Cellular, bone-like tissue in the bucklers and thorns of the thornback ray

Chondrichthyans (cartilaginous fishes) have lost the cellular bone characteristic of other jawed vertebrate skeletons. However, we identify cellular bone-like tissue in modified scales with enlarged bases, called 'bucklers' and 'thorns', which are distinctive for one group of extant batoids (rays). As placoid scales, they possess spines of orthodentine and osteodentine, but a unique basal structure. This consists of a cell-rich material, previously misidentified as an acellular tissue. Newly formed basal tissue grows appositionally and episodically from a cell-rich periosteum-like layer and closely resembles cellular bone, with entombed cells situated between bundles of attachment fibres anchoring the scale to the underlying dermal tissue and the 'periosteum' to the scale surface. In histologically more mature tissue, the cell spaces and attachment fibres are remodelled, forming enlarged, elongated spaces. The result is a unique mineralized tissue in these rays, initially sharing similarities with cellular bone, but with a mature state where cell spaces are modified throughout the base, by proposed remodelling of the matrix. Our findings of cellular bone forming the attachment tissues in ray scales demonstrate the chondrichthyan capacity to deposit bone-like tissues within the odontode module, contrary to previous understandings of hard tissue evolution in vertebrates.