Reconstructed skulls from an extinct group of Australian snakes may provide the much-needed missing link in the slippery story of snake evolution, Eleanor Lawrence explains.
New finds of fossil snake skull fragments indicate that the sinuous, legless snake body from a large predatory lizard-like ancestor, perhaps for swimming, or as an adaptation for sliding through dense vegetation. The finds are reported this week by John D. Scanlon from the University of Queensland, Brisbane, and Michael S.Y. Lee from the University of New South Wales, Sydney, Australia.
Skulls and jaws hold the main clues to the course of snake evolution, but for one important group of extinct snakes they have been missing until now. The skull and jaw fragments, which Scanlon and Lee describe in Nature1, come from two related species of extinct snake, Wonambi naracoortensis and Wonambi barrei, which lived in the Pleistocene, just a blink away in evolutionary time. These species belong to a group of medium-sized to giant snakes known as madtsoiids, which are now all extinct.
The madtsoiids have a 90-million-year history in the fossil record, from the mid-Cretaceous, around 100 million years ago, to the Pleistocene, which ended around 100,000 years ago. Their relationships to other snakes, both extinct and living, are crucial to understanding snake evolution. But almost nothing was known about the skulls of madtsoiids -- until now.
Enough bits of skull and jaw have now been found to reconstruct a skull of W. naracoortensis and to identify a new species, W. barrei. And these skulls have many primitive features. In particular, they lack the fully developed flexible joint in the upper jaw and have no flexibility in the front of the lower jaw. These are features that all 'advanced' snakes possess, and that give them the ability to extend their mouths around prey many times larger than themselves.
As snakes evolved from lizards, they gradually acquired more and more flexible and extensible jaws. In true lizards, both upper and lower jaws are inflexible. In the extinct lizard-like mosasurs and in the most primitive extinct snakes, a loose joint has developed in the bottom jaw. In more advanced snakes, a flexible joint has also appeared in the upper jaw, while the most advanced snakes, such as boas and pythons, have, in addition, a joint in the front of the lower jaw which enables the two halves of the jaw to move apart.
With their new information on skull and jaw features, Scanlon and Lee were able to make an extensive skeletal comparison between the madtsoiids and other fossil and living snakes. Their analysis clearly shows that the madtsoiids most closely resemble the more primitive snakes from the early Cretaceous than they do any living snakes. Because all these primitive snakes are fairly sizable and none shows any adaptation to burrowing, Scanlon and Lee conclude that the 'small and subterranean' theories of snake origins must be incorrect. These theories arose because the most primitive of living snakes are indeed small burrowers.
Scanlon,J. D. & Lee, M. S. Y. The Pleistocene serpent Wonambi and the early evolution of snakes. Nature 403,416 2000.
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Lawrence, E. A twist in the tail of snake evolution. Nature (2000). https://doi.org/10.1038/news000127-8