- Ryosuke Motani, doi:10.1371/journal.pone.0088640.g002
- Detail of the fossilized birth from a photo included in the original article published in Plos One. Click for full image and more info.
This exceptional specimen captures an articulated embryo in birth position, with its skull just emerged from the maternal pelvis. Its headfirst birth posture, which is unlikely to be a breech condition, strongly indicates a terrestrial origin of viviparity, in contrast to the traditional view. The tail-first birth posture in derived ichthyopterygians, convergent with the conditions in whales and sea cows, therefore is a secondary feature.
In other words: most marine animals are born tail-first, so the fact that the fossil shows a head-first birth suggests its ancestors were land-dwellers.
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