Snakes are everywhere in our legends and mythology . Yet for most of us, our blood runs cold whenever we encounter these strangely undulating, scaly tubes of muscle slithering through the leaf litter.
Author
- Roy Ebel
Research Officer Herpetology, Museums Victoria Research Institute
The loss of an arm or leg poses a challenge. Yet snakes do perfectly well without all four of them. This makes them suspect. It also leaves us unable to fathom how this radical transition produced one of the most successful vertebrate body plans .
More than 4,000 species of snakes are alive today, on every continent except Antarctica , from thread-thin burrowers to massive pythons, on land and in the sea. We have studied them since antiquity , and still the oldest question about them has no answer: what turned snakes into snakes?
We have long suspected their peculiar, limbless body plan is explained by how the earliest snakes lived. In a new study , published today in Nature, my colleagues and I describe a small, exquisitely preserved fossil that brings us closer to an answer than ever before.
Sea or soil?
Snakes are essentially highly modified lizards. Sometime in the age of dinosaurs, one lizard lineage lost its limbs and stretched out its body. The key question is why.
For more than a century, several ideas have competed with each other.
One says the first snakes colonised the water , with a long, limbless body suited for swimming, much like an eel.
A second says they lived on the surface of the land , among leaf litter and vegetation, where reduced limbs could have eased their movement through dense ground cover.
A third says they went underground , losing their limbs and elongating their bodies to pursue a head-first burrowing lifestyle, much like modern blind snakes.
The trouble is that the evidence has rested on a tiny handful of fossils. Fewer than ten early snake skeletons are known from that era.
The skull of a burrower
Our fossil is a rare new witness. It comes from roughly 80-million-year-old rock in São Paulo state, Brazil. We named it Tametara mirim, meaning "adorned" and "small" in the local Indigenous language.
The fossil is one of the best-preserved early snakes found anywhere on Earth. It is also the first articulated snake skeleton from Brazil, with its bones still connected in their original arrangement. We found its lifestyle encoded in its bones - in particular, its skull.
Head-first burrowers build denser, thicker bone in this region. This trait has independently evolved across numerous burrowing lizard lineages. We presume this consolidates the skull against the strain exerted during its use as a digging tool.
Tametara shows exactly this trait combination. In fact, our lifestyle reconstruction positions the fossil among the most specialised head-first burrowers living today, and well away from any lizards and snakes that pursue a more generalist lifestyle.
But the skull roof was not our only line of evidence.
We also digitally rebuilt its brain cavity, the most detailed such reconstruction yet for any early snake. This cavity would have closely matched the shape of the brain in the living animal. The reduced visual centres and simplified forebrain point the same way the bones do: underground.
Tuning their senses
But here is where the story turns.
We ran the same analyses on Dinilysia, a famous early snake from Argentina , and we found something astonishing. The two oldest snakes we can study in this detail had brains more different from each other than most snake lineages alive today. Dinilysia was no burrower. It lived on the surface.
Their brains hint at how differently these two animals sensed their world.
Tametara had the reduced eyesight of a creature that spent its life in the dark. Dinilysia, on the other hand, had senses that were shaped for the open. At the very dawn of their history, snakes were already tuning their senses to very different habitats.
How, then, does this fit with the skull bones, which place the ancestral snake lineage in subterranean space? The truth may lie somewhere in between.
As my colleague Tiago Simões from Princeton University, who led the study, says: "Our findings reframe a long-running debate about how the earliest snakes lived."
Redrawing the debate
We set out to settle the century-old question of what turned snakes into snakes with the firmest reconstruction yet of how the earliest of these reptiles lived.
Instead, we may need to reframe the question: it was never simply sea or soil.
Indeed, our best-preserved early snake was a committed burrower. But the pathway towards modern snakes branched and meandered. No single fossil, however exquisite, can capture the whole of that journey. Each is only a spotlight thrown across a bigger, more convoluted scene.
Shaped underground or not, the snake sliding through the grass today is the survivor of a long, restless experiment, recorded in the bones and brains of animals that have been dead for 80 million years.
This discovery alone has brought us closer than ever to understanding how evolution gave us these iconic yet feared creatures.
Roy Ebel received funding through an Australian Government Research Training Program Scholarship.
