A Long Lizard with very Short Arms
12:22 am
A 95,000,000-year-old fossil shows the earliest example of vestigial limbs in lizards and may provide insight into the way snakes lost their legs, according to a new paper in the Journal of Vertebrate Paleontology.
The fossil was quarried over a century ago in what is now Slovenia, but recent preparation has shown it to be a new species of the extinct aquatic lizard Adriosaurus. Adriosaurus was well-adapted for cruising the Cretaceous seas. It had heavy, thickened (pachyostotic) ribs to keep it from floating to the surface of the water. It added several extra vertebrae to its spinal column, giving it a long, flexible body. And it had shortened both its front and back legs, allowing it to tuck its limbs along its body and swim with graceful, snakelike undulations.
Detail of the holotype of Adriosaurus microbrachis
Credit: University of Alberta. From the EurekAlert press release.
This newly-described fossil appears to have taken this limb-shortening to another level. Other Adriosaurus specimens possess short yet fully-developed arms, complete with five fingers on a small hand. But this latest specimen shows no sign of hands or forearms, and its humeri (upper-arm bones) are reduced to mere slivers. Drs. Alessandro Palci and Michael Caldwell have named the new species A. microbrachis—the “small-armed” Adriosaurus.
Because of the slender shape and excellent preservation of the existing arm bones, Palci and Caldwell believe that the missing forearms reflect the actual skeleton of A. microbrachis, and were not lost in fossilization. “There was a moment when I said, ‘I think we stumbled on a new fossil illustrating some portion of the aquatic process of losing limbs,’” said Caldwell. “There are lots of living lizards that love to lose their forelimbs and then their rearlimbs, but we didn’t know it was being done 100 million years ago and we didn’t know that it was happening among groups of marine lizards.”
The forelimbs of A. microbrachis were so severely reduced that they could not have played any role in locomotion, either in water or on land. In both environments, A. microbrachis must have slithered like a snake, perhaps using its short, yet fully-developed, hindlimbs to assist in pushing its body forward.
A. microbrachis might provide a snapshot of the morphological changes that occurred during the transition from lizards to snakes. The origin and early evolution of snakes is, at present, riddled with controversy, but one prominent hypothesis suggests that the limblessness and elongation of snakes first evolved as adaptations to an aquatic lifestyle.
Some of the best evidence supporting this hypothesis comes from the fossil record, which preserves an easy-to-follow continuum from terrestrial varanid lizards to the aquatic aigialosaurs and mosasaurs, to the elongated aquatic dolichosaurs, through elongated aquatic forms with reduced forelimbs (including Adriosaurus), finally reaching primitive snakes like Pachyrhachis, which lost their forelimbs but retained vestigial hindlimbs and are widely believed to have been aquatic in habit. Evidence from other fossils, and from myological, developmental, and molecular studies of modern-day lizards and snakes throw their fair share of kinks into this transitional series. But the discovery of Adriosaurus microbrachis adds an interesting data point to future discussions on the origins of snakes.
Further reading:
Palci, Alessandro and Caldwell, Michael W. 2007. Vestigial forelimbs and axial elongation in a 95 million-year-old non-snake squamate. Journal of Vertebrate Paleontology 27 (1): 1–7.
The EurekAlert press release, and a similar story at Science Daily.
Dr. Michael Caldwell’s faculty page, with more info on his work on snake origins. (And I’d be remiss if I didn’t mention his enviable membership in the Luxuriant Flowing Hair Club for Scientists.)
And a tip of the toupee to Afarensis, who brought the story to my attention.
Snakes lost their legs when they were cursed by THE CREATOR GOD to crawl on their bellies (Genesis 3.)
The snake was the most subtile (small but crafty) beast created by THE CREATOR GOD. Other articles on this topice reveal he also had forearms and fingers.
These articles also report his legs were for attacking, but he crawled on his belly and used his shoulders for pushing through the dirt. How silly. If you had arms and legs why would you burrow through the ground or only use your legs to attack.
A reply is appreiciated.
(This is in addition to reply sent minutes ago)
P.S. The legs and arms were shortened when those parts broken & scattered when whatever event occured that made his remains a fossil.
No such disaster has occured on this Earth to create fossils of human or animal remains. When human or animal remains are dug up on this Earth they are in one skeleton piece and not fossil state — and this will be true a billion years from now. Fossils are the host of the original Earth that (as Genesis 1:2 record) was without form and void – gone.
A reply is requested.
The limblessness of snakes appears to have begun during the Cretaceous period, around 100,000,000 years ago by the fossil record, and in some species is still incomplete. Pythons and boas, for instance, still retain some vestigial limbs.
While I tend to view the stories of Genesis as allegorical in nature (i.e., not an accurate representation of past events), some researchers have taken a more literal reading and suggested that the transitional fossils showing limblessness in serpents can help us pinpoint the Fall of Man to somewhere within the Cretaceous period (145,000,000–65,000,000 years ago +/- six days).
Perhaps the most pertinent question for biology is this: did THE CREATOR GOD, as you put it, directly remove the snakes’ limbs as part of (H)is/(H)er curse, forcing them to crawl on their bellies due to limblessness; or, did the curse only require the wayward members of Class Serpentes to crawl upon their bellies, with their subsequent limblessness being an evolutionary adaptation to the divine restrictions placed on their locomotor habits?
I just stumbled onto this page in researching legless lizards, and I have to say that was one of the funniest exchanges I’ve ever read on the Internet. Thanks for the information AND entertainment!