Some unusual imaging work done on
a fossil snake has uncovered vestigial legs pretty well confirming that the
snake evolved from land based reptiles and ending alternative explanations.
The method of imaging is also of
great interest as it makes work on fossils possible, although I am certain that
it is impossibly expensive. So at first
we answer the big questions.
The interesting question is whether
we actually have multiple evolutionary routes for the snake types. The convenience of a legless body is not
unique to a narrow niche and it would not be surprising to have multiple parallel
evolutionary 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 the
previously hidden second leg ofEupodophis. The bones are artificially colored
to highlight the internal structure of the bone and show how the snake's leg
grew. Credit: A. Houssaye
A novel X-ray imaging technology is helping scientists better
understand how in the course of evolution snakes have lost their legs. The
researchers hope the new data will help resolve a heated debate about the
origin of snakes: whether they evolved from a terrestrial lizard or from one
that lived in the oceans. New, detailed 3-D images reveal that the internal
architecture of an ancient snake's leg bones strongly resembles that of modern
terrestrial lizard legs. The results are published in the 8 February issue of
the Journal of Vertebrate Paleontology.
The team of researchers was led by Alexandra Houssaye from the Museum
National d'Histoire Naturelle (MNHN) in Paris, France, and included scientists
from the European Synchrotron Radiation Facility (ESRF) in Grenoble, France,
where the X-ray imaging was performed, and the Karlsruhe Institute of
Technology (KIT), Germany, where a sophisticated technique and a dedicated
instrument to take the images were developed.
This movie from computer graphics images illustrates the
fossil Eupodophis descouensi and the 3-D model of the snake leg
reconstructed using synchrotron-radiation computed laminography. Credit: P.
Tafforeau/ESRF
Only three specimens exist of fossilised snakes with preserved leg
bones.Eupodophis descouensi, the ancient snake studied in this experiment, was
discovered ten years ago in 95-million-year-old rocks in Lebanon
The high-resolution 3-D images, in particular the fine detail of the
buried small leg, suggest that this species lost its legs because they grew
more slowly, or for a shorter period of time. The data also reveal that the
hidden leg is bent at the knee and has four ankle bones but no foot or toe
bones.
"The revelation of the inner structure
of Eupodophis hind limbs enables us to investigate the process of
limb regression in snake evolution," says Alexandra Houssaye.
This is a photograph of Eupodophis descouensi, a fossil snake from
the Cretaceous Period (95 million years ago) of Lebanon
The scientists used synchrotron laminography, a recent imaging
technique specially developed for studying large, flat samples. It is similar
to the computed tomography (CT) technique used in many hospitals, but uses a
coherent synchrotron X-ray beam to resolve details a few micrometers in
size--some 1000 times smaller than a hospital CT scanner. For the new
technique, the fossil is rotated at a tilted angle in a brilliant high-energy
X-ray beam, with thousands of two-dimensional images recorded as it makes a
full 360-degree turn. From these individual images, a high-resolution, 3-D
representaton is reconstructed, which shows hidden details like the internal
structures of the legs.
"Synchrotrons, these enormous machines, allow us to see
microscopic details in fossils invisible to any other techniques without damage
to these invaluable specimens," says Paul Tafforeau of the ESRF, a
co-author of the study.
Provided by European Synchrotron Radiation Facility
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