An ancient snake known as Najash rionegrina once roamed the lands of Argentina some 100 million years ago, but it didn’t slither along like the snakes of today. Rather, it boosted itself up with two “small but perfectly formed hind legs” – also known as your worst nightmare – and a recent analysis of its three-dimensional skeleton is helping scientists fill gaps in evolutionary understanding.
Snakes are one of the “most dramatic examples” of evolutionary versatility in vertebrate species. Over time, their bodies got longer and their limbs gradually disappeared while their skulls underwent kinesis. Study author Alessandro Palci told IFLScience that the evolution of snakes is a "great example of how a new complex body plan can evolve" as well as highlights some of the basic mechanisms of evolution.
"For example, how can the environment push a lineage of organisms into evolving different shapes, anatomies, and behaviors? Why do organisms lose their limbs, what are the genetic mechanisms behind it, and are there ecological drivers leading to these transformations?" said Palci.
But our understanding of these adaptations has long been limited by a lack of fossil record.
To fill that gap, an international team of researchers from Argentina and the University of Alberta performed high-resolution computed tomography (CT) scanning and light microscopy of preserved Najash skulls to visualize the ancient snake’s structure and pathways of nerves and blood vessels. It is the first time that researchers have had a complete, three-dimensional fossil of one of the earliest snakes with skull features that are intermediate between those of lizards and of modern snakes, which Palci notes is "kind of expected if you believe in Darwin's theory of evolution."
"Among the most notable anatomical features is an L-shaped cheekbone in the skull of Najash. Many lizards have an L-shaped cheekbone called 'jugal bone'. Modern snakes have an I-shaped bone behind their orbits and most scholars have always thought it cannot possibly be the equivalent of the jugal bone of lizards," said Palci. "In truth, Najash shows us that that bone behind the orbit of modern snakes is a jugal that has simply lost the lower bar of the 'L' in the course of evolution likely to increase [the] flexibility of the jaws."
After 160 years of “getting it wrong,” the study authors write in Science Advances that their research “revolutionizes” what we know about jugal bones in a way that is “based not on guesswork, but on empirical evidence.”
"Our findings support the idea that the ancestors of modern snakes were big-bodied and big-mouthed – instead of small burrowing forms as previously thought," explained Fernando Garberoglio, from the Fundación Azara at Universidad Maimónide, in a statement. "The study also reveals that early snakes retained their hindlimbs for an extended period of time before the origin of modern snakes which are for the most part, completely limbless."
At some point throughout the first 70 million years of the snake's evolutionary history, the jugal bone gradually disappeared, allowing for the evolution of the uniquely flexible and specially adapted skull that is made for consuming larger prey. This separated snakes from their reptilian lizard cousins.
"Unlike most other organisms on the planet that feed regularly on relatively small amounts of food, snakes feed much more sporadically but ingest large prey items whole. These prey items can be almost as heavy as the snakes themselves!" said Palci, adding that if an organism finds itself in an environment where food is scarce, having the ability to feed sporadically can become advantageous.
"This may, in fact, be the reason why snakes survived the mass extinction that wiped out the dinosaurs at the end of the Cretaceous. Of course, in order to swallow large prey items, you need a very flexible, highly mobile skull," added Palci. "Now, why other lizards did not evolve such ability is a mystery to me."