When you ask an average person what a theropod is you will most likely get either a blank stare, or “Tyrannosaurus!!!” or “RAPTOR!!!!” yelled at you. Carnivores, both of them, and quite potentially hypercarnivores at that. Other people will start talking about “raptors” and so forth – and it will all be killing and scavenging and so on. Even if you happen to meet one of these enlightened ones who knows that extant birds are theropods, very likely you’ll hear about Zombie Tits eating bat and bird brains, about falcons impact-killing other birds in the air, and whatnot. Yes, the vast majority of people is aware that many birds are not carnivorous, but if you use the term “theropod” or “dinosaurs” they will instantly forget.
Extant theropods, however, differ enormously in their diets. Yes, lots of them are pure carnivores, just think of all the eagles and falcons and so on out there. Insectivorous species abound, too. And then there is the enormous range of birds who will basically eat anything, with a strong preference for meat – invertebrate or vertebrate. Most gulls come to mind, and many corvids, too. The rest? Lots of highly specialized or generalist herbivorous species, fruiteaters, seedeaters, whatnoteaters.
And in the fossil record? Are there indications that all this diversity in diet existed before the K/T boundary? Before active flapping flight? Before any kind of flight? Or way further down the tree, let’s say end-Triassic?
No, I won’t really answer those questions in this post. Rather, they lead me to an issue Darren Naish of TetZoo fame recently brought up in a comment on a Facebook post of the photo of a Pal-nut vulture (Gypohierax angolensis) below:
“To those who don’t know: Gypohierax is an obligate frugivore. I’ve been trying to find out whether we’d known this from its anatomy.”
I don’t know much about vultures, but the German wikipedia page on the Palmgeier has some helpful info:
“Palmgeier sind mittelgroße, recht kräftig gebaute Greifvögel mit breiten, aber relativ kurzen Flügeln, einem sehr kurzen, gerundeten Schwanz, recht kleinem Kopf und kräftigem und langem Schnabel.”
(from de.wikipedia.org/wiki/Palmgeier under CC-by-sa-3.0 license)
So, Palm-nut vultures are mid-sized, rather sturdily built raptors with a broad but relatively short wings, a very short and rounded tail, a rather small head and strong and long beak.
Hm, wing proportions. The depth of a wing is hard to tell from a fossil, unless the feathers are preserved. Tail shape and length? Same. Overall size is not a tell-tale sign of adaptation to an unusual diet either. A small head? The beak size and shape? Hm, especially the latter might trigger me to think “odd for a vulture, very loooong”. So if we found this animal as a feathered fossil, we might start to think that golly, this does differ quite a bit from many other Old World vultures. But would we conclude that adults’ diet consists of up to 2/3 fruit, and juveniles even over 90% fruit? I am quite sure we would not!
One more – note that the bird is blinking!
So overall, should we expect to recognize atypical diets in fossils? Not normally, if the species in question hasn’t had much time to adapt. Because, as commentor Dartian wrote in response to a TetZoo post on vultures here, usually, it takes a while for morphology to catch up with ecology.
To emphasise the point, here is another atypically feeding raptor, the Pern or European Honey Buzzard (Pernis apivorus). Look at it and tell me you can tell that this bird’s diet consists mostly of bee and wasp larvae!
So the take-home message today is: you can deduce the typical diet of a major group from the features you find consistently on the skeletons of the group. But you can’t tell the diet of individual species with any kind of certainty!