title: Mammalian development does not recapitulate suspected key transformations in the evolutionary detachment of the mammalian middle ear.
abstract: The ectotympanic, malleus and incus of the developing mammalian middle ear (ME) are initially attached to the dentary via Meckel's cartilage, betraying their origins from the primary jaw joint of land vertebrates. This recapitulation has prompted mostly unquantified suggestions that several suspected--but similarly unquantified--key evolutionary transformations leading to the mammalian ME are recapitulated in development, through negative allometry and posterior/medial displacement of ME bones relative to the jaw joint. Here we show, using µCT reconstructions, that neither allometric nor topological change is quantifiable in the pre-detachment ME development of six marsupials and two monotremes. Also, differential ME positioning in the two monotreme species is not recapitulated. This challenges the developmental prerequisites of widely cited evolutionary scenarios of definitive mammalian middle ear (DMME) evolution, highlighting the requirement for further fossil evidence to test these hypotheses. Possible association between rear molar eruption, full ME ossification and ME detachment in marsupials suggests functional divergence between dentary and ME as a trigger for developmental, and possibly also evolutionary, ME detachment. The stable positioning of the dentary and ME supports suggestions that a 'partial mammalian middle ear' as found in many mammaliaforms--probably with a cartilaginous Meckel's cartilage--represents the only developmentally plausible evolutionary DMME precursor.
authors.affiliation: School of Biological Sciences, University of Queensland, Goddard Building 8, St Lucia 4072, Australia h.ramirezchaves@uq.edu.au.
authors.forename: Héctor E
authors.initials: HE
authors.lastname: Ramírez-Chaves
authors.affiliation: Function, Evolution and Anatomy Research Laboratory, Division of Zoology, School of Environmental and Rural Sciences, University of New England, New South Wales 2351, Australia.
authors.forename: Stephen W
authors.initials: SW
authors.lastname: Wroe
authors.affiliation: School of Biosciences, University of Melbourne, Melbourne, Victoria 3010, Australia.
authors.forename: Lynne
authors.initials: L
authors.lastname: Selwood
authors.affiliation: CSIRO Biosecurity Flagship, GPO Box 1700, Canberra, Australian Capital Territory 2601, Australia.
authors.forename: Lyn A
authors.initials: LA
authors.lastname: Hinds
authors.affiliation: Anatomical Sciences, Adelaide University, North Terrace, South Australia 5000, Australia.
authors.forename: Chris
authors.initials: C
authors.lastname: Leigh
authors.affiliation: The University Museum, The University of Tokyo, Hongo 7-3-1, Tokyo 113-0033, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Koyabu
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz Berlin 114109, Germany.
authors.forename: Nikolay
authors.initials: N
authors.lastname: Kardjilov
authors.affiliation: School of Biological Sciences, University of Queensland, Goddard Building 8, St Lucia 4072, Australia v.weisbecker@uq.edu.au.
authors.forename: Vera
authors.initials: V
authors.lastname: Weisbecker
mesh_terms: D000818:Animals; D005075:Biological Evolution; D003817:Dentition; D004432:Ear, Middle / Q000033:anatomy & histology* / Q000254:growth & development; D008322:Mammals / Q000033:anatomy & histology* / Q000254:growth & development; D008394:Marsupialia / Q000033:anatomy & histology / Q000254:growth & development; D009007:Monotremata / Q000033:anatomy & histology / Q000254:growth & development
keywords: Synapsida; allometry; detachment; middle ear bones