Extant cartilaginous fishes share trabecular and areolar mineralization patterns, but not tesserae, and evidence for a paedomorphic chimaera skeleton

A comprehensive study is lacking that clearly defines and directly compares the diverse mineralized endoskeletal tissues exhibited by extant chondrichthyans (elasmobranchs, such as sharks and skates, and holocephalans, such as chimaeras). Tiles of mineralized polygonal structures called tesserae occur at cartilage surfaces in chondrichthyans, but recent studies showing trabecular structures suggest that tesserae are not as common as previously thought. A specific region of tesserae termed cap zone and continuous (not tiled) mineralized elasmobranch neural arches demonstrate bone-like tissues. Areolar mineralized tissue in elasmobranchs is generally considered a unique chondrichthyan feature. Despite these reports, it remains unclear what mineralized endoskeletal features define extant chondrichthyans. To address this question, adult skeletal tissues in two elasmobranchs (little skate and small-spotted catshark) and a chimaera (spotted ratfish) were characterized using synchrotron radiation and desktop micro-CT imaging, and histological and immunofluorescent assays. Data from these extant chondrichthyan representatives suggested that trabecular and areolar mineralization, but not tesserae and bone-like tissues, are shared features of the extant chondrichthyan endoskeleton. Interestingly, three separate analyses argued that the chimaera endoskeleton retains ancestral embryonic features (i.e., paedomorphic). This study further proposes general terminology for character states of the extant chondrichthyan endoskeleton and infers those states in ancestral chondrichthyans.