Very interesting news on BBC Nature today about this paper by Ryosuke Motani et al., published about a new ichthyosaur fossil.
Ichthyosaurs were marine reptiles that lived in the sea at the same time as the dinosaurs. They were so well adapted to marine life that they evolved a body very much like a modern fish or dolphin; with such a body, they couldn’t come on land to lay eggs like turtles, so instead they gave birth to live young. We know this because of a fossil of a baby ichthyosaur that was fossilised as it left the mother’s birth canal (photographed in the Natural History Museum).
So, did ichthyosaurs evolve live birth after they moved into the water, so that they could subsequently evolve a body more efficient for swimming? No, it seems.
The fossil described in the recent study belongs to another species of ichthyosaur named Chaohusaurus, which lived 248 million years ago and is one of the first ichthyosaurs known. The skeleton contains three embryos – and it looks like they were being born head first.
Note that the embryo in the image above, from a much later ichthyosaur than Chaohusaurus, is being born tail first. This is typical of whales today; like whales, ichthyosaurs were air-breathers, and tail-first birth helped the babies avoid drowning as they were born. Terrestrial animals, on the other hand, tend to give birth head first.
What does this mean? It suggests that the ancestors of ichthyosaurs were already bearing live young (as many reptiles still do today) before they moved into the sea – and that terrestrial reptiles were giving birth over 100 million years before existing fossil evidence indicated they were!
Motani R, Jiang D-y, Tintori A, Rieppel O, Chen G-b (2014) Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils. PLoS ONE 9(2): e88640. doi:10.1371/journal.pone.0088640