The oldest known amniotic embryos suggest viviparity in mesosaurs

Abstract

The earliest undisputed crown-group amniotes date back to the Late Carboniferous, but the fossil record of amniotic eggs and embryos is very sparse, with the oldest described examples being from the Triassic. Here, we report exceptional, well-preserved amniotic mesosaur embryos from the Early Permian of Uruguay and Brazil. These embryos provide the earliest direct evidence of reproductive biology in Paleozoic amniotes. The absence of a recognisable eggshell and the occurrence of a partially articulated, but well-preserved embryo within an adult individual suggest that mesosaurs were viviparous or that they laid eggs in advanced stages of development. Our finds represent the only known documentation of amniotic embryos in the Paleozoic and the earliest known case of viviparity, thus extending the record of these reproductive strategies by 90 and 60 Ma, respectively.

Keywords:

• Mesosauridae
• Early Permian
• Gondwana
• reproductive biology
• oldest amniotic embryos
• viviparity

Acknowledgements

G.P. is indebted to Elizabeth Morosi, Alejandro Ramos and Guillermo Roland for their collaboration in the field work, to Santiago Carreira and Federico Achaval for supplying informative comments on the biology of extant reptiles, to Andrés Rinderknecht and David Peters for helpful critical comments and bibliography, and to Inés Castiglioni and Gustavo Lecuona for the illustrations. Silvia Villar, Alejandro Márquez and Jorge Tróccoli (Facultad de Ciencias) offered advice on the SEM analyses. M.L. thanks David Marjanović for bibliographic information and comments on the draft. We especially thank Marcello Ruta for language revisions and Erin Maxwell and an anonymous reviewer for criticism and suggestions that greatly improved the manuscript. This contribution was supported by ANII, FCE2007 96 (G.P.), and CNRS (UMR 7207) and a CNRS/CONICET Grant 22977 (M.L.).