Dinosaurs are Mesozoic superstars. The largest literally overshadowed other forms of life during their prehistoric heyday, and even now they attract far more attention than any other group of ancient organisms. It’s easy to forget the diverse and disparate species that wove together the ecology that helped support the dinosaurs we are so enchanted by.
This is especially true of the Late Jurassic Morrison Formation. These rocks yielded some of the first dinosaurian superstars – Diplodocus, Stegosaurus, Allosaurus, Ceratosaurus, and more – but in 1987 paleontologist George Callison wanted to remind his colleagues that there was an entire array of “wee fossils” that were often forgotten about. In a paper published by the Museum of Western Colorado in Fruita, Callison highlighted the mammals, smaller crocodiles, pterosaurs, lizards, and other diminutive players that inhabited the same floodplains among the likes of Apatosaurus. Among the lot were a few bones that seemed to mark the early days of a lineage still around us today – fossils that looked as if they belonged to an archaic snake.
The serpent wasn’t published when Callison wrote his paper. And other experts weren’t so sure the bones belonged to a snake. Utah state paleontologist Jim Kirkland, who helped fill in some of the background for this post, remembers that the fossils were too ambiguous to definitively assign to a snake. A lizard seemed a better fit, and this made sense given that snakes and lizards are close evolutionary cousins of each other in the reptile group called squamates.
Almost three decades later, though, Callison has been vindicated. The specimen he alluded to has just been confirmed as among the earliest known snakes. Together with three other species, the Jurassic reptile helps draw back the origin of snakes much further back in time.
From previous finds in Africa, North America, Europe, and South America, paleontologists knew that snakes had evolved by about 100 million years ago and could be found around the globe. They weren’t quite like serpents alive today – some still had hind limbs sticking out from their bodies – but they were undergoing a rapid radiation. From this diversity, paleontologists suspected that they weren’t looking at the origin of snakes so much as an evolutionary bloom already in progress. There were probably older, more archaic snakes. The trick was finding them.
The dawn snakes had been hiding in plain sight for years. A problematic block of 140 million year old fossils from England had helped conceal them. The Jurassic slab contains a variety of small reptile bones, most of which seemed to be from lizards. Some of these bones were given the name Parviraptor and were interpreted as those as a lizard, and they became the standard for interpreting similar fossils found elsewhere. Little bones from Colorado and Portugal, for example, were interpreted as lizards because of their similarity to the bones from England. But University of Alberta paleontologist Michael Caldwell and colleagues have now recognized these fossils as the earliest snakes.
Caldwell and coauthors have named four new snake species spanning 167-143 million years ago, drawing the origin of snakes back over 67 million years into the heart of the Jurassic. The oldest – Eophis underwoodi – is represented by parts of 167 million year old jaws, while Portugalophis lignites lived 155 million years ago in Portugal and the reinterpreted Parviraptor estesi inhabited England about 140 million years ago. And Callison’s fossils have finally been confirmed as falling in the ophidian ranks – the bones he alluded to have been named Diablophis gilmorei, a snake that slithered over fern-covered floodplains about 155 million years ago.
All of these snakes were small, but their exact size is uncertain. Too little is left of them to tell; just pieces of jaw and vertebrae from the front half of their bodies. But these seemingly sparse remains are still enough to tell that Diablophis and kin really are snakes. Even though snakes are modified lizards, they can be distinguished by features of their skulls and teeth. For example, snakes both ancient and modern have short, strongly-recurved teeth with shallow roots and three-sided tooth sockets.
Even though these ancient snakes probably looked different than those sliding along their bellies today – they likely still had hind legs, for starters – Caldwell and colleagues argue that the fossils show the typical snake skull evolved very early in the group’s history. A snake is not defined by a long, legless body, but rather by shared features that show up in the skull. (The same is true of other groups of animals – whales are not united by blowholes or blubber, but by a thickening of part of their ear bones.) This means that the very first snakes were probably almost indistinguishable from their lizard ancestors, identifiable primarily by subtle skull features. As paleontologists continue to search for early serpents, Caldwell and coauthors write, “the fossil record of snake evolution will likely reveal four legged, short bodied ‘stem snakes’ that possess ‘snake’ skull anatomies.” The hunt for the four-legged snakes is on.
Caldwell, M., Nydam, R. Palci, A., Apesteguía, S. 2015. The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution. Nature Communications. doi: 10.1038/ncomms6996
Callison, G. 1987. Fruita; A place for wee fossils, in Averett, W.R., ed., Paleontology and Geology of the Dinosaur Triangle: Museum of Western Colorado, Grand Junction, Colorado. pp. 91–96.