title: A primitive placoderm sheds light on the origin of the jawed vertebrate face.
abstract: Extant vertebrates form two clades, the jawless Cyclostomata (lampreys and hagfishes) and the jawed Gnathostomata (all other vertebrates), with contrasting facial architectures. These arise during development from just a few key differences in the growth patterns of the cranial primordia: notably, the nasal sacs and hypophysis originate from a single placode in cyclostomes but from separate placodes in gnathostomes, and infraoptic ectomesenchyme migrates forward either side of the single placode in cyclostomes but between the placodes in gnathostomes. Fossil stem gnathostomes preserve cranial anatomies rich in landmarks that provide proxies for developmental processes and allow the transition from jawless to jawed vertebrates to be broken down into evolutionary steps. Here we use propagation phase contrast synchrotron microtomography to image the cranial anatomy of the primitive placoderm (jawed stem gnathostome) Romundina, and show that it combines jawed vertebrate architecture with cranial and cerebral proportions resembling those of cyclostomes and the galeaspid (jawless stem gnathostome) Shuyu. This combination seems to be primitive for jawed vertebrates, and suggests a decoupling between ectomesenchymal growth trajectory, ectomesenchymal proliferation, and cerebral shape change during the origin of gnathostomes.
authors: Uppsala University, Department of Organismal Biology, Subdepartment of Evolution and Development, Norbyvägen 18A, SE-752 36, Uppsala, Sweden.
authors: Vincent
authors: V
authors: Dupret
authors: 1] Uppsala University, Department of Organismal Biology, Subdepartment of Evolution and Development, Norbyvägen 18A, SE-752 36, Uppsala, Sweden [2] European Synchrotron Radiation Facility, 6 rue Jules Horowitz, 38043 Grenoble Cedex, France.
authors: Sophie
authors: S
authors: Sanchez
authors: Muséum national d'Histoire naturelle, UMR 7207 CR2P CNRS/MNHN/UPMC, 8 rue Buffon, CP 38,75231 Paris Cedex 05, France.
authors: Daniel
authors: D
authors: Goujet
authors: European Synchrotron Radiation Facility, 6 rue Jules Horowitz, 38043 Grenoble Cedex, France.
authors: Paul
authors: P
authors: Tafforeau
authors: Uppsala University, Department of Organismal Biology, Subdepartment of Evolution and Development, Norbyvägen 18A, SE-752 36, Uppsala, Sweden.
authors: Per E
authors: PE
authors: Ahlberg
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001921:Brain / Q000033:anatomy & histology; D005145:Face / Q000033:anatomy & histology; D005399:Fishes / Q000033:anatomy & histology* / Q000145:classification; D005580:Fossils; D007568:Jaw / Q000033:anatomy & histology; D007798:Lampreys / Q000033:anatomy & histology; D009432:Neural Crest / Q000033:anatomy & histology; D010802:Phylogeny