Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.06.09.132027v1?rss=1 Authors: Brazeau, M., Giles, S., Dearden, R., Jerve, A., Ariunchimeg, Y., Zorig, E., Guillerme, T., Castiello, M. Abstract: Endochondral bone is the main internal skeletal tissue of nearly all osteichthyans1,2 -- the group comprising more than 60,000 living species of bony fishes and tetrapods. Chondrichthyans (sharks and their kin) are the living sister group of osteichthyans and have cartilaginous endoskeletons, long considered the ancestral condition for all jawed vertebrates (gnathostomes)3,4. The absence of bone in modern jawless fishes and the absence of endochondral ossification in early fossil gnathostomes appears to lend support to this conclusion. Here we report the discovery of extensive endochondral bone in a new genus of placoderm-like fish from the Early Devonian (Pragian) of western Mongolia described using x-ray computed microtomography (XR-CT). The fossil consists of a partial skull roof and braincase with anatomical details providing strong evidence of placement in the gnathostome stem group. However, its endochondral space is filled with an extensive network of fine trabeculae resembling the endochondral bone of osteichthyans. Phylogenetic analyses place this new taxon as a proximate sister group of the gnathostome crown. These results provide direct support for theories of generalised bone loss in chondrichthyans5,6. Furthermore, they revive theories of a phylogenetically deeper origin of endochondral bone and its absence in chondrichthyans as a secondary condition7,8. Copy rights belong to original authors. Visit the link for more info