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
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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
