A 248 million-year-old fossil Ichthyosaur shows a stunningly preserved female with three embryos inside. This pushes the timeline for vertebrate live births back 10 million years. The discovery comes a team led by Ryosuke Motani of UC Davis and was published in PLOS One.
The development of offspring inside the mother is known as viviparity, which is used by most mammals along with certain examples of arthropods, sharks, and snakes. This method is opposed to oviparity, which is when a mother does not facilitate development aside from laying eggs, which is seen in birds and most reptiles, insects, and fish. Many animals utilize a variation that combines certain aspects of these two techniques. While nearly all modern reptiles are oviparous, this has not been not always been the case. The discovery of a fossil in China from the ancient species ichthyosaur has now shown that the earliest known instance of viviparity actually came from a reptile.
The preservation of the fossil was so clear, scientists were able to gather quite a bit of information about the birthing process and found several surprising features. The fossil dates back to the Mesozoic era, 248 million years ago. This is about 10 million years older than the previous record holder for the oldest known evidence of vertebrate live birth.
Scientists were also surprised to find that the embryos appear to emerge through the birth canal head first. This is a feature commonly seen in terrestrial animals, while most marine live births occur tail first. If the ichthyosaurs gave birth on land, this challenges previous notions that live births first occurred in the water and eventually came about on land.
The female was found with her three offspring: one still inside of her, one halfway out of the pelvis, and one outside of the mother, not far away. This has led scientists to believe that the mother died during the childbirth and that the first neonate was stillborn.
The occurrence of live births in reptiles has come and gone no less than 100 times throughout their 320 million-year-long evolutionary history, though gaps in the fossil record leave scientists unable to connect the dots about what may or may not have brought about these changes.
This image shows a color-coded guide to examining the fossil. The maternal backbone is viewed in black, her ribs in green, and her pelvis in blue. The neonate assumed to have been stillborn is shown in red, while the yellow skeleton is was in the process of emerging through the pelvis, and the orange was the third embryo who had been trapped inside. Credit: Motani et al.