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Re: Therizinosaurs and the Maniraptora
In a message dated 97-09-04 19:21:46 EDT, firstname.lastname@example.org writes:
<< Oh it is not such a remarkable or unbelieveable occurance. a genetis
quirk can reevolve a certain body part. If what you say is true then the
occasional horse with 3 toes and the whale with large and presumeably
functional hindlimbs is impossible. Evolution is not a one way street it
is a many way street with many options. While looking at the
therizinosaur foot it just looks rebroadened and metatarsal 1 is
lenghthned slightly. >>
I know that there are circumstances under which lost body parts can reappear
in an individual, because such lost parts are retained in various early
stages of embryological development and typically lost during later stages.
When normal digital loss is retarded in the embryo, the digits will be
expressed postnatally. This, however, is a pathological or congenital
occurrence in isolated individuals that, like polydactyly, seldom if ever
will spread through the entire species. I maintain not that regaining a lost
digit is impossible, but that it is highly improbable.
In view of the great number of characters shared by segnosaurs and
prosauropods, especially including the configuration of the hind-foot bones
but also in the skull, I consider placing segnosaurs into Theropoda is
presently quite unwarranted. If you read and reread, with a skeptic's eye,
Clark et al's paper on the skull of _Erlikosaurus_, which I've done many
times now, each time finding more inconsistencies and vague character
definitions, you'll be far less convinced of the closeness of
segnosaur-theropod relationships. Although they were concerned with the skull
and their analysis uses skull characters almost exclusively, it would have
been nice if they had explained how they think segnosaur feet might have
developed from typical theropod feet.
Segnosaurs aren't bullatosaurs, nor coelurosaurs, nor manuraptorans, nor
tetanurans, despite sharing a few minor, random characters here and there
with each of those groups. Their best placement, taking into account all
available evidence, cranial and postcranial, is somewhere between
prosauropods and ornithischians in Phytodinosauria.
It's too bad transmitting pictures is such a nuisance via e-mail. Otherwise
I'd prepare some illustrations of prosauropod body parts alongside
corresponding segnosaur body parts, so that you all could see just what I'm
talking about. Maybe next year, when my personal circumstances are a bit less
chaotic, I'll put up a dinosaur phylogeny web site with illustrations; then I
won't have to repeat these arguments time after time.