Mesosaurus Embryos!

Good friend, Dr. Graciela Piñeiro et al. (2012) just described the oldest known amniotic embryos. They belong to mesosaurs (Gervais 1865). From their 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.”

Embryo Mesosaurus

Figure 1. Embryo Mesosaurus curled up within its ellipsoid amniotic membrane, but no egg shell was preserved. Left: specimen; Middle: tracing of specimen; Right: restoration of specimen.

The embryo was ~10% the size of an average adult and coiled as if in an ellipsoid egg. The snout was relatively short. The head was relatively large. Despite the elliptical shape of the embryos, no shell was preserved. Some embryos were found within their mother. Only one and rarely two were carried at a time. These data support the large reptile family tree that recovered a mesosaur/thalattosaur/ichthyosaur relationship in which ichthyosaurs are known to exhibit live birth (viviparity) emerging from their amniotic sac prior to birth.

Lizards typically carry the embryo within the uterus for extended periods. Many exhibit viviparity, but lizards and mesosaurs are not related.

Ichthyosaurs and sauropterygians also exhibit viviparity and are closer to mesosaurs. All three are distantly related to both therapsids (basal mammals lay eggs) and archosaurs (both birds and crocs lay eggs). So viviparity in this clade seems to have had its genesis in mesosaurs.

There’s more big news on mesosaurs to come.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.
Piñeiro G, Ferigolo J, Meneghel M and Laurin M 2012. The oldest known amniotic embryos suggest viviparity in mesosaurs. Historical Biology: An International Journal of Paleobiology, DOI:10.1080/08912963.2012.662230

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