Heads or tails first? Evolution of fetal orientation in ichthyosaurs, with a scrutiny of the prevailing hypothesis

BMC Evolutionary Biology

article

Miedema, Feiko¹  Klein, Nicole³  Blackburn, Daniel G.⁵  Sander, P. Martin⁴  Maxwell, Erin E.¹  Griebeler, Eva M.⁷  Scheyer, Torsten M.³ 

[1] Staatliches Museum für Naturkunde Stuttgart;Institut für Biologie, Fachgebiet Paläontologie, Hohenheim University;Paläontologisches Institut, Universität Zürich;Abteilung Paläontologie, Institut für Geowissenschaften, Universität Bonn;Dept. of Biology and Electron Microscopy Facility, Trinity College;Dinosaur Institute;Institut für Organismische und Molekulare Evolution, Universität Mainz

关键词: Ichthyopterygia;    Fetal orientation;    Viviparity;    Asphyxiation risk;    Aquatic birth;    Mixosaurus;    Stenopterygius;   

DOI  :  10.1186/s12862-023-02110-4

学科分类：护理学

来源: BioMed Central

【 摘 要 】

According to a longstanding paradigm, aquatic amniotes, including the Mesozoic marine reptile group Ichthyopterygia, give birth tail-first because head-first birth leads to increased asphyxiation risk of the fetus in the aquatic environment. Here, we draw upon published and original evidence to test two hypotheses: (1) Ichthyosaurs inherited viviparity from a terrestrial ancestor. (2) Asphyxiation risk is the main reason aquatic amniotes give birth tail-first. From the fossil evidence, we conclude that head-first birth is more prevalent in Ichthyopterygia than previously recognized and that a preference for tail-first birth likely arose in derived forms. This weakens the support for the terrestrial ancestry of viviparity in Ichthyopterygia. Our survey of extant viviparous amniotes indicates that fetal orientation at birth reflects a broad diversity of factors unrelated to aquatic vs. terrestrial habitat, further undermining the asphyxiation hypothesis. We propose that birth preference is based on parturitional mechanics or carrying efficiency rather than habitat.

【 授权许可】

CC BY|CC0