A new operculate symmoriiform chondrichthyan from the Late Mississippian Fayetteville Shale (Arkansas, United States)

We describe a new genus of symmoriiform chondrichthyan from the Late Mississippian Fayetteville Shale of Arkansas, United States, and include this fossil in a phylogenetic analysis of chondrichthyans. This taxon possesses elongate cartilaginous rays extending from the gill arches, forming an operculate structure that covers at least two of the branchial arches farther posteriorly. Although presence of a ‘hyoid operculum’ has been postulated in at least two unrelated Paleozoic sharks (e.g., Triodus, Tristychius), subsequent investigations failed to corroborate those claims. The new fossil therefore provides the first evidence of an endoskeletal operculum formed by elongate, fused pharyngeal arch rays in a chondrichthyan.