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Re: Removing segnosaurs from ...



In a message dated 95-08-31 10:53:47 EDT, jdharris@lust.isem.smu.edu (Jerry
D. Harris) writes:

>        But with the prosauropod/therizinosaur problem, the fourth
>(actually, the first) digit never really disappeared!  You claimed that the
>functionally tetradactyl pedes of therizinosauroids could never be derived
>from that of a theropod pes because theropods are tridactyl.  Untrue:  most
>are _functionally_ tridactyl, but are in reality tetradactyl.  The reversal
>in question would come from re-enlarging that first digit and making it
>functional once again.  This is far, far easier to believe, it is true,
>than having the toe reappear if it had originally disappeared
>entirely...but, of course, that never happened.

The "disappearing" part of theropod foot anatomy is the proximal end of
metatarsal I, which articulated with the tarsus. I don't see that as coming
back. Originally, I thought that metatarsal I in theropods separated in the
middle into two parts, a proximal and a distal segment, with the distal
segment, carrying the digit with it, retroverting and the proximal gradually
becoming lost, like the splint metatarsal V in some dinos. But I've since
learned that the proximal Mt I illustrated in some theropod-foot drawings may
be a misidentified Mt V, so I'm still open on just how the Mt I retroverted.

Otherwise, the "disappearing digit" is digit V of sauropods, which had at
least two phalanges, which became lost in certain (not all) prosauropods and
segnosaurs. The discussions may have confused the two situations somewhere
along the line.

G.O.