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The braincase and jaws of a Devonian ‘acanthodian’ and modern gnathostome origins


Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era1, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic ‘acanthodians’ are possibly the earliest2,3 gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution1. Their relationships with modern gnathostomes have remained mysterious, partly because their un-mineralized endoskeletons rarely fossilized. Here I present the first-known braincase of an Early Devonian (approximately 418–412 Myr bp) acanthodian, Ptomacanthus anglicus4, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes5, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes5 in exhibiting several plesiomorphic features shared with placoderms6,7 and some early chondrichthyans8,9. Most striking is its extremely short sphenoid region and its jaw suspension, which displays features intermediate between some Palaeozoic chondrichthyans and osteichthyans. Phylogenetic analysis resolves Ptomacanthus as either the most basal chondrichthyan or as the sister group of all living gnathostomes. These new data alter earlier conceptions of basal gnathostome phylogeny and thus help to provide a more detailed picture of the acquisition of early gnathostome characters.

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Figure 1: Ptomacanthus anglicus NHM P 24919a.
Figure 2: Comparison of neurocranial proportions (ventral view) in early gnathostome taxa drawn to same anterior–posterior length.
Figure 3: Results of phylogenetic analyses and selected gnathostome palatoquadrates.


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I thank M. Richter for loans of specimens and access to collections; S. Davis, M. Friedman, D. Goujet, J. Long, J. Maisey and G. Young for discussions on early gnathostome endocranial anatomy; P. Ahlberg and H. Blom for discussions and support during all phases of this work. Financial support for this work was provided by a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship (NSERC PGS-D 331758-2006) awarded to the author. Thanks to J. Maisey, R. Miller and S. Turner for access to unpublished data on Doliodus.

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Correspondence to Martin D. Brazeau.

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Brazeau, M. The braincase and jaws of a Devonian ‘acanthodian’ and modern gnathostome origins. Nature 457, 305–308 (2009).

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