It is of course inevitable that similar features undergoing similar evolutionary pressures will convergently acquire similar form. Here’s a nice example from the theropods. Above is the foot of Tyrannosaurus which is a pretty classic ‘running’ foot. The metatarsals are compressed into a single main spar (which will add efficiency), and the toes are quite well spread (giving grip and stability). And below we have the foot of a moa. Despite the fact that there’s quite an evolutionary distance between the two, and of course that after the tyrannosaurs came long theropods had kinda become birds and been living in trees and flying for a good few million years before coming back down. But again we see similar adaptations, the metatarsals are now fused fully into a single unit, and the toes are rather well spread out.
Is this really convergence though? The forms are related, its not /merely/ because of similarity of function that they have a similarity of form.
On the other hand, presumably there were plenty of non-running forms between the last-common-ancestor-of-Moa’s-and-Tyrannosaurs and Moas themselves, so its not a retained form, inherited unchanged.
Whats interesting also is that, as far as I know, we /don’t/ see too many examples of similar structure in, say, antelolopes or other fast running creatures, as far as the splayed digits. Begs the question, to an extent, why no hoofed-theropods or single pedal digit theropods.
It’s definitely convergence, as the last common ancestor of tyrannosauroids and maniraptors (including birds) would have been a very basal coelurosaur. As for hoofed/single pedal digit theropods, the closest is probably the ostrich.
There aren’t any cursorial carnivorans with hooves either. Presumably hooves are advantageous for herbivores but not carnivores which use their claws for getting food and preventing it from running away.
Well as you say there is a divergence in foot form between tyrannosaurs and moas before the return of the shape in a moa. You could call this parallelism as they areancestrally quite similar, but that’s pretty subject too. I certainly don;t think it’s *wrong* to call it convergence.
As for the footshape, this is partly down to substrate – horses don’t do too well on ground that isn’t firm, where a spread out foot will give better grip and not sink in wasting energy. And horses are pretty rare too, there’s only a handful of species that have jsut one solid hoof. Look at the way artiodactyl feet splay out in soft ground, it does make a difference.
“Begs the question, to an extent, why no hoofed-theropods or single pedal digit theropods.”
I thought this would obviously follow from two vs. four feet: animals on two feet can hardly have hoofs, or they’d have severe balancing problems.
Good point, though it partly depends how the foot is built. I don’t think there’s a reason you couldn’t make a hoof the same size and shape as as ostriche’s toes, so you might be able to do it. Just keep making that last ungual bigger and bigger with most keratin over it, and shortening the preceeding phalanges and you might get to something close to a true hoof.
Were moa cursorial? I know there was some variation between them, but I do not recall any moa being truly cursorial. In fact, moa like *Pachyornis* were extremely graviportal. So, I would think any resemblance between the feet of *Tyrannosaurus* and moa would be caused by plesiomorphies rather than convergence.
As for avian ‘hoofs’, the didactyl ostrich is clearly the best living analog. However, fossil ergilornithids were similarly didactyl cursors and at least some dromornithids had very hooflike toes, even if they were still tridactyl.
The relatively tiny, compressed feet of bustards may also be comparable to hoofs.
Well cursorial only really means ‘capable of running’ and even graviportal elephants can break into a pseudo sprint, so I don’t think there’s a terminology issue here. These really can’t be plesiomorphies though – that would require moa inheriting this condition from a shared ancestor with tyrannosaurs and they don’t. There’s obviously a nice long line of fully perching and flying birds with very different feet in between (Archaeopteryx, confuscisornithiforms, enanrnithines etc. etc.) so the foot shape ancestral to moas will have to have come from a perching foot at some point. This is convergence.
I think a real hoof (as much as the definition can be used, as with cursorial and great many other terms, they are all a bit fuzzy) is something that is effectively unguligrade and walking only on the ungual, and there are no birds that do that.