It is a rare occurrence indeed for paleontologists to find solid evidence of life processes in action, preserved for millions of years. But this is precisely what paleontologist Ryosuke Motani and a team of international scientists found and reported on in a February study in PLOS ONE.
In Majiashan China, Motani and colleagues uncovered a number of ichthyosaur fossils – the most interesting of which may be the earliest proof of a live marine reptile birth occurring on dry land.
Among more than 80 ichthyosaur skeletons analyzed by Motani and his team was one partial skeleton belonging to Chaohusaurus, the oldest Mesozoic marine reptile, which contained within it embryo fossils – presumably fossilized mid-birth.
Chaohusaurus, lived 248 million years ago in China. It looked somewhat like a cross between a porpoise and a large lizard. Measuring in at about six feet long, Chaohusaurus was in fact one of the smallest ichthyosaurs to have swum the oceans of the ancient world. Like some other ichthyosaurs who came about later in the Triassic, Chaohusaurus was thought to be viviparous – or gave birth to live young.
To be exact, three Chaohusaurus embryos at various stages in birth were discovered within the fossilized mother ichthyosaur. But the fossil of interest was the second embryo in line out of the mother’s pelvis: this embryo is exiting skull-first.
Motani et al. 2014
Paleontologists have traditionally come to the consensus that live birth by marine reptiles originated in the water, and later became a terrestrial adaptation. But the new Chaohusaurus find says otherwise. The second embryo in succession to leave the mother ichthyosaur reveals the most important discovery of the study: the headfirst exit strategy by this particular baby ichthyosaur is an indication of terrestrial birth.
Since this fossil predates existing Mesozoic marine reptiles by ten million years, Motani’s team thinks land-birth may have come before aquatic vivipartiy in marine reptiles. Aquatic birth, thought originally to have been the first mode of birth in the evolutionary path of marine reptiles, occurs in a tail-first birth position. This finding may rewrite marine reptile evolution to include an older terrestrial viviparity characteristic, and a water-birth trait that evolved in later organisms.
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.
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