title: Head size, weaponry, and cervical adaptation: Testing craniocervical evolutionary hypotheses in Ceratopsia.
abstract: The anterior cervical vertebrae form the skeletal connection between the cranial and postcranial skeletons in higher tetrapods. As a result, the morphology of the atlas-axis complex is likely to be shaped by selection pressures acting on either the head or neck. The neoceratopsian (Reptilia:Dinosauria) syncervical represents one of the most highly modified atlas-axis regions in vertebrates, being formed by the complete coalescence of the three most anterior cervical vertebrae. In ceratopsids, the syncervical has been hypothesized to be an adaptation to support a massive skull, or to act as a buttress during intraspecific head-to-head combat. Here, we test these functional/adaptive hypotheses within a phylogenetic framework and critically examine the previously proposed methods for quantifying relative head size in the fossil record for the first time. Results indicate that neither the evolution of cranial weaponry nor large head size correlates with the origin of cervical fusion in ceratopsians, and we, therefore, reject both adaptive hypotheses for the origin of the syncervical. Anterior cervical fusion has evolved independently in a number of amniote clades, and further research on extant groups with this peculiar anatomy is needed to understand the evolutionary basis for cervical fusion in Neoceratopsia.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada. cv300@cam.ac.uk.|Department of earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom. cv300@cam.ac.uk.
authors.forename: Collin S
authors.initials: CS
authors.lastname: VanBuren
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada.|Department of Earth Sciences (Palaeobiology), University of Uppsala, Villavägen 16, SE-752 36, Uppsala, Sweden.|Department of Organismal Biology (Subdepartment of Evolution and Development), Norbyvägen 18A, SE-752 36, Uppsala, Sweden.
authors.forename: Nicolás E
authors.initials: NE
authors.lastname: Campione
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada.|Department of Natural History, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario, Canada, M5S 2C6.
authors.forename: David C
authors.initials: DC
authors.lastname: Evans
mesh_terms: D000220:Adaptation, Biological; D000818:Animals; D005075:Biological Evolution; D002574:Cervical Vertebrae / Q000033:anatomy & histology*; D025061:Dinosaurs / Q000033:anatomy & histology*; D005580:Fossils / Q000033:anatomy & histology*; D006257:Head / Q000033:anatomy & histology; D009333:Neck / Q000033:anatomy & histology; D010802:Phylogeny; D012886:Skull / Q000033:anatomy & histology
keywords: Adaptation; fossils; macroevolution; morphological evolution; paleobiology