An analysis of the first three-dimensionally preserved skulls and skeletons of the extinct legged snake Najash rionegrina shows that nearly 100 million years ago (Cretaceous period), legged snakes still had a cheekbone — also known as a jugal bone — that has all but disappeared in their modern descendants, and that snakes possessed hind legs during the first 70 million years of their evolution.

The evolution of the snake body has captivated researchers for a long time, representing one of the most dramatic examples of the vertebrate body’s ability to adapt. But a limited fossil record has obscured our understanding of their early evolution until now.

“Snakes are famously legless, but then so are many lizards,” said Dr. Alessandro Palci, a researcher at Flinders University.

“What truly sets snakes apart is their highly mobile skull, which allows them to swallow large prey items.”

“For a long time we have been lacking detailed information about the transition from the relatively rigid skull of a lizard to the super flexible skull of snakes.”

“Our findings support the idea that the ancestors of modern snakes were big-bodied and big-mouthed — instead of small burrowing forms as previously thought,” added Dr. Fernando Garberoglio, a scientist in the Fundación Azara at the Universidad Maimónides.

Dr. Palci, Dr. Garberoglio and their colleagues performed high-resolution (CT) scanning and light microscopy of Najash rionegrina skulls from northern Patagonia, Argentina, to reveal substantial new anatomical data on the early evolution of snakes.

“Najash has the most complete, three-dimensionally preserved skull of any ancient snake, and this is providing an amazing amount of new information on how the head of snakes evolved,” Dr. Palci said.

“It has some, but not all of the flexible joints found in the skull of modern snakes. Its middle ear is intermediate between that of lizards and living snakes, and unlike all living snakes it retains a well-developed cheekbone, which again is reminiscent of that of lizards.”

“Najash shows how snakes evolved from lizards in incremental evolutionary steps, just like Darwin predicted,” added Professor Mike Lee, from Flinders University and South Australian Museum.

“This research revolutionizes our understanding of the jugal bone in snake and non-snake lizards,” said Professor Michael Caldwell, from the University of Alberta.

“After 160 years of getting it wrong, this paper corrects this very important feature based not on guesswork, but on empirical evidence.”

The new snake family tree created by the team also reveals that snakes possessed small but perfectly formed hind legs for the first 70 million years of their evolution.

“The study reveals that early snakes retained their hindlimbs for an extended period of time before the origin of modern snakes which are for the most part, completely limbless,” Dr. Garberoglio said.

“These primitive snakes with little legs weren’t just a transient evolutionary stage on the way to something better,” Professor Lee said.

“Rather, they had a highly successful body plan that persisted across many millions of years, and diversified into a range of terrestrial, burrowing and aquatic niches.”

The results were published in the November 20, 2019 issue of the journal Science Advances.

_____

Fernando F. Garberoglio et al. 2019. New skulls and skeletons of the Cretaceous legged snake Najash, and the evolution of the modern snake body plan. Science Advances 5 (11): eaax5833; doi: 10.1126/sciadv.aax5833