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Re: turtles, mammals and rattlesnakes...



Mickey wrote:
>> According to the NH article, turtles evolved during the Permian period
>> from a group of animals known as pareiasaurs.  What I'm wondering is
>> a) how secure is the conclusion that turtles arose via that lineage,
>> and b) given that we accept it, how much help does that give us in
>> determining where the turtle/snake/mammal splits occurred?

Tony wrote:
>This probably refers to the work by Michael Lee, who has described several
>shared derived characteristics linking turtles to pareiasaurs.  I believe it
>has been published in Science.

Yes:
Lee, M. S. Y. (1993) The origin of the turtle body plan: bridging a famous
morphological gap. Science, 261: 1716 - 1720.

> I have seen him present this work, and
>discussed it with him, and it seems very convincing.

I agree, very good paper.

> As pareiasaurs were
>previously thought to be a sister group to Chelonia, I don't see how this
>adds to the mammal/reptile relationship debate..

(rehash of an e-mail message to Mickey):

Basically, it was always a problem trying to quantify turtle relationships
with the rest of the reptiles, mainly because they suddenly appear in the
fossil record in the Mesozoic. Best guesses as to the phylogeny placed them
as a sister group to the diapsids and placed their 'splitting off point'
close to the point where the diapsids radiated (hence the expectation that
sequencing data would pull the turtle & snake together and split off the
mammals). One problem was that the earliest turtle, _Proganochelys_,
although intermediate between all other turtles and the more generalized
amniotes, was thought to have many specializations which precluded it from
being the ancester to all other turtles. A close relationship was
postulated between turtles and another little known group, the pareiasaurs,
from the mid-late Permian of China, Russia and South Africa. However, Lee
has shown that the pareiasaurs are, in fact, the nearest relatives of the
turtles. They share a number of derived characters, some of which - a
reduced presacral count (would you belive a reduction in the number of
vertebrae above the pelvic area?), an acromion process (would you believe
ill tempered? Oh all right , the point of attachment of the clavicle to the
scapular) and a trochanter major (umm the rounded point of attachment for
the iliac muscle on the upper end of the shaft of the femur, honest) - are
absent from other basal amniotes. Also, at least 4 of the apparent
specializations found in  _Proganochelys_ are also present in the
pareiasaurs, making _Proganochelys_ much more likely to be the true
ancestral form of all other turtles.

With a link estabilshed between the turtles and the pareiasaurs, the
turtles now have a much longer fossil record. The splitting off point for
turtles can now be removed from near the more recent diapsid specialization
period and placed farther back at the pareiasaur group origins. This is
closer to the synapsid split, postulated for the late Carboniferous.

Chris Nedin
cnedin@geology.adelaide.edu.au