Résumé
Abstract Specific character traits of mineralized endoskeletal tissues
need to be clearly defined and comprehensively examined among extant
chondrichthyans (elasmobranchs, such as sharks and skates, and
holocephalans, such as chimaeras) to understand their evolution. For
example, tiles of mineralized polygonal structures called tesserae occur
at cartilage surfaces in chondrichthyans, but recent studies showing
trabecular mineralization at elasmobranch cartilage surfaces suggest that
tesserae are not as common as previously thought. Also, while areolar
mineralized tissue in elasmobranchs is generally considered a unique,
shared chondrichthyan feature, some chondrichthyan species demonstrate
bone-like tissues in both a specific region of tesserae termed the cap
zone and continuous (not tiled) mineralized neural arches. To clarify the
distribution of specific endoskeletal features among extant
chondrichthyans, adult skeletal tissues in a holocephalan chimaera
(spotted ratfish) and two elasmobranchs (small-spotted catshark and little
skate) were characterized using synchrotron radiation and desktop micro-CT
imaging, and histological and immunofluorescent assays. Endoskeletal
mineralization in the ratfish, catshark, and little skate varied both
quantitively in tissue mineral density (TMD), and qualitatively in the
morphology and localization of mineralized structures and tissues. For
example, TMD of several skeletal elements was significantly lower in
ratfish, compared to catshark and little skate. Trabecular and areolar
mineralization were shared among these extant chondrichthyan species, but
tesserae and bone-like tissues were not. Interestingly, three separate
analyses argued that the adult chimaera endoskeleton has features of the
embryonic little skate endoskeleton. Generally, this study proposes
specific terminology for character states of the extant chondrichthyan
endoskeleton and infers those states in ancestral chondrichthyans with
reference to fossil data.