I recently got an angry email from a BANDit (or rather, MANIAC [maniraptorans are not in actuality coelurosaurs]) believer when I called a bird a theropod. Well, for a good take-down of BANDitry up to that date, see Prum 2003. Since then, the BANDits have been active, but not successful in convincing anyone. That’s because they have no new arguments that stand the simplest tests, and because they ignore all criticism. Oh well, I don’t really care much about kooks (want a really crazy kook? Lookie here), I care about dinosaurs. Here’s the largest extant theropod, male and female.
Struthio camelus (REPTILIA: ARCHOSAUROMORPHA: ARCHOSAURIA: DINOSAUIRA: SAURISCHIA: THEROPODA: COELUROSAURIA: MANIRAPTORA: PARAVES: AVIALAE: AVES: ORNITHURAE: CARINATAE: NEORNITHES: PALAEOGNATHAE: STRUTHIONIFORMES), photo taken at the Berlin Zoologischer Garten.
and a close-up of the feet of the female.
Some important things to note:
1) Ostriches have the second-most reduced feet in all walking Dinosauria, with one main toe bearing the largest part of the weight, and a second, much-reduced toe providing additional stability. (thanks to Anonymous for pointing Sarmientichnus scagliai Casamiquela 1964 out in a comment!)
2) Ostrich feet, as all bird feet, are very slim, very little muscle mass is located on the feet. All the toe flexors and extensors and so on operate the joints via long tendons, so that the muscle bellies can be far up the limb. That moves the center of mass up, and thus closer to the point of rotation (the hip), which reduces the rotational inertia of the limb. We see the same thing in cursorial mammals, btw.
3) Ostriches are fully digitigrade. The metatarsus is off the ground – period! Yes, that long, near-vertical things is the middle foot, not the shank. And its distal end is off the ground! OFF! If non-avian dinosaurs walked this way (and the feet sure look the same), what, then, created the heel impressions we see in fossil footprints?
EDIT: Darren Naish’s post at the famous TetZoo about ostrich dissection is worth a thorough read!