title: Copulation in antiarch placoderms and the origin of gnathostome internal fertilization.
abstract: Reproduction in jawed vertebrates (gnathostomes) involves either external or internal fertilization. It is commonly argued that internal fertilization can evolve from external, but not the reverse. Male copulatory claspers are present in certain placoderms, fossil jawed vertebrates retrieved as a paraphyletic segment of the gnathostome stem group in recent studies. This suggests that internal fertilization could be primitive for gnathostomes, but such a conclusion depends on demonstrating that copulation was not just a specialized feature of certain placoderm subgroups. The reproductive biology of antiarchs, consistently identified as the least crownward placoderms and thus of great interest in this context, has until now remained unknown. Here we show that certain antiarchs possessed dermal claspers in the males, while females bore paired dermal plates inferred to have facilitated copulation. These structures are not associated with pelvic fins. The clasper morphology resembles that of ptyctodonts, a more crownward placoderm group, suggesting that all placoderm claspers are homologous and that internal fertilization characterized all placoderms. This implies that external fertilization and spawning, which characterize most extant aquatic gnathostomes, must be derived from internal fertilization, even though this transformation has been thought implausible. Alternatively, the substantial morphological evidence for placoderm paraphyly must be rejected.
authors.affiliation: 1] School of Biological Sciences, Flinders University, 2100, Adelaide, South Australia 5001, Australia [2] Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 9007, USA [3] Museum Victoria, PO Box 666, Melbourne, Victoria 3001, Australia.
authors.forename: John A
authors.initials: JA
authors.lastname: Long
authors.affiliation: Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia.
authors.forename: Elga
authors.initials: E
authors.lastname: Mark-Kurik
authors.affiliation: Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK.
authors.forename: Zerina
authors.initials: Z
authors.lastname: Johanson
authors.affiliation: 1] South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia [2] School of Earth and Environmental Sciences, The University of Adelaide, South Australia 5005, Australia.
authors.forename: Michael S Y
authors.initials: MS
authors.lastname: Lee
authors.affiliation: Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory 0200, Australia.
authors.forename: Gavin C
authors.initials: GC
authors.lastname: Young
authors.affiliation: Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China.
authors.forename: Zhu
authors.initials: Z
authors.lastname: Min
authors.affiliation: Department of Organismal Biology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18A, 752 36 Uppsala, Sweden.
authors.forename: Per E
authors.initials: PE
authors.lastname: Ahlberg
authors.affiliation: Vine Lodge, Vine Road, Johnston, Haverfordwest, Pembrokeshire SA62 3NZ, UK.
authors.forename: Michael
authors.initials: M
authors.lastname: Newman
authors.affiliation: 6 Burghley Road, Wimbledon, London SW19 5BH, UK.
authors.forename: Roger
authors.initials: R
authors.lastname: Jones
authors.affiliation: University of Amsterdam, Science Park 904, 1098XH, Amsterdam, The Netherlands.
authors.forename: Jan
authors.initials: J
authors.lastname: den  Blaauwen
authors.affiliation: School of Biological Sciences, Flinders University, 2100, Adelaide, South Australia 5001, Australia.
authors.forename: Brian
authors.initials: B
authors.lastname: Choo
authors.affiliation: 1] Western Australian Organic and Isotope Geochemistry Centre, Department of Chemistry, Curtin University, Perth, Western Australia 6102, Australia [2] Earth and Planetary Sciences, Western Australian Museum, Perth, Western Australia 6000, Australia.
authors.forename: Kate
authors.initials: K
authors.lastname: Trinajstic
mesh_terms: D000818:Animals; D005075:Biological Evolution; D003307:Copulation / Q000502:physiology*; D005260:Female; D005306:Fertilization / Q000502:physiology*; D005399:Fishes / Q000033:anatomy & histology* / Q000502:physiology*; D005580:Fossils; D007568:Jaw; D008297:Male; D008954:Models, Biological; D010802:Phylogeny; D012727:Sex Characteristics; D014714:Vertebrates / Q000033:anatomy & histology / Q000502:physiology*