What it is
A small reptile that lived in the water, in a salty inland sea in what are now Brazil and Uruguay, about 278 million years ago. Few kinds of animal lived in that sea: mesosaurs, and the small crustaceans they hunted.
Two embryos
One adult from Brazil has a second, tiny skeleton between its ribs, with a very large skull, a few small vertebrae and most of a foot. Stomach acid would have marked a swallowed meal, and these little bones are clean, so the scientists read the adult as a pregnant female. In Uruguay, a separate tiny skeleton, less than 150 millimeters long, lies curled up like a baby inside an egg, with no eggshell around it.
Born or hatched?
The fossils fit two readings. Mesosaurus may have given birth to live young, or kept its eggs inside the body until they were almost ready to hatch. Either way, these are the oldest reptile embryos anyone has found.