How did snakes evolve? Fossil discovery holds clues.
The fossils of four previously undiscovered snake species, the earliest known snakes, could challenge biologists' perception of reptilian evolution.
What came first, snake heads or snake tails?
The world's oldest known snake fossils, identified by a team led by University of Alberta biologist Michael Caldwell and described this week in Nature Communications, might answer that, as well as many other questions about the origins of our slithery friends.
Until now, the oldest snake fossils on record were estimated to be about 100 million years old. But Dr. Caldwell’s study describes four new snake species, all dated between 170 and 140 million years ago. The oldest of these remains could be placed in the mid-Jurassic period, at the peak of the age of dinosaurs. By comparison, modern humans first appeared on this planet about 200,000 years ago.
The new specimens come from a wide range of geographic locations. Diablophis gilmorei and Portugalophis lignites were found 150 meters below the Earth’s surface, from coal mines in Colorado and Leira, Portugal, respectively. Eophis underwoodi, from a quarry in Oxfordshire, England.
The location of a fossil can tell us a lot about the animal itself, Caldwell says. For example, the fossil now dubbed Parviraptor estesi was excavated from a limestone formation in Dorset, England. Rich in calcium from snail shells and the skeletal fragments of other marine organisms, this limestone was probably composed of lake sediment.
“So we can conclude that at least the animal died in a lake,” Caldwell says, “or at the very least was preserved in one. But snakes are quite often habitat generalists. Many rattlesnakes are completely comfortable swimming in lakes, but you also find them in deserts.”
Caldwell is hesitant to say too much about what these snakes might have looked like – so far, his team has dug up only skull fragments and a few vertebrae. Without a full or partial skeleton, describing the lengths and body types of these new species would be mostly speculation.
That job goes to scientific illustrators. Canadian artist Julius Cstonyi, who was tasked with drawing the prehistoric snakes, describes his process:
"Sometimes there is direct evidence for color patterns in fossils, amazingly enough," says Cstonyi. "However, when only skeletal elements are preserved, I have to assign color patterns to the restoration. In that case, I consider commonly seen color pattern trends among the closest living relatives and among species that fill a similar ecological niche. Color patterns are frequently utilized by life forms to hide in their environment in order to increase their chances of survival, and we often know something about the environment in which a prehistoric species lived, such as the plant species that typified its habitat. Many animals today have evolved color patterns that resemble some aspect of these elements of their environments."
But as a paleobiologist, Caldwell is more reluctant to say what the snake looks like “All we’ve done for certain is prove that snakes were around 167 million years ago, and that we have to go even deeper in time to find their origins,” he says.
But Caldwell says that he is “absolutely certain” that the discovery is a novel one. Because the fossils are so much older than any other known specimens – and because they are “characteristically different” yet still snakelike – Caldwell asserts that “there is no uncertainty that these are new taxons.”
But what makes a snake “snakelike”? According to Caldwell, it’s all in the skull.
“Snake skulls are very distinctive,” Caldwell says. “You would never confuse being bitten by a snake with being bitten by an iguana.”
When it comes to reptilian evolution, the shape of the skull largely indicates eating habits. Snakes have a maxilla, or upper jaw bone, that can move independently of the rest of the skull. Flexible jaw motion, along with inward-curving teeth, allows snakes to swallow large prey. In contrast, a lizard maxilla is sutured to other skull bones, maintaining a tight cranial structure. So despite having only a few skulls to work from, Caldwell was able to conclude that the fossils did belong to early snakes, rather than lizard relatives.
“[The fossils] all show those snake features, so it indicates that the animals were likely eating and behaving the way snakes do today,” Caldwell says.
The discovery of much older fossils suggests that the origin of snakes came earlier than previously thought. In the mid-Jurassic, reptiles experienced a boom of adaptive radiation – the rapid diversification of organisms into new forms. It occurs when new environments become accessible, thereby presenting new environmental challenges and opportunities to specialize. When the supercontinent Pangea broke up into separate land masses, it allowed the squamates – a reptilian order that includes all lizards and snakes – to diversify.
“One of the great cliches in paleontology is, if you find it in the fossil record, it originated before that time,” Caldwell says. “We’ve long pinpointed that period in the Jurassic when skinks, anguimorphs, and geckos popped up. The middle Jurassic appears to be a radiation point for small-bodied lizards, and we’re finally able to add snakes to that point.”
The discovery of new species extends the fossil record of snake evolution by 70 million years. In doing so, it may also challenge previous notions about snake evolution.
So did the heads evolve after the tails? Previous researchers have suggested that snake precursors first developed long, limbless bodies, and only later developed distinctive cranial features. Caldwell posits that the opposite is true. In his study, he predicts that the fossil record will eventually show “four legged, short bodied, ‘stem-snakes’ that possess ‘snake’ skull anatomies.”
“It’s very much an argument by analogy,” Caldwell says. “There are a large number of completely legless lizards today. They’re long, skinny, and they look just like snakes. But their skulls maintain qualities we use to describe lizards. All of the legless lizards are derived from a four legged form – and the same thing happens for amphibians and mammals. We know they all have four legged forms in history, but some lost them in evolution. There’s no reason to conclude snakes developed any other way.”
It’s a sound theory, but Caldwell is going to need more than a few jawbones to make the case conclusively.
“I’m still waiting for the day when we have a four legged snake specimen,” Caldwell adds.
Until then, Caldwell says this discovery can teach us a lot about biodiversity. The differences between a boa constrictor and a python might seem minute – especially to those of us who tremble at the mention of snakes – but the species as a whole is incredibly diverse.
“It makes it very clear that a lot of the snakes we see when we walk outside our doors – in terms of adaptations and anatomy – are very young,” Caldwell says. “The oldest known viper goes back about 20 million years. That’s it. So all this spectacular diversity in modern snakes is very new.”
“Paleontology always provides perspective on biodiversity,” Caldwell adds. “When you discover that time is so deep for a certain group, you begin to get a grand sense of deep time and how it affects modern biology.”