Live birth has evolved dozens of times in vertebrates, but has never been seen in archosauromorphs, which emerged around 260 million years ago. This group comprises dinosaurs as well as extant birds and crocodiles.
A team led by Jun Liu at the Hefei University of Technology in China analysed the fossil — found in 2008 in southwestern China — and concluded that it was a long-necked marine reptile called Dinocephalosaurus. A relatively large creature found inside its rib cage was curled up and positioned in a way that is typical of vertebrate embryos.
The fossil suggests that no genetic or developmental barriers prevented live births in archosauromorphs, the authors say."