral arch morphology. Using CT technology, we have investigated the branchial arch morphology of Gladbachus in detail. The gill basket encompasses four arches each comprising a pharyngobranchial and an epibranchial dorsally, and a ceratobranchial ventrally. Midline accessory cartilages are also preserved. The first three ceratobranchials are long and thin with ventral grooves, while the fourth is wide and flat, and articulated with the pectoral girdle. Teeth might be present in this supposedly toothless fish, and the cranium is wide and short. Here, we suggest that this early gnathostome was a filter feeder, making Gladbachus the first Paleozoic representative of such a unique ecomorphotype. The skeleton of this fish is specialized and exceptionally complete, and warrants careful comparison with modern analogs, such as the basking shark and paddlefish, to identify convergent functional morphology. As the earliest known fish with a complete gill skeleton, Gladbachus also offers insights into ancestral conditions of the vertebrate visceral skeleton.