title: A caseian point for the evolution of a diaphragm homologue among the earliest synapsids.
abstract: The origin of the diaphragm remains a poorly understood yet crucial step in the evolution of terrestrial vertebrates, as this unique structure serves as the main respiratory motor for mammals. Here, we analyze the paleobiology and the respiratory apparatus of one of the oldest lineages of mammal-like reptiles: the Caseidae. Combining quantitative bone histology and functional morphological and physiological modeling approaches, we deduce a scenario in which an auxiliary ventilatory structure was present in these early synapsids. Crucial to this hypothesis are indications that at least the phylogenetically advanced caseids might not have been primarily terrestrial but rather were bound to a predominantly aquatic life. Such a lifestyle would have resulted in severe constraints on their ventilatory system, which consequently would have had to cope with diving-related problems. Our modeling of breathing parameters revealed that these caseids were capable of only limited costal breathing and, if aquatic, must have employed some auxiliary ventilatory mechanism to quickly meet their oxygen demand upon surfacing. Given caseids' phylogenetic position at the base of Synapsida and under this aquatic scenario, it would be most parsimonious to assume that a homologue of the mammalian diaphragm had already evolved about 50 Ma earlier than previously assumed.
authors.affiliation: Institut für Zoologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany.|Sektion Herpetologie, Zoologisches Forschungsmuseum Alexander Koenig, Bonn, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Lambertz
authors.affiliation: Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany.|Palaeobiology Research Group, Department of Biological Sciences, University of Cape Town, Rhodes Gift, South Africa.
authors.forename: Christen D
authors.initials: CD
authors.lastname: Shelton
authors.affiliation: Institut für Geologie, Technische Universität Bergakademie Freiberg, Freiberg, Germany.|Dinosaurier-Park Altmühltal, Denkendorf, Germany.
authors.forename: Frederik
authors.initials: F
authors.lastname: Spindler
authors.affiliation: Institut für Zoologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany.
authors.forename: Steven F
authors.initials: SF
authors.lastname: Perry
mesh_terms: D000818:Animals; D005075:Biological Evolution; D003964:Diaphragm / Q000033:anatomy & histology* / Q000502:physiology; D053476:Extinction, Biological; D010802:Phylogeny
keywords: breathing mechanics; evolution; functional morphology; respiratory biology; ventilatory apparatus