Thursday, 23 July 2015

Now! A Four-Legged Transitional Lizard Snake!

Four-legged snake, Tetrapodophis amplectus
Source: Science Daily.
Image credit: University of Portsmouth.
A four-legged snake from the Early Cretaceous of Gondwana | Science.

I'm getting a bit tired of all these transitional fossils. It's getting so you can't pick up a decent biology or palaeontology journal without another team of researchers having found yet more of them. It's also getting so you can't turn to any creationist website, blog or Facebook group without seeing creationists flatly denying that they exist and refusing to look at the contradictory evidence.

I'm now seriously thinking of starting a new blog called No Transitional Fossils. The problem is I'm not sure I'd have time to write about them all and still have time for this one.

Tetrapodophis amplectus (artist's impression)
Source: New Scientist
Image credit: Julius T. Cstonyi
The latest one, discovered by a team from the University of Portsmouth, UK, is this one from the Early Cretaceous Era, found in Brazil in a geological formation that was once part of Gondwana. It is a snake with four limbs which lived about 110 million years ago.

Snakes are a remarkably diverse and successful group today, but their evolutionary origins are obscure. The discovery of snakes with two legs has shed light on the transition from lizards to snakes, but no snake has been described with four limbs, and the ecology of early snakes is poorly known. We describe a four-limbed snake from the Early Cretaceous (Aptian) Crato Formation of Brazil. The snake has a serpentiform body plan with an elongate trunk, short tail, and large ventral scales suggesting characteristic serpentine locomotion, yet retains small prehensile limbs. Skull and body proportions as well as reduced neural spines indicate fossorial adaptation, suggesting that snakes evolved from burrowing rather than marine ancestors. Hooked teeth, an intramandibular joint, a flexible spine capable of constricting prey, and the presence of vertebrate remains in the guts indicate that this species preyed on vertebrates and that snakes made the transition to carnivory early in their history. The structure of the limbs suggests that they were adapted for grasping, either to seize prey or as claspers during mating. Together with a diverse fauna of basal snakes from the Cretaceous of South America, Africa, and India, this snake suggests that crown Serpentes originated in Gondwana.

As though to taunt creationists with this refutation there is an interesting Editor's Summary:

Editor's Summary
It may surprise you to learn that snakes, like us, are tetrapods derived from an ancient four-legged ancestor. Martill et al. describe a fossil from the Brazilian Cretaceous period that contains a snakelike species that is elongate and serpentine, with both hind- and forelimbs (see the Perspective by Evans). This species appears to have been a burrower and shows clearly the early transitional stages from a lizardlike body plan to the smooth legless snakes we know today.

The team have classified this specimen as a snake rather than a lizard on the basis that:
  • A skull with distinctly snake-like features.
  • Typical snake dentition.
  • An elongated body (rather than a tail) with 150 vertebrae.
  • A single row of scales on the underside, typical of snakes but unlike lizards.

A four-legged snake seemed fantastic and as an evolutionary biologist, just too good to be true, it was especially interesting that it was put on display in a museum where anyone could see it... It is a perfect little snake, except it has these little arms and legs, and they have these strange long fingers and toes.

The hands and feet are very specialised for grasping. So when snakes stopped walking and started slithering, the legs didn't just become useless little vestiges -- they started using them for something else. We're not entirely sure what that would be, but they may have been used for grasping prey, or perhaps mates.

Dr Nicholas R. Longrich
Quoted in Science Daily
So there is little doubt that this is a four-legged snake, not a long, thin lizard, although it is obviously transitional between the two, even having lizard-like grasping toes.

It has been widely believed in paleontological and evolutionary biology circles that snake evolved from lizards, but there was no general consensus about when they did so, what type of lizard they evolved from, and more importantly, why they did so. This fossil appears to clarify several of these points. For example, it appears to be a skeleton adapted for burrowing which resolves the question of whether snakes evolved from a burrowing or a marine lizard. The ability to burrow would have given access to a new source of food for a lizard as well as a degree of protection.

This in turn would give an advantage to reduced limbs and an elongated body. So, it seems probable that the basic body plan for snakes evolved for burrowing. Only later did this body plan turn out to be useful for living above ground, tree-climbing, swimming, etc. enabling snakes to radiate into the many families, genuses and species we see today.

And, most disconcertingly for creationists, especially given their hilarious fairytale version of a magic speaking serpent being condemned to crawl on it's belly and eat dust (sic) for telling Eve the truth about their god's lie, we can now see a clear transitionary progress from lizards to snakes over millions of years, which, incidentally, places snakes firmly in the same major branch as the other tetrapod vertebrates.

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