Ancient lizard pregnant for 120 million years

The fossilised lizard Yabeinosaurus tenuis was pregnant with at least fifteen babies at the time it died (Original photo: Jie Zhang/Susan Evans)

Scientists have discovered the oldest-known fossil of a live-bearing lizard. The fossilised remains of the ancient pregnant lizard, dated between 120 and 125 million years old, show the skeletons of more than fifteen developing embryos in utero.

The earliest live-bearing (viviparous) lizards were thought to have originated during glacial episodes in the Pleistocene, no earlier than two million years ago. However, the recent discovery of the fossil suggests live birth evolved at a similar time in both lizards and mammals.

Susan Evans, a professor from the University College London and co-author of the journal article published in Naturwissenschaft, discovered the fossil in Lower Cretaceous geological deposits in the Jehol region of Northeast China.

“We previously thought that lizards adapted to live birth after mammals, but now it looks like it happened at roughly the same kind of time,” Evans told PhysOrg.com.

The fossilised lizard, identified as Yabeinosaurus tenuis, was most certainly viviparous; Evans found no trace of an eggshell surrounding the embryos and their skeletons indicate they were almost fully developed and probably within days of being born.

Susan Evans, co-author of the research, determined the developmental stage of the embryos by studying their tiny skeletons (Original photo: Jie Zhang/Susan Evans)

There are very few examples of fossilised pregnant reptiles, since it is incredibly rare that the skeletal remains of embryos are well-preserved in the fossil. Previous fossils have revealed that some ancient aquatic reptiles were also viviparous, but this is the most ancient primarily land-dwelling viviparous lizard to be discovered.

In the past, some “pregnant” fossils were in fact cases of cannibalism, where the remains of a juvenile reptile in the gut of an adult were mistaken for an embryo. However, there is no mistaking these embryos for this fossilised lizard’s lunch - the positions of the fifteen little skeletons correspond to the location of the paired uteri found in modern-day lizards.