An enigmatic chondrichthyan spine from the Visean of Indiana, USA that resembles a median rostral cartilage of Squaloraja (Holocephali, Chimaeriformes)

Abstract

Squaloraja is a genus of chimaeriform fishes known from the Early Jurassic. It has a dorsoventrally flattened body and a long median rostral cartilage. Males have a lance-like tenaculum that articulates with a central groove on the dorsal face of the median rostral cartilage. The genus is the only member of the family Squalorajidae, itself the only family in the suborder Squalorajoidei of the order Chimaeriformes. Phylogenetic analyses suggest a divergence of the Squalorajoidei from other Chimaeriformes in the Mississippian, implying the existence of a ghost lineage of more than 130 My. A spine that resembles a median rostral cartilage of a male Squaloraja was found in the St. Louis Formation (Visean) of Indiana, USA. As in Squaloraja, the dorsal face has a long narrow groove that would have articulated with a long narrow tenaculum. The spine is designated as the holotype of Sulcacanthus schachti, n. gen. et sp. Sulcacanthus is tentatively assigned to the Squalorajoidei based on morphology, but the possibility of convergence cannot be eliminated. A holocephalan tenaculum from the same locality might belong to the same taxon as the median rostral cartilage but could also belong to the suborder Myriacanthoidei.