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turtles, mammals and rattlesnakes...

This question isn't directly about dinosaurs, but I think it's close
enough that some people here might have answers.

There's an on-going conversation in talk.origins about a protein
sequence (for cytochrome c, I think) which indicates that humans are
more closely related to rattlesnakes than rattlesnakes are to turtles.
Apparently there is still some dispute about the relative affinities
of the animals in these three groups, so it's unclear whether or not
the sequences are consistent with what we know based on other
information.  However, I just read an article in the June issue of
_Natural History_ which might help.

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?  

Any help would be appreciated.


Mickey Rowe     (rowe@lepomis.psych.upenn.edu)