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Re: fore limbs of dinosaurs
In response to Aravind L.:
> In the Mesozoic archosaur evolution has been characterized by
> experiments with the forelimbs. One trend which is very prevalent is
> that they have undergone shortening in a progressive fashion along
> some lines. This was first seen in the rauisuchians and the
> ornithosuchians. Subsequently in the theropod lines like the
> ceratosaurs, allosaurids and maniraptorans it occured independently.
Apart from several clades of flightless avians (hesperornithids and
kiwis spring to mind), reduction in the size of the forelimbs has
occurred several times in theropod evolution - abelisaurs
(_Carnotaurus_ at least), torvosaurids, _Giganotosaurus_ (?an
allosauroid), and tyrannosaurs. In torvosaurids it was principally
the lower elements that were shortened. These bones were also thick
and massive, and coupled with the robustness of the pectoral
girdle, suggests that the forelimbs of these beasts were capable of
As for _Giganotosaurus_, I'm not too sure if the _entire_ forelimb
I wouldn't nominate forelimb reduction as an overall trend in the
megalosauroids, allosauroids, or maniraptorans. It occurred
independently in certain lineages within these groups.
> In many cases like the tyrannosaurs and abelisaurs no apparent
> selective advantage is visible.
Maybe there was no selective advantage. Maybe the arms of abelisaurs
and tyrannosaurs were vestigial and on track to being lost
It is not known if tyrannosaur didactyly occurred before or after or
even simultaneously with reduction of the forelimbs. (George
Olshevsky has some interesting ideas on the reasons on the
development of didactyly.) The Tyrannosauroidea is the only group to
exhibit didactyl mani - _Compsognathus_' forelimbs are too poorly
known to be sure of what they looked like.
Anyway, that's my 2c worth (2.5c Australian)