On the basis of well preserved specimens from the Lower Silurian of the Tarim Basin, Xinjiang Uygur Autonomous Region and Shiqian County, Guizhou Province, People's Republic of China we describe in detail the histological structure 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 spine structure is composed of an outer layer of atubular dentine and an inner layer of globular calcified cartilage, and the nature and distribution of these tissues indicates that the spines were formed as a result of interaction between the endoskeleton and dermoskeleton. The tissue distribution and style of growth described herein places the sinacanthids crownwards of the placoderms, and possibly within the total group Chondrichthyes. However, before they can be firmly placed within a phylogenetic scheme, further evidence is required both on the general anatomy of sinacanthids and on the nature of chondrichthyan apomorphies. © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society, 2005, 144, 379–386.