Spiny chondrichthyan from the lower Silurian of South China

Abstract

Modern representatives of chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods) have contrasting skeletal anatomies and developmental trajectories^1-4 that underscore the distant evolutionary split^5-7 of the two clades. Recent work on upper Silurian and Devonian jawed vertebrates^7-10 has revealed similar skeletal conditions that blur the conventional distinctions between osteichthyans, chondrichthyans and their jawed gnathostome ancestors. Here we describe the remains (dermal plates, scales and fin spines) of a chondrichthyan, Fanjingshania renovata gen. et sp. nov., from the lower Silurian of China that pre-date the earliest articulated fossils of jawed vertebrates^10-12. Fanjingshania possesses dermal shoulder girdle plates and a complement of fin spines that have a striking anatomical similarity to those recorded in a subset of stem chondrichthyans^5,7,13 (climatiid 'acanthodians'¹⁴). Uniquely among chondrichthyans, however, it demonstrates osteichthyan-like resorptive shedding of scale odontodes (dermal teeth) and an absence of odontogenic tissues in its spines. Our results identify independent acquisition of these conditions in the chondrichthyan stem group, adding Fanjingshania to an increasing number of taxa^7,15 nested within conventionally defined acanthodians¹⁶. The discovery of Fanjingshania provides the strongest support yet for a proposed⁷ early Silurian radiation of jawed vertebrates before their widespread appearance⁵ in the fossil record in the Lower Devonian series.