Hidden Leg Revealed in Snake Fossil

Some unusual imaging work done ona fossil snake has uncovered vestigial legs pretty well confirming that thesnake evolved from land based reptiles and ending alternative explanations.

The method of imaging is also ofgreat interest as it makes work on fossils possible, although I am certain thatit is impossibly expensive.  So at firstwe answer the big questions.

The interesting question is whetherwe actually have multiple evolutionary routes for the snake types.  The convenience of a legless body is notunique to a narrow niche and it would not be surprising to have multiple parallelevolutionary lines from other reptiles.

X-rays reveal hidden leg of an ancient snake

February 7, 2011

This is a 3-D reconstruction from synchrotron X-ray images of thepreviously hidden second leg ofEupodophis. The bones are artificially coloredto highlight the internal structure of the bone and show how the snake's leggrew. Credit: A. Houssaye

A novel X-ray imaging technology is helping scientists betterunderstand how in the course of evolution snakes have lost their legs. Theresearchers hope the new data will help resolve a heated debate about theorigin of snakes: whether they evolved from a terrestrial lizard or from onethat lived in the oceans. New, detailed 3-D images reveal that the internalarchitecture of an ancient snake's leg bones strongly resembles that of modernterrestrial lizard legs. The results are published in the 8 February issue ofthe Journal of Vertebrate Paleontology.

The team of researchers was led by Alexandra Houssaye from the MuseumNational d'Histoire Naturelle (MNHN) in Paris, France, and included scientistsfrom the European Synchrotron Radiation Facility (ESRF) in Grenoble, France,where the X-ray imaging was performed, and the Karlsruhe Institute ofTechnology (KIT), Germany, where a sophisticated technique and a dedicatedinstrument to take the images were developed.

This movie from computer graphics images illustrates thefossil Eupodophis descouensi and the 3-D model of the snake legreconstructed using synchrotron-radiation computed laminography. Credit: P.Tafforeau/ESRF

Only three specimens exist of fossilised snakes with preserved legbones.Eupodophis descouensi, the ancient snake studied in this experiment, wasdiscovered ten years ago in 95-million-year-old rocks in Lebanon. About 50 cm long overall,it exhibits a small leg, about 2 cm long, attached to the animal's pelvis. Thisfossil is key to understanding the evolution of snakes, as it represents anintermediate evolutionary stage when ancient snakes had not yet completely lostthe legs they inherited from earlier lizards. Although the fossil exhibits justone leg on its surface, a second leg was thought to be concealed in the stone,and indeed this leg was revealed in full detail thanks to synchrotron X-rays.

The high-resolution 3-D images, in particular the fine detail of theburied small leg, suggest that this species lost its legs because they grewmore slowly, or for a shorter period of time. The data also reveal that thehidden leg is bent at the knee and has four ankle bones but no foot or toebones.

"The revelation of the inner structureof Eupodophis hind limbs enables us to investigate the process oflimb regression in snake evolution," says Alexandra Houssaye.

This is a photograph of Eupodophis descouensi, a fossil snake fromthe Cretaceous Period (95 million years ago) of Lebanon. The black scale bar at thebottom right equals 1 cm. Credit: A. Houssaye

The scientists used synchrotron laminography, a recent imagingtechnique specially developed for studying large, flat samples. It is similarto the computed tomography (CT) technique used in many hospitals, but uses acoherent synchrotron X-ray beam to resolve details a few micrometers insize--some 1000 times smaller than a hospital CT scanner. For the newtechnique, the fossil is rotated at a tilted angle in a brilliant high-energyX-ray beam, with thousands of two-dimensional images recorded as it makes afull 360-degree turn. From these individual images, a high-resolution, 3-Drepresentaton is reconstructed, which shows hidden details like the internalstructures of the legs.

"Synchrotrons, these enormous machines, allow us to seemicroscopic details in fossils invisible to any other techniques without damageto these invaluable specimens," says Paul Tafforeau of the ESRF, aco-author of the study.

Provided by European Synchrotron Radiation Facility

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