Over Eons, Snakes and Lizards Switch Between Laying Eggs and Live Birth

A biologist at The George Washington University contends that new revelations in the reproductive habits of ancient reptiles may lead to a sea change in the school of thought behind ancient animal behavior. 

A study published Tuesday in the journal Ecology Letters reports that the ancestors of modern-day squamates - the collective term for snakes and lizards - likely gave birth to live young, rather than laying eggs like their modern-day kin. Furthermore, the researchers contend, these species have switched back and forth in their preferred reproductive mode over time.

"This is a very unusual and controversial finding, and a major overturn of an accepted school of thought," said Alex Pyron, an assistant professor of biology at George Washington University. "Before, researchers long assumed that the ancestor of snakes and lizards laid eggs, and that if a species switched to live birth, it never reverted back. We found this wasn't the case."

To conduct their research, Pyron and his cohorts analyzed an evolutionary tree containing all groups of squamates. The tree uses DNA sequencing technology to group a huge band of creatures including thousands of lizard and snakes, including all families and subfamilies and most genus and species groups.

In total, about 115 squamate groups - amounting to nearly 2,000 species - gave live birth. The other 8,000 species lay eggs, at least for now.

The major takeaway from the research is that live birth, or viviparity, in lizards and snakes has a much more ancient past than previously believed. Pyron and his colleagues traced viviparity in squamates back in time to 175 million years ago. Their research is backed by several recent fossil finds in which plesiosaur and mosasaur fossils, as well as the fossils of a few Cretaceous Period lizards revealed embryos inside the mother, which the researchers contend makes a case for live birth in the creatures.

Future research into the reproductive modes of other animal groups, such as tetrapods, may shed more light on ancient animal behavior. Pyron intends to explore this group next, as well as to test the genetics at play when a species switches from egg birth to live birth.