Cryptovaranoides fossil reclaims title of world's oldest known lizard

The oldest known lizard has regained its place in the crown clade. After re-examining a disputed fossil, palaeobiologists have identified it as the earliest known member of the lizard and snake group.

The animal at the heart of the argument is Cryptovaranoides microlanius, a name meaning “hidden lizard, small butcher”. The latter part describes its size and diet; the former refers to the fossil having spent nearly 70 years in a storage room before receiving a scientific description.

Cryptovaranoides and the origins of Squamata

Scientists at the University of Bristol finally examined the skeleton in 2022 and assigned it to Squamata, the reptile order encompassing every species of lizard and snake. As the specimen is at least 202 million years old, that classification would move the group’s origins back by approximately 35 million years.

The decision proved contentious, although the researchers felt they had thoroughly assessed the species’ physiology.

“We were therefore surprised, perhaps even shocked, that in 2023 another team of academics suggested that Cryptovaranoides was not a lizard or even a lizard relative, but in fact an archosauromorph, more closely related to crocodilians and dinosaurs,” says vertebrate palaeontologist Michael Benton.

In a new paper, the Bristol team has examined the fossil more closely to address those objections. X-rays, CT scans and phylogenetic analysis offer further support for its initial finding: Cryptovaranoides is a squamate, placing the group’s origins in the Late Triassic period.

“All the details of the skull, the jaws, the teeth, and the limb bones confirm that Cryptovaranoides is a lizard, not an archosauromorph,” says Benton. “In our new paper, we provide great detail of every criticism made and we provide more photographs of the specimen and 3D images from the scans, so everyone can check the detail.”

The fossil evidence and evolutionary analysis

The dispute’s details are technical, but the new paper systematically answers the points raised by the other researchers, who were led by Yale evolutionary biologist Chase Brownstein.

The Bristol scientists first set out what they consider major observational errors in the rival team’s examination of the fossil. Brownstein and colleagues apparently overlooked features of the limbs and skull that point to a squamate, which the Bristol team highlights in new images. The researchers then discuss a series of bone characteristics supporting their original classification.

Their strongest counterargument, however, comes from a phylogenetic analysis. This method establishes where a species sits on the evolutionary tree by examining traits it shares with related organisms.

“This is where we code hundreds of anatomical features in Cryptovaranoides and other modern and fossil lizards, as well as various archosauromorphs,” says Bristol geologist David Whiteside.

The team used this approach to identify three principal features placing Cryptovaranoides in the superorder Lepidosauria, which includes Squamata but excludes archosauromorphs.

A further ten features place it in the Pan-Squamata clade, while two assign it specifically to Squamata. Finally, four features narrow its position to Anguimorpha, a squamate suborder that includes monitor lizards, beaded lizards and Komodo dragons.

By contrast, the team argues that assigning the fossil to Archosauromorpha, as Brownstein and colleagues proposed, would mean dismissing all those features. The Bristol researchers say the few characteristics that might support that classification resulted from mistakes by the other team.

“We ran the analysis time after time, and it gave our original result, that the little Bristol reptile is indeed the world's oldest modern-type lizard,” says Whiteside.

Whichever classification is correct, the modest reptile could not have known what debate it would provoke 200 million years later as it scurried beneath the feet of early dinosaurs.

The research appeared in the journal Royal Society Open Science.