Did any theropod dinosaur have smaller forelimbs (in proportion to body-size) than Tyrannosaurus Rex?
The T-Rex had pretty tiny arms, but I think that I remember reading that they were fairly muscular, for some reason, so they were not exactly useless or vestigial. Am I remembering that right?
Thanks in advance for any info that you can give me.
Well, many flightless birds might qualify, depending on the degree to which wing atrophy has occurred. Monokynus in particular seems to have a forelimb consisting of nothing but a humerus (buried within the breast) and a single stout claw, with appears to project from the breast of the animal.
On your subsidiary question, I can be more help. Tyrannosaurus obviously did not spend its entire life standing bipedally. If you’ve ever observed a biped getting up from a recumbent position (human or otherwise), you’ll notice that forelimbs play a part. It’s speculative but reasonably well settled that the small, strong forelimbs of Tyrannosaurus and its allies were used to anchor the torso as the hind legs moved forward to lift the abdomen, following which the tail counterbalanced the lifting of the head and chest. Without them, the result of scrabbling with the mighty hind legs would be to send the prone tyrannosaur sliding through the dirt face first; with them, small but strong, anchoring it in place, it stayed in one place as the strong hind legs lifted first abdomen and then the rest of the body.
I’ve also heard it speculated (though I don’t know if this is taken seriously among paleontologists) that T. rexes (well, I suppose, a T. rex and a T. regina) would use their forelimbs to hold together while mating.
From Calvin & Hobbes:
…And now back to your regularly scheduled thread
Thank you, Sir.
I’m glad to know that someone on this board has their priorities straight.
Wasn’t there a theory from a few years back that the Tyrannosaurus body was adapted for eating carrion, and the arms may have helped when entering the abdomen of a large herbivore?
You are remembering correctly for Tyrannosaurus.
The most reduced, useless-looking arms among non-avian theropods belonged to the carnotaurine abelisaurs, dinosaurs like Carnotaurus and Aucasaurus. Here’s a diagram of Aucasaurus, notice how the elbow is basically connected to the wrist because the lower arm is so reduced. Very short fingers, too. Detail of the fossil arm.
I prefer the balance theory. The weight of forelimbs create a CG that shifts with posture, and requires more energy in the T-form bipedal stance, and limiting the size and agility of a dinosaur. Once size becomes a greater survival factor than forelimbs, the forelimbs will tend not to develop, and even reduce in size. T. Rex arms have been compared in size to a large human arm. If the limbs had the same proportion of a velociraptor, this would have added a couple of hundred pounds of weight far forward of the balance point above the hind legs. Notice in velociraptors how the body is shorter than the T. Rex, and the arms are much closer to the legs.
The balance theory adds to the T. Rex as efficient predator theory. Scavengers would have less need for either size or agility.
Sure, that explains why the limbs would be small, but the fossil evidence is also that they were heavily muscled. If they were completely vestigial, then you’d expect that the muscles would have atrophied, too.
Most modern predators scavenge or steal kills from other predators when they can. T. Rex probably scavenged, but I think it’s ridiculous to assume that anything with teeth and jaws like that was primarily a scavenger.
That shows that the T. Rex forelimbs weren’t totally useless. The various theories about standing and grasping still hold water. I think grasping for mating would be a big plus. As for gutting a carcass, I think the massive jaw and huge teeth would do a better job.
It’s not the teeth and jaws that are a problem. Scavengers need teeth and jaws just as strong as a predator’s, since they are consuming the same food.
The problem is that it’s highly unlikely that an obligate terrestrial scavenger is even viable. They just couldn’t find fast enough to keep themselves alive. Some birds manage to be dedicated scavengers because flight allows them to scan huge distances to find food, and allows them to reach that food before the scavenging predators beat them to it. For a ground based animals there would need to be a huge number of carcasses being produced each day just to keep them alive.
Yes, “Carnotaurus” was the dinosaur that I was thinking about! I had not heard of Aucasaurus, though.
Thanks for the info!
Motivated solely by intellectual curiosity, I entered Monokynus into a Google image search. 'Twas a fine-looking animal, indeed.
The dinosaur’s name is Mononykus (emended from the original Mononychus, which turned out to already be an insect’s name). Here’s a diagram of a Mononykus arm. It’s short, but there’s more to it than just a humerus and a claw.
edit: wrong thread
And even at that, a vulture will also go after live animals, if they’re weak enough to be easy prey. I don’t think there’s any animal which is a scavenger but not predator, or vice-versa.
There are insects, worms, and various sea life forms that seem to be pure scavengers, dependent on decaying material. These animals may not distinquish between dead animal and plants though. Otherwise, predators seem to be mostly opportunistic, and happy to consume some other creatures kill.
In a sense though, humans sort of fit the scavenger role. Many humans may never kill or eat a freshly killed animal, consuming only those that have died days beforehand. We even use decay to make meat more palatable, as in ham or aged beef. So we are the apex predator in terms of killing, but we prefer to eat meat killed by others and left to rot for a while. Noble savages indeed.
There were both large theropod dinosaurs with large forearms (e.g., Deinocheirus) and small theropod dinosaurs with relatively small forearms (e.g., Compsagnathus). As such, I doubt that maintaining center of gravity was a primary driver in forearm evolution in theropods.
Tyrannosaurs had already begun down the path of reduced forearms well before they became giants. Guanlong, for example, had smaller-than-average forearms, despite being only about 9 feet long (compared with T. rex’s 40-foot length). Dilong, another early tyrannosaur which predates Tyrannosaurus by a good 60 million years, was smaller yet, and also had reduced forearms.
I assume the path to smaller forelimbs started early in the evolution of dinosaurs, and was a necessity in developing T-form bipedalism in the first place. The examples cited just show a continuation of the trend extending into animals of various size and nature. Overall size wouldn’t be a factor in the balance equation, concentrating the body weight around the CG will still make a more agile and efficient animal.
