Endochondral bone is the main internal skeletal tissue of nearly all osteichthyans—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 primarily cartilaginous endoskeletons, long considered the ancestral condition for all jawed vertebrates (gnathostomes). The absence of bone in modern jawless fishes and the absence of endochondral ossification in early fossil gnathostomes appear to lend support to this conclusion. Here we report the discovery of extensive endochondral bone in Minjinia turgenensis, a new genus and species of ‘placoderm’-like fish from the Early Devonian (Pragian) of western Mongolia described using X-ray computed microtomography. 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 generalized bone loss in chondrichthyans. Furthermore, they revive theories of a phylogenetically deeper origin of endochondral bone and its absence in chondrichthyans as a secondary condition.
Bibliographical noteFunding Information:
M. Bolortsetseg generously assisted M.D.B. with contacts and field experience in Mongolia. Fieldwork was supported by National Geographic Society grants CRE 8769-10 and GEFNE35-12 to M.D.B. The field contributions of A.J. were supported by funds from the Anna Maria Lundin’s stipend from Smålands Nation, Uppsala University. The field contributions of R.S. were supported by a Royal Society Research Grant and the University of Manchester. The majority of this work was supported by the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement number 311092 to M.D.B. R.P.D. was also supported by the Île-de-France DIM (Domaine d’Intérêt Majeur) Matériaux Anciens et Patrimoniaux grant PHARE. S. Walsh is thanked for access to and loan of a specimen at the National Museums of Scotland. Synchrotron tomography was performed at the European Synchrotron Radiation Facility (application LS 2451) with the assistance of P. Tafforeau. S.G. was supported by a Royal Society Dorothy Hodgkin Research Fellowship. M. Friedman is thanked for undertaking the X-ray computed microtomography analysis. This study includes data produced in the CTEES facility at University of Michigan, supported by the Department of Earth and Environmental Sciences and College of Literature, Science, and the Arts. TNT was made available with the support of the Willi Hennig Society.
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ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics