Re: Giant carnivorous geese

[longrich@alumni.princeton.edu]

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