Mineralisation of theCallorhinchusvertebral column (Holocephali; Chondrichthyes)

Chondrichthyes (Elasmobranchii and Holocephali) are distinguished by their largely cartilaginous endoskeleton that comprises an uncalcified core overlain by a mineralised layer; in the Elasmobranchii (sharks, skates, rays) this mineralisation takes the form of calcified polygonal tiles known as tesserae. In recent years, these skeletal tissues have been described in ever increasing detail in sharks and rays but those of Holocephali (chimaeroids) have been less well-described, with conflicting accounts as to whether or not tesserae are present. During embryonic ontogeny in holocephalans, cervical vertebrae fuse to form a structure called the synarcual. The synarcual mineralises early and progressively, anteroposteriorly and dorsoventrally, and therefore presents a good skeletal structure in which to observe mineralised tissues in this group. Here we describe the development and mineralisation of the synarcual in an adult and stage 36 elephant shark embryo ( Callorhinchus milii ). Small, discrete, but irregular blocks of cortical mineralisation are present in stage 36, similar to what has been described recently in embryos of other chimaeroid taxa such as Hydrolagus , while in Callorhinchus adults, the blocks of mineralisation have become more irregular, but remain small. This differs from fossil members of the holocephalan crown group ( Edaphodon ), as well as from stem group holocephalans (e.g., Symmorida, Helodus , Iniopterygiformes), where tessellated cartilage is present, with tesserae being notably larger than in Callorhinchus and showing similarities to elasmobranch tesserae, for example with respect to polygonal shape.