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Endochondral bone is the main internal skeletal tissue of nearly all osteichthyans 1,2 -26 the group comprising more than 60,000 living species of bony fishes and tetrapods. 27 Chondrichthyans (sharks and their kin) are the living sister group of osteichthyans and 28 have cartilaginous endoskeletons, long considered the ancestral condition for all jawed 29 vertebrates (gnathostomes) 3,4 . The absence of bone in modern jawless fishes and the 30 absence of endochondral ossification in early fossil gnathostomes appears to lend 31 support to this conclusion. Here we report the discovery of extensive endochondral bone 32 in a new genus of 'placoderm'-like fish from the Early Devonian (Pragian) of western 33 Mongolia described using x-ray computed microtomography (XR-µCT). The fossil 34 consists of a partial skull roof and braincase with anatomical details providing strong 35 evidence of placement in the gnathostome stem group. However, its endochondral space 36 is filled with an extensive network of fine trabeculae resembling the endochondral bone 37 of osteichthyans. Phylogenetic analyses place this new taxon as a proximate sister group 38 of the gnathostome crown. These results provide direct support for theories of 39 generalised bone loss in chondrichthyans 5,6 . Furthermore, they revive theories of a 40 phylogenetically deeper origin of endochondral bone and its absence in 41 chondrichthyans as a secondary condition 7,8 . 42 43