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(Tip of the hat to Tim Williams for the title.)
Carnotaurus arms are ridiculous. They’re tiny, with three stubby clawed fingers and one that isn’t even clawed. And there’s barely any forearm, so the hand sprouts almost directly from the elbow. Short and inflexible.
But Carnotaurus and other abelisaurids aren’t alone in this regard. Other members of the Silly Little Arms Club are spinosauroids, carcharodontosaurids, tyrannosaurids, ratites (ostriches, kiwi birds, etc.), phorusrhacids (”fear cranes”), diatrymatids, etc. Why did this reduction happen so many times across so many different lineages? What’s the common factor?
The most obvious commonality is that these are all theropods. The other major dinosaur groups, the generally herbivorous sauropodomorphs and ornithischians, actually trended in the opposite direction, starting out as facultatively bipedal runners and developing into quadrupeds with bulky forelimbs. This happened at least four times: in sauropods, in coronosaurian ceratopsids, in hadrosauriform ornithopods (which retained some degree of bipedality), and in eurypods (ankylosaurs and stegosaurs).
Why are these theropods different? For some, it may be the predatory lifestyle. In many groups, as body size increases, the heads grow larger as the arms grow smaller. Abelisaurids, spinosauroids, carcharodontosaurids, tyrannosaurids, phorusrhachids, and (possibly) diatrymatids all probably used their massive jaws, lined with sharp teeth (or, in the last two cases, sharp beaks) to catch prey, with assistance from taloned feet. No need for arms.
Interestingly, two other types of large theropod, therizinosaurids and ornithomimosaurs, had the opposite condition: gigantic forelimbs and tiny little heads. Perhaps not coincidentally, these are two of the few Mesozoic theropod groups thought to be potentially herbivorous.
Where does this leave ratites, with tiny heads and tiny arms? Perhaps because they are secondarily flightless, the forelimbs had become too modified for any other use, and so they were reduced when flight was abandoned or completely lost, in the case of dinornithid ratites, a.k.a. moas. Penguins (spheniscids) are the only long-flightless birds to have retained substantial forelimbs, and they arguably still fly, albeit in a different, more viscous medium than most avians.
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