Re: [dinosaur] Etrigansauria, new clade of Ceratosauria (free pdf)

[Tim Williams <tijawi@gmail.com>]

Mickey Mortimer <mickey_mortimer111@msn.com> wrote:

> Etrigansauria is just a junior synonym of Neoceratosauria, which is
> basically ignored by Delcourt.

I agree; using Neoceratosauria for the Ceratosauridaeâ+âAbelisauridae
clade is not dependent upon Noasauridae being part of this clade.

The phylogenetic taxonomy might not be ideal, but Delcourt's
paleobiological inferences regarding abelisaurids are interesting.
One suggestion (raised by DelCourt, and others) is that because
abelisaurid forelimbs were so reduced, especially the distal elements
(and in some cases the digits apparently lacked claws), the forelimbs
may only have been used for display.

I can't argue with that.  In fact, I'd go further, and say that
display might have been the principal purpose of the forelimbs in many
non-avialan theropods - not just short-armed theropods, but their
long-armed relatives as well.  Even for long-armed theropods, the
mobility and anterior reach of the forelimbs was quite limited.  So
for predatory theropods, the apparently strong, clawed forelimbs were
probably not much use for catching prey, or for bringing food to the
mouth.  Maybe the clawed forelimbs were used to help subdue or disable
prey already caught - but only against 'large' prey (i.e., prey big
enough that it could be grasped or impaled with both hands).  The
forelimbs would have been useless for seizing or dispatching small
prey.  For many theropods (carnivores that targeted small prey, or
herbivores), I suspect the claws were retained for close-range
intraspecific combat, or as a defense of last resort.

Abelisaurids (like tyrannosaurids) have large scapulocoracoids, in
contrast to the rest of the forelimb.  The large size of the
scapulocoracoids might suggest that the forelimbs were capable of
great strength; but this may not be true, and scapuloccoracoid size
could be irrelevant to the precise function of the forelimbs. Delcourt
cites possible reasons for the lack of reduction of abelisaurid
scapulacoracoids.

In terms of 're-purposing' the forelimbs, abelisaurids went in one
direction - the forelimbs were completely reduced, and the hands
possibly became vestigial.  Paravians went in another direction - the
forelimbs (including the hands) became lengthened and furnished with
long feathers, all for the purpose of ornamentation.  One hypothesis
is that large feathered 'wings' then became co-opted for locomotion
(possibly multiple times).  But in both abelisaurids and bird
ancestors the extreme reduction and extreme elongation of the
forelimbs, respectively, could reflect a long-standing absence of
predatory/grasping function.  Certain deinonychosaurs might have used
their forelimbs for dual purposes (predation and display), but I doubt
this was true of the first birds (avialans).  (I also highly doubt
that basal birds used their forelimbs for tree-climbing.)