Yet Another Transistional Fossil - Snake Origins

[UTEOTW]

There has been a long standing debate on the origin of snakes.

Some have thought that they came from smaller, burrowing lizards while others advocated an origin from within a group of larger lizards.

Well a new transitional snake fossil may have answered the question. It still maintains many lizard like features which points the way towards the right basal group.

Skull of the large non-macrostomatan snake Yurlunggur from the Australian Oligo-Miocene, Scanlon, Nature 439, 839-842 (16 February 2006)

Abstract

Understanding the origin and early evolution of snakes from lizards depends on accurate morphological knowledge of the skull in basal lineages, but fossil specimens of archaic snakes have been rare, and either fragmentary or difficult to study as a result of compression by enclosing sediments. A number of Cenozoic fossil snakes from Australia have vertebral morphology diagnostic of an extinct group, Madtsoiidae, that was widespread in Gondwana from mid-Cretaceous (Cenomanian) to Eocene times, and also reached Europe in the late Cretaceous period. Despite this long history, only about half the skull is known from the best-known species Wonambi naracoortensis, and the few known cranial elements of other species have added little further evidence for phylogenetic relationships. Conflicting hypotheses have been proposed for their relationships and evolutionary significance, either as basal ophidians with many ancestral (varanoid- or mosasaur-like) features, or advanced (macrostomatan) alethinophidians of little relevance to snake origins. Here I report two partial skeletons referred to Yurlunggur, from the late Oligocene and early Miocene of northern Australia, which together represent almost the complete skull and mandible. The exceptionally preserved skulls provide new evidence linking Yurlunggur with Wonambi and other madtsoiids, falsifying predictions of the macrostomatan hypothesis, and supporting the exclusion of Madtsoiidae from the clade including all extant snakes.

Some selected quotes from the paper.

The nasals, septomaxillae and vomers are similar to those of Cylindrophis except that the nasals are narrow and separated from the prefrontals by posterior extensions of the external nares reaching the frontals, as in varanoid and mosasauroid lizards but not Dinilysia or any basal modern snakes.
...
The stepped, oblique overlap of the upper and lower elements is comparable to the plesiomorphic postorbital–jugal contact retained in many lizards.
...
The ectopterygoid clasps the large lateral process of the pterygoid both dorsally and ventrally, as in most lizards but unlike any modern snake.
...
Yurlunggur possesses ancestral or intermediate states for each of these characters, previously unobserved in madtsoiids or disputed in Wonambi.

And for a less complicated look, a journalistic view.

New fossils found in Australia tell us that enormous snakes evolved from predatory lizards like goannas, scientists say.

Palaeontologist Dr John Scanlon describes the well-preserved 25 to 20 million-year old snake skulls from the Riversleigh World Heritage site, in today's issue of the journal Nature.

"There's been quite a bit of controversy about what sort of lizards snakes evolved from," says Scanlon, of the Riversleigh Fossils Centre in Mount Isa.

He says one idea is that snakes evolved from small burrowing insect-eating lizards that lost their legs and developed an elongated body.

Another idea is that snakes evolved from relatively large predatory lizards such as goannas.

Scanlon says he hopes the skulls, from a now extinct snake known as Yurlunggur, will settle the debate on the evolution of snakes.

The structure of the jaws and face prove that Yurlunggur, an Arnhem Land Aboriginal term for 'rainbow serpent', is a very primitive snake, says Scanlon.

"It has so many features in common with lizards rather than other modern snakes."

For example, Yurlunggur has a clear jugal bone, similar to a cheekbone, not present in modern snakes.

And, in particular, the skulls show the snake is more closely related to large predatory lizards than to smaller insectivorous ones, says Scanlon.

"It basically shows a relationship between the most primitive snakes and the varanoid lizards such as goannas and mosasaurs," he says, referring to the giant acquatic goannas that lived during the Cretaceous period.

http://www.abc.net.au/science/news/stories/s1567127.htm

 

[Petrel]

Interesting. Goannas are those big toothy lizards that Steve Irwin is always throwing himself upon in the Australian deserts. Better him than me!