Meeting Abstract
An accepted uniting character of cartilaginous fishes (sharks, rays, chimaera) is the presence of mineralized tiles (tesserae) on the outside of the cartilage skeleton. Tesserae have, however, never been demonstrated in modern chimaera and it is debated whether the skeleton mineralizes at all. We use materials and biological tissue characterization techniques to show, for the first time, the presence of a tessellated mineralized layer in chimaeroid fish, in several skeletal elements (jaws, cranium, vertebral column) and three genera. The mineralized “tiles” are irregular and not uniformly distributed, unlike most shark and ray tesserae, yet share several features with tesserae. The mineralized layer is peripheral in the unmineralized cartilage and seems to grow by periodic accretion of mineral at edges, forming laminated patterns of mineral density variation similar to those in shark and ray tesserae (e.g. in Liesegang lines, hypermineralized “spokes”). Chimaeroid mineralized cartilage, however, appears to lack the network of cell spaces that characterize tesserae, although we observe poorly mineralized regions suggesting infilled cell spaces. Significant is the apparent absence of the cell- and fiber-rich joints that link shark and ray tesserae, suggesting that cells and true intertesseral joints may be vital to the development of more geometric tessellations. Our data indicate that skeletal mineralization is more widespread and diverse in extant cartilaginous fishes than previously thought; developmental studies of chimaeroid mineralization are necessary to determine the mechanisms underlying skeletal patterning and their conservation across cartilaginous fishes.
