About 248 million years ago, this sea beast died while giving birth – and left these gorgeously preserved fossil remains that say a lot about evolution of live birth in the earliest marine reptiles of the Mesozoic:
The fossil belongs to Chaohusaurus (Reptilia, Ichthyopterygia), which is the oldest of Mesozoic marine reptiles (ca. 248 million years ago, Early Triassic). 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.
They had true flippers, not just webbed feet, but looked pretty lizard-like with no dorsal fin. And they were small: Adults were only about 100 cm long. In the colored diagram, black indicates the maternal vertebral column; blue, the maternal pelvis and hind flipper; green, maternal ribs and gastralia; orange and yellow are the bones of two different embryos; and red, those of a newborn. The scale bar is 1 cm.
This fossil is one of more than 80 skeletons of early marine reptiles recently uncovered at a single quarry site in China’s Anhui province.
Source: Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils, by Ryosuke Motani, Da-yong Jiang, Andrea Tintori, Olivier Rieppel, and Guan-bao Chen; PLoS ONE (2014)