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Giant carnivorous geese
Sure - the takahe has very powerful jaws - as do many other herbivorous
birds as you point out. So on this criterion alone your right to conclude
that by themselves powerful jaws do not a carnivorous bird make. But the
argument first posited for carnivorous Diatryma by Witmer & Rose - and
later invoked re dromornithids by me does not rest on this premise. The
reasoning goes as follows - birds do not process food in their mouths as do
mammals - therefor their skull's need not be larger than necessary to
acquire (not process) their food of choice. Based on this - the prediction
is that as mostly herbivorous avian taxa get larger we would expect the
skulls to become RELATIVELY smaller. This is certainly consistent with what
we know of giant terrestrial birds that definitely eat mostly plant foods -
emu - ostrich - cassowary - moa - elephant bird etc. So the point is not
that dromornithids had large heads and powerful beaks - but that they were
very BIG birds with RELATIVELY large heads and powerful beaks. Put another
way - the Australian palm cockatoo has a large head the size of a man's
fist and with this it can crack the largest of known Australian nuts. If
we up-sized the palm cockatoo by a few hundred kg it would not be necessary
for it to develop a head much larger than the one it has - in fact it would
be gross energetic inefficiency to upscale the head at the same rate unless
it's diet shifted to larger nuts. But in Australia there is no evidence of
larger nuts in the past.
Its also worth noting that there are many species of largely carnivorous
birds that lack hooked beaks or terrifying talons - although I'd agree that
the most specialised typically do sport these features.
On Thursday, August 15, 2002, at 01:18 PM, Williams, Tim wrote:
There may be some variation within the family on that score. According to
one source, amino acid analysis of eggshells attributed to _Genyornis_
indicates a herbivorous diet. And dromornithid talons don't appear designed
for shredding prey. But look at the jaws of _Bullockornis_ - could it be
anything other than a carnivore?
Well, the herbivorous takahe has a very robust skull and mandible, as in
Diatryma or Bullockornis, and much moreso than carnivores like
phorusrhacids or hawks, and parrots or seed-eating finches also have
massive beaks, so it is consistent with more than one interpretation. In
addition, phorusrhacids, owls, and hawks have very well developed hooking
of the upper mandible, which is absent from Diatryma and Bullockornis. One
would predict that a specialized carnivore would show this.
Now parrots are pretty interesting. I was sure that the elongate quadrate
condyle was giving them a sliding-jaw motion of the dentary as in
oviraptorosauria and dicynodonts... the similarity between the shape of the
lower jaws of _Citipati_ and a macaw is just uncanny. From what I can tell,
though, this isn't the case- the elongate quadrate condyles have their long
axes inclined relative to each other so that sliding is pretty restricted
and the jaw acts more like a hinge, maybe with some left-right rotational
movement. The guys with the oviraptorosaurian/dicynodont-like
quadrate-articular complexes are, get this...
Its dangerous to overgeneralize, but generally galliformes are eating a
large amount of plant matter, with some other stuff like insects, fruits,
seeds etc. thrown in, although none that I know of have particularly
elongate dentary symphyses or deep jaws like in the oviraptorosauria.
Interesting implications for the oviraptorosauria.
Stephen Wroe - Homepage - http://www.bio.usyd.edu.au/staff/swroe/swroe.htm