title: A Palaeozoic shark with osteichthyan-like branchial arches.
abstract: The evolution of serially arranged, jointed endoskeletal supports internal to the gills--the visceral branchial arches--represents one of the key events in early jawed vertebrate (gnathostome) history, because it provided the morphological basis for the subsequent evolution of jaws. However, until now little was known about visceral arches in early gnathostomes, and theories about gill arch evolution were driven by information gleaned mostly from both modern cartilaginous (chondrichthyan) and bony (osteichthyan) fishes. New fossil discoveries can profoundly affect our understanding of evolutionary history, by revealing hitherto unseen combinations of primitive and derived characters. Here we describe a 325 million year (Myr)-old Palaeozoic shark-like fossil that represents, to our knowledge, the earliest identified chondrichthyan in which the complete gill skeleton is three-dimensionally preserved in its natural position. Its visceral arch arrangement is remarkably osteichthyan-like, suggesting that this may represent the common ancestral condition for crown gnathostomes. Our findings thus reinterpret the polarity of some arch features of the crown jawed vertebrates and invert the classic hypothesis, in which modern sharks retain the ancestral condition. This study underscores the importance of early chondrichthyans in resolving the evolutionary history of jawed vertebrates.
authors.affiliation: Department of Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, USA.
authors.forename: Alan
authors.initials: A
authors.lastname: Pradel
authors.affiliation: Department of Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, USA.
authors.forename: John G
authors.initials: JG
authors.lastname: Maisey
authors.affiliation: European Synchrotron Radiation Facility, BP 220, 6 rue Jules Horowitz, 38043 Grenoble Cedex, France.
authors.forename: Paul
authors.initials: P
authors.lastname: Tafforeau
authors.affiliation: Department of Geological Sciences, Ohio University, Athens, Ohio 45701, USA.
authors.forename: Royal H
authors.initials: RH
authors.lastname: Mapes
authors.affiliation: School of Biological Sciences, Washington State University, Pullman, Washington 99164-4236, USA.
authors.forename: Jon
authors.initials: J
authors.lastname: Mallatt
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001934:Branchial Region / Q000033:anatomy & histology; D002356:Cartilage / Q000033:anatomy & histology; D005580:Fossils; D005880:Gills / Q000033:anatomy & histology*; D010802:Phylogeny; D012754:Sharks / Q000033:anatomy & histology* / Q000145:classification
references.citation: J Morphol. 2000 Mar;243(3):219-45
references.pmid: 10681469
references.citation: Nature. 2012 Jun 13;486(7402):247-50
references.pmid: 22699617
references.citation: Evol Dev. 2012 Jan-Feb;14(1):76-92
references.pmid: 23016976
references.citation: Nature. 2013 Oct 10;502(7470):188-93
references.pmid: 24067611
references.citation: PLoS One. 2013 Jun 17;8(6):e66988
references.pmid: 23799138
references.citation: PLoS One. 2011;6(9):e24938
references.pmid: 21980367
references.citation: J Fish Biol. 2012 Apr;80(5):918-51
references.pmid: 22497368
references.citation: PLoS One. 2013 Apr 18;8(4):e62389
references.pmid: 23638061