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Re: polarity of bipedality in dinosaurs
In a message dated 96-09-23 13:21:50 EDT, longrich@phoenix.Princeton.EDU
> No one has ever answered this question for me: are digits IV
> and V clawed?? I know they are unclawed in some dinosaurs. If they
> are unclawed in theropoda, then there is little reason to keep them
> since they are of little use in walking (possibly why ornithopods
> retain them) and little use in grasping. If these digits were
> unclawed, then this would seem to me to be a more parsimonious
> explanation of their loss.
Manual digits IV and V were generally unclawed, that is, the ungual phalanx
was not strongly curved and sharply pointed, in any dinosaur that had them.
In some dinosaurs, however, notably _Iguanodon_, these unclawed digits
retained considerable grasping function. For that matter, >all< the manual
digits are unclawed in primates (including ourselves) without this affecting
their grasping function. So grasping and the presence of claws are not
particularly interdependent. Likewise walking.
The point is that, yes, there are a number of reasons why manual digits might
be lost. But how many of these reasons can >also< account for, say, stiffened
theropod tails, hollow theropod bones, keeled theropod sternum, retroverted
hallux, etc.? If you need a different reason for each major character that
arose in theropods as they evolved into birds, you haven't got an
explanation, just an _ad hoc_, incoherent collection of reasons, i.e., BADD
When you find a >unified, economical< hypothesis that purports to explain all
the items I mentioned in my earlier post (and more), and it is different from
BCF, then BCF will indeed have some worthy competition.