Thursday, April 10, 2008

Remember when snakes had legs?

ResearchBlogging.orgCan't remember? Well, it was a while back, and fossils of legged snakes have been hard to come by. Another one of those pesky gaps in the fossil record, although we know from the precious few specimens we have and a whole range of other evidence that snakes evolved from lizards that had legs. Not just that - loss of limbs is a recurring theme among a bunch of squamate lineages with snakes being the most prominent. And one of the problems with fossils, especially when you don't have many specimens to play with, is that its not easy to study anatomical detail without risking destroying the fossil! And without such detail, it is difficult to reconstruct exactly what might have happened in the evolutionary trajectory from lizards to snakes. What to do? Well, high energy physics to the rescue, of course!


As reported today by the BBC, a new study used intense X-rays - with much higher energy, one guesses, than found in your dentist's office - to examine one such rare snake fossil to look beneath the surface of the fossil and take a picture of a hidden leg! Here's an excerpt (with photo below the fold):


Researchers at the European Light Source (ESRF) in Grenoble, France, used intense X-rays to confirm that a creature imprinted on a rock, and with one visible leg, had another appendage buried just under the surface of the slab.


"We were sure he had two legs but it was great to see it, and we hope to find other characteristics that we couldn't see on the other limb," said Alexandra Houssaye from the National Museum of Natural History, Paris.


The 85cm-long (33in) creature, known as Eupodophis descouensi, comes from the Late Cretaceous, about 92 million years ago.


Unearthed near the village of al-Nammoura [in Lebanon], it was originally described in 2000.


Its remains are divided across the two interior faces of a thin limestone block that has been broken apart.


A portion of the vertebral column is missing; and in the process of preservation, the "tail" has become detached and positioned near the head.


But it is the unmistakable leg bones - fibula, tibia and femur - that catch the eye. The stumpy hind-limb is only 2cm (0.8in) long, and was presumably utterly useless to the animal in life.



Still how much can one do with one such tantalizing glimpse from a single fossil?


_44555496_snake_fossil466x400.gif
The top picture is a synchrotron view of the visible snake leg;
Synchrotron light in the bottom view illuminates the hidden limb



"Every detail can be very important in establishing the great relationships and that's why we must know them very well," explained Ms Houssaye.


"I wanted to study the inner structure of different bones and so for that you would usually use destructive methods; but given that this is the only specimen [of E. descouensi ], it is totally impossible to do that.


"3D reconstruction techniques were the only solution. We needed a good resolution and only this machine can do that," she told BBC News.


That machine is the European Synchrotron Radiation Facility. This giant complex on the edge of the Alps produces an intense, high-energy light that can pierce just about any material, revealing its inner structure.


For this study, the fossil snake was clamped to an inclined table and rotated in front of the facility's brilliant X-ray beam.


In a process known as computed laminography, many hundreds of 2D images are produced which can be woven, with the aid of a smart algorithm, into a detailed 3D picture.


The finished product, which can be spun around on a computer screen, reveals details that will be measured in just millionths of a metre.


The E. descouensi investigation shows the second leg hidden inside the limestone is bent at the knee.


"We can even see ankle bones," ESRF's resident palaeontologist Paul Tafforeau said.


"In most cases, we can't find digits; but that may be because they are not preserved or because, as this is a vestigial leg, they were never present."


Cool, huh? The only uncool part of the story is that the BBC does not tell us where this is being published in the peer-reviewed literature! I suppose we'll know soon enough. Meanwhile, check out these earlier papers, linked above:


Apesteguí­a, S., Zaher, H. (2006). A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature, 440(7087), 1037-1040. DOI: 10.1038/nature04413


Wiens, J.J., Brandley, M.C., Reeder, T.W. (2006). Why does a trait evolve multiple times within a clade? Repeated evolution of snakelike body form in squamate reptiles. Evolution, 60(1), 123. DOI: 10.1554/05-328.1



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