New
Fossil Snake With Legs, Reported In Science
Washington, D.C.
American Association For The
Advancement Of Science
Appearing like the punchline
to an evolutionary riddle, a new fossil snake with legs has emerged
from 95 million year-old deposits near Jerusalem. Its sedimentary
surroundings suggest a seafaring lifestyle for this ancient reptile,
but its advanced anatomy could overturn a current theory about the
marine origin of snakes.
This intriguing new species,
dubbed Haasiophis terrasanctus in the 17 March issue of Science,
is the second limbed snake to come from the site of Ein Yabrud, an
ancient marine environment broadly similar to the still, coastal waters
of today's Bahamian reef.
The first such species, Pachyrhachis
problematicus, plays a pivotal role in a scenario that places
the ancestor of snakes in the sea. In support of Pachyrhachis'
position at the base of the serpent family tree, some paleontologists
have noted features in its skull that they believe single it out as
a transitional link between mosasaurs--gigantic swimming lizards of
the Cretaceous (144-65 million years ago)--and true snakes. This view
contrasts dramatically with the traditional view of small terrestrial
or burrowing lizards as snake ancestors.
A group of scientists, led
by Olivier Rieppel of the Field Museum in Chicago, Illinois and Eitan
Tchernov of the Hebrew University of Jerusalem in Israel, brought
Haasiophis into the midst of this origins controversy after
the fossil had spent years in nameless limbo in a museum drawer. Their
description of the extremely well-preserved fossil, along with an
analysis of its evolutionary relationships, led the scientists to
conclude that their new species was close kin to Pachyrhachis.
Their analysis also indicates, however, that these two snakes were
not primitive ancestors, but advanced snakes similar to modern boas
and pythons.
The new anatomical interpretation
suggests that neither Pachyrhachis nor Haasiophis have
anything to do with snake origins. The finding does undermine the
theory that Pachyrhachis represents an evolutionary link between
marine reptiles and true snakes. Snakes like boas and pythons have
a distinctively mobile skull structure that allows them to nearly
unhinge their jaw in a formidable gape and "walk" their skull over
their prey, dining on meals larger than the diameter of their own
head. The two species of fossil snakes from Ein Yabrud appear to have
skull architecture similar to these modern serpents. Previous studies
of Pachyrhachis had concluded that the snake was incapable
of such kinetic feats, instead adopting a modified gape similar to
that of the mosasaurs as an intermediate step between the rigid skull
of lizards and the mobile skull of higher snakes. "We went back
and looked very carefully at the skulls of Pachyrhachis, Haasiophis,
and lizards like mosasaurs, especially features like the braincase,
the dentition, and the joint in the middle of the lower jaw,"
says Rieppel. "The better preservation of Haasiophis allowed
us to use its anatomy as a guide, and gave us the background to see
just how much these fossils looked like advanced snakes."
But a riddle remains: why
do these two snake species have hind limbs? If legs were the norm
for snake ancestors, it would make sense to see the species' advanced
anatomy as only superficially similar to more modern snakes. On the
other hand, the stubby limbs on the fossil snakes might represent
an evolutionary reversal, where snakes with advanced skull design
regain hindlimbs that were lost or perhaps greatly reduced in their
ancestors.
Rieppel and his colleagues
counted the number of evolutionary steps involved in each possible
scenario, and concluded that the redevelopment of limbs was a more
likely story. "We know of at least 62 lizard and snake lineages
that have undergone some degree of limb reduction," Rieppel notes.
"Since our fossil record of snakes is very poor, we can't exclude
the possibility that limbs in snakes were lost not just once in the
beginning, but several times throughout their history."
Rieppel said that it is difficult
to tell how the legs themselves might have been used, since they are
too small in relation to the animal's whole body to have any locomotor
function. Modern pythons have a rudimentary hindlimb, usually little
more than a "claw" of cartilage tipped with bone that they use during
mating and occasional fighting, and it is possible that Haasiophis'
leg served a similar purpose.
Related books and videos
The
Encyclopedia of Snakes
I
Wonder Why Snakes Shed Their Skin and Other Questions About Reptiles
Living
Snakes of the World in Colour
The
Mini-atlas of Snakes of the World
Nature
Watch: Snakes
Snakes:
a Natural History
Gaby
Roslin Predators - Snakes ~ VHS
Snakes:
The Armed Warriors (1989) ~ VHS
