title: A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes.
abstract: Chimaeroid fishes (Holocephali) are one of the four principal divisions of modern gnathostomes (jawed vertebrates). Despite only 47 described living species, chimaeroids are the focus of resurgent interest as potential archives of genomic data and for the unique perspective they provide on chondrichthyan and gnathostome ancestral conditions. Chimaeroids are also noteworthy for their highly derived body plan. However, like other living groups with distinctive anatomies, fossils have been of limited use in unravelling their evolutionary origin, as the earliest recognized examples already exhibit many of the specializations present in modern forms. Here we report the results of a computed tomography analysis of Dwykaselachus, an enigmatic chondrichthyan braincase from the ~280 million year old Karoo sediments of South Africa. Externally, the braincase is that of a symmoriid shark and is by far the most complete uncrushed example yet discovered. Internally, the morphology exhibits otherwise characteristically chimaeroid specializations, including the otic labyrinth arrangement and the brain space configuration relative to exceptionally large orbits. These results have important implications for our view of modern chondrichthyan origins, add robust structure to the phylogeny of early crown group gnathostomes, reveal preconditions that suggest an initial morpho-functional basis for the derived chimaeroid cranium, and shed new light on the chondrichthyan response to the extinction at the end of the Devonian period.
authors.affiliation: Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60637-1508, USA.
authors.forename: Michael I
authors.initials: MI
authors.lastname: Coates
authors.affiliation: Geology Department and Albany Museum, Rhodes University, Grahamstown 6139, South Africa.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Gess
authors.affiliation: UCD School of Biology and Environmental Science, UCD Science Education and Research Centre, University College Dublin, Belfield, Dublin 4, Ireland.|UCD Earth Institute, University College Dublin, Belfield, Dublin 4, Ireland.
authors.forename: John A
authors.initials: JA
authors.lastname: Finarelli
authors.affiliation: Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60637-1508, USA.
authors.forename: Katharine E
authors.initials: KE
authors.lastname: Criswell
authors.affiliation: Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60637-1508, USA.
authors.forename: Kristen
authors.initials: K
authors.lastname: Tietjen
mesh_terms: D000818:Animals; D005399:Fishes / Q000033:anatomy & histology* / Q000145:classification*; D005580:Fossils; D010802:Phylogeny; D012754:Sharks / Q000033:anatomy & histology / Q000145:classification; D012886:Skull / Q000033:anatomy & histology*; D013019:South Africa