The earliest known crown-group amniotes date back to the Late Carboniferous, with the oldest fossil examples of amniotic eggs and embryos being from the Triassic. However, Historical Biology: An International Journal of Paleobiology (2012) authors, Graciela Pineiro, Jorge Ferigolo, Melitta Meneghel, and Michel Laurin, wrote an article claiming that researchers have found fossils of an isolated embryo of the Early Permian Mesosaurus tenudines and have also found a single embryo inside an adult mesosaur in Uruguay.
I couldn't really make it out...so I thought this pic would help. :)
DESCRIPTION OF EMBRYO:
The well preserved isolated embryo consists of an almost fully developed, very small individual (150mm). It was identified by possessing common characteristics of the adult M. tenudine fossils (ie. long snout and presence of pachyostotic ribs). The shell of the egg was not preserved; however, a light halo circles the embryo almost completely. The bone development of this embryo suggest adaptive advantages at very early stages of development.
THE IN UTERO SPECIMEN:
The skeleton, preserved between the pectoral and pelvic girdles, represents a medium sized adult mesosaur skeleton associated with a very very small mesosaur. This suggests a gravid female carrying a single embryo in utero. The in utero embryo is about 1/2 the size of the isolated embryo. Because the fossilization of the in utero embryo was incomplete, paleobiologist suggests that the ossification of the skeleton was incomplete or in very early stages of development.
DISCUSSION:
The well developed skeletal structures of the isolated embryo suggest egg retention in mother mesosaurs so that the eggs are laid just before hatching time. The partially ossified embryo in utero supports this hypothesis.
Another interesting tid bit, the mother mesosaur was only carrying one embryo, which is characteristic of organisms that provide extensive parental care, such as some mammal (ie. humans, chimps, and whales). This implies that not only did amniotes occur 60Ma before the previous records, but that parental care also evolved before expected. The oldest known case of parental care is represented by Middle Permian varanoid synapsids, about 20Ma after the mesosaur record.
CONCLUSION:
These fossils represent the geologically oldest record of extensive embryo retention in amniotes, as well as the equivocal evidence of parental care. These finding are important for scientists better understanding and groupings of phylogeny and understanding reproductive evolution over time.
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1 comment:
Fascinating find! It's interesting that the moms carried only one embryo at a time.
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