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Re: Therizinosaurs and the Maniraptora



In a message dated 97-09-06 22:37:39 EDT, m_troutman@hotmail.com (Matthew
Troutman) writes:

<< While looking at the 
 therizinosaur foot it just looks rebroadened and metatarsal 1 is 
 lenghthned slightly. >>

As I've noted repeatedly here, there's much more to the changes in segnosaur
foot morphology than simply "rebroadening" and "slightly lengthening" MT I. I
think I'll just post this stuff on my web site one of these days so I don't
have to constantly fend off these misguided missiles.

<< Come on Dinogeorge the therizinosaur manus is not as specialized as you 
 claim and obviously in early forms the manus is same as the typical 
 maniraptoran manus. >>

Ah, but it's >not< a typical maniraptoran (or manuraptoran) manus. For one
thing, there are those claws... For another thing, the carpus of _Alxasaurus_
doesn't have a semilunate radiale and has more distal elements than in other
maniraptorans. And for another thing, it doesn't have those very diagnostic
short 3rd and 4th phalanges of digit 3. In _Therizinosaurus_ the carpals are
>sutured< to the metacarpals. As described in _Dinosauria_, the forelimb,
girdles, and phalanges of _Therizinosaurus_ are considered "unique," even
though the genus is referred to Theropoda _incertae sedis_.

The concept of _Therizinosaurus_ (and thus segnosaurs in general) as
theropods goes back to Rozhdestvensky's paper of 1970, so I'm not surprised
there's great resistance to removing segnosaurs from Theropoda. They
undeniably had acquired an assortment of theropod characters convergently
here and there during their 100+ million years of cryptic evolution from
prosauropod-like ancestral forms.

<< Well here is my rant. Though nothing is for certain in paleontology it 
 can be said that we have certainty on the therizinosaur issue. They are 
 simply maniraptorans. Phylogenies can be sticky but that's the way its 
 always been. >>

You're right about nothing being for certain, and wrong abuot there being
certainty on the therizinosaur issue. It is almost impossible that segnosaurs
are maniraptorans. They're derived sauropodomorphs in which the forelimb
shares a few maniraptoran features by convergence. No other part of the
segnosaur skeleton appears maniraptoran to any significant extent, neither
the skull, nor the hind foot, nor the pelvis, nor the hind limb, nor the
axial skeleton, nor the tail (what's known of it). All of these skeletal
regions, as well as the forelimbs, derive straightforwardly from the
prosauropod condition without reversals.