The histology and affinities of sinacanthid fishes: primitive gnathostomes from the Silurian of China

Abstract

Craniofacial Development, Dental Institute, Guy’s Tower, London, SE1 9RT, UKReceived October 2003; accepted for publication March 2005On the basis of well preserved specimens from the Lower Silurian of the Tarim Basin, Xinjiang Uygur AutonomousRegion and Shiqian County, Guizhou Province, People’s Republic of China we describe in detail the histologicalstructure of sinacanthid spines, the only known remains of a group of fishes common in Silurian strata from China.The sinacanthids have previously been assigned either to the acanthodians or to the chondrichthyans. The spinestructure is composed of an outer layer of atubular dentine and an inner layer of globular calcified cartilage, and thenature and distribution of these tissues indicates that the spines were formed as a result of interaction between theendoskeleton and dermoskeleton. The tissue distribution and style of growth described herein places the sina-canthids crownwards of the placoderms, and possibly within the total group Chondrichthyes. However, before theycan be firmly placed within a phylogenetic scheme, further evidence is required both on the general anatomy ofsinacanthids and on the nature of chondrichthyan apomorphies. © 2005 The Linnean Society of London, ZoologicalJournal of the Linnean Society, 2005, 144, 379–386.