Fossilized remains of four ancient snakes between 140 and 167 million years old are changing the way we think about the origin of snakes, and how and when it happened.
Ancient snakes: (top left) Portugalophis lignites (Upper
Jurassic) in a gingko tree, from coal swamp deposits at
Guimarota, Portugal; (top right) Diablophis gilmorei
(Upper Jurassic), hiding in a ceratosaur skull, from the
Morrison Formation in Fruita, Colorado; (bottom)
Parviraptor estesi (Upper Jurassic/Lower Cretaceous)
swimming in freshwater lake with snails and algae, from
the Purbeck Limestone in Swanage, England. Artist
Credit: Julius Csotonyi
The discovery by an international team of researchers, including University of Alberta professor Michael Caldwell, rolls back the clock on snake evolution by nearly 70 million years.
"The study explores the idea that evolution within the group called 'snakes' is much more complex than previously thought," says Caldwell, professor in the Faculty of Science and lead author of the study published today in Nature Communications. "Importantly, there is now a significant knowledge gap to be bridged by future research, as no fossils snakes are known from between 140 to 100 million years ago."
The oldest known snake, from an area near Kirtlington in Southern England, Eophis underwoodi, is known only from very fragmentary remains and was a small individual, though it is hard to say how old it was at the time it died. The largest snake, Portugalophis lignites, from coal deposits near Guimarota in Portugal, was a much bigger individual at about a metre long. Several of these ancient snakes (Eophis, Portugalophis and Parviraptor) were living in swampy coastal areas on large island chains in western parts of ancient Europe. The North American species, Diablophis gilmorei, was found in river deposits from some distance inland in western Colorado.
This new study makes it clear that the sudden appearance of snakes some 100 million years ago reflects a gap in the fossil record, not an explosive radiation of early snakes. From 167 to 100 million years ago, snakes were radiating and evolving toward the elongated, limb-reduced body shape characterizing the now well known, ~100-90 million year old, marine snakes from the West Bank, Lebanon and Argentina, that still possess small but well-developed rear limbs.
Caldwell notes that the identification of definitive snake skull features reveals that the fossils -- previously associated with other non-snake lizard remains -- represent a much earlier time frame for the first appearance of snakes.
These ancient snakes share features with fossil and modern snakes (for example, recurved teeth with labial and lingual carinae, long toothed suborbital ramus of maxillae) and with lizards (for example, pronounced subdental shelf/gutter). The paleobiogeography of these early snakes is diverse and complex, suggesting that snakes had undergone habitat differentiation and geographic radiation by the mid-Jurassic. Phylogenetic analysis of squamates recovers these early snakes in a basal polytomy with other fossil and modern snakes, where Najash rionegrina is sister to this clade. Ingroup analysis finds them in a basal position to all other snakes including Najash.
"Based on the new evidence and through comparison to living legless lizards that are not snakes, the paper explores the novel idea that the evolution of the characteristic snake skull and its parts appeared long before snakes lost their legs," he explains.
He adds that the distribution of these newly identified oldest snakes, and the anatomy of the skull and skeletal elements, makes it clear that even older snake fossils are waiting to be found.
Caldwell MW, Nydam RL, Palci A, Apesteguía S. 2015. The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution. Nature Communications, 2015; 6: 5996 DOI: 10.1038/ncomms6996