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The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins.

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Title: The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins.
Author(s): Brazeau, MD
Item Type: Journal Article
Abstract: Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic acanthodians are possibly the earliest gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution. 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 anglicus, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes in exhibiting several plesiomorphic features shared with placoderms and some early chondrichthyans. 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.
Publication Date: 15-Jan-2009
URI: http://hdl.handle.net/10044/1/17971
DOI: http://dx.doi.org/10.1038/nature07436
Start Page: 305
End Page: 308
Journal / Book Title: Nature
Volume: 457
Issue: 7227
Copyright Statement: ©2009 Macmillan Publishers Limited. All rights reserved
Conference Place: England
Appears in Collections:Faculty of Natural Sciences



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