On Thursday, August 15, 2002, at 06:12 PM, Stephen Wroe wrote:
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.
Roadrunners and marabou stork come to mind as pretty
unspecialized, though they are sort of weirdos in being solo offshoots of
noncarnivorous groups. Where larger clades exploit carnivory (excepting
fish... fisheaters sometimes do, sometimes don't have a hooked beak), a
strongly hooked beak seems to be the rule, e.g. Falconiformes, the New
World Vultures (if they are a separate lineage... and wouldn't "The New
World Vultures" be a cool name for a punk group?), owls; then the shrikes
and skuas have a stronger development of this feature than their songbird
and gull relatives.
The robusticity of the mandible still kinda bugs me. To my
knowledge, carnivores don't show this kinda thing- eagles, hawks, owls,
vultures, etc. all have mandibles which aren't exactly weak, but aren't
exactly bone-crushers either. There's a phorushracid on p. 238 of
Feduccia's book that looks fairly robust but its at an angle so is that
the foreshortening?; _Phorusrhacos_ itself looks fairly eagle-like, i.e.
not particularly robust.
Another thing is the eyes. Owls and eagles of course have
enormous eyes because they are vision-based hunters, and phorusrhacids
have pretty large eyes as well despite being so much larger. The eyes of
_Diatrima_ and _Bullockornis_ are really tiny compared to the huge skull;
and while the eyes are pretty big in some ratites (ostriches and rheas
e.g.) in some others, such as moas and elephant birds, they are
proportionately very small. A lot of theropods look to have very small
eyes but that's at least in part an allometric function of having skulls
up to four feet long. The smaller theropods all seem to show relatively
large eyes.
Yeah the pinhead nature of most ratites does imply that Diatryma
and Bullockornis wouldn't be doing the same thing, but I wonder if these
things could represent a different type of herbivory. The takahe,
Porphyrio mantelli, has a larger head than the swamphen P. porphyrio,
despite its larger size. Swamphens I guess feed on reeds and grasses
among other things; the larger head of the takahe presumably would allow
more and tougher vegation to be taken. The still larger _Apterornis_ also
has a relatively large head and a powerful beak. Is there anything else
out there which is takahe-like in its ecology or form? These things sure
are bizarre, interesting, perplexing critters.
>>>
Sorry Mike - I was contributing to the debate over diet in giant 'geese'
(dromornithids). The presence of gastroliths has been forwarded as an
argument against a carnivorous habit for these birds. In response I point
out that this is anything but a watertight argument given the presence of
gastroliths in some carnivorous dinosaurs.
<<<
Have they found any in unambiguous carnivores? I know they're
seen in coelurosaurs like ornithomimids and Caudipteryx, but both of
these guys look like herbivores to me- ornithomimids being really similar
to ostriches, rheas and emus in a lot of respects, not to mention far and
away more abundant than any of the other, unquestionably carnivorous
theropods, and Caudipteryx is a relative of Oviraptorosauria which
themselves are pretty likely to turn out to be herbivores or omnivores
for a number of different reasons (for one thing, they are pretty
abundant compared to other theropods in Mongolia).
Nick