The front legs are held close to the body where they won’t get in the way. The head and jaws are very big and heavy on reptiles, and way at the front, and must be counterbalanced.

Primates have lost their tails by adopting a vertical posture and a complex spine, hip and balance arrangement which is quite unique in the animal kingdom.

My best power-walking speed is four mph. For all that the NatGeo article tried to make T Rex sound slow, I think I’d have a hard time maintaining 12.1 mph for long enough to get away. And I’d have to go around barriers T Rex would just lumber through. I thought citing the extreme case of the fastest human runner at 27mph was deceptive–I’ll bet my “sprint” speed is barely 12.

As one scientist in the article noted, though, morphology only takes you so far. T Rex looks to me like a machine for boring holes in jungle, but that’s just morphology.

It just now occurred to me, listening carefully to both our arguments, we can certainly bend the facts to fit the kind of evolution we favor to have occurred. Just like in politics, eh?

Roy Chapman Andrews found the bones of an Allosaurus mixed up with those of a duckbill dinosaur. The former had probably been killed and buried by a flash flood while dining on the latter. Indeed, the Allosaur’s teeth fit the tooth marks in the duckbill’s bones. But that doesn’t prove the Allosaur actually brought down the duckbill, just that he was eating it.

The deciding facts will have to come from the bones. If it can be convincingly demonstrated by biomechanical analysis that T.Rex’s legs were incapable of moving that body at high speed through heavy undergrowth, then the open country carrion eater hypothesis makes sense. (I actually read a discussion of a biologist who feels he has proven this.) If he is mistaken, the jungle predator concept still holds. But its going to take anatomical observation and calculation to settle that, not reason and “logic” which is what we’re doing. Unfortunately, if sufficient evidence is not available in the fossil record, we may never know for sure. I don’t think we can figure it out using “common sense”.

As for the tiny hands, who knows? I’m inclined to believe they were festooned with colorful feathers used for courtship dances, like coquettish Japanese fans, but no one really knows. BTW, you do know T. Rex was covered with feathers, don’t you?

In a related theme, I recently read some paleontologists now believe the Triceratops bony crest was too weak to be used for defense, it was brittle, porous with blood vessels, and would have bled profusely if damaged; they suggested it might have been covered with chromatophores, like a a chamaeleon, for signaling and camouflage.

As for where the big herbivores live…today in Africa, all the big herbivores live in the grasslands, not the rainforests. The big predators follow them, they live in the Serengeti, not the Congo.

I guess the point I’m trying to make is that evolution makes, or fails to make, adaptations in ways that don’t always seem logical or reasonable to us. I know, the idea of bad-ass T Rex as a craven vulture (not a fierce and noble predator) doesn’t fit in with our preconceptions. But even among modern hunters, dead prey that doesn’t fight bank is always preferable to a food source that can injure you. Lions and bald eagles will always go for road kill, if available.

Recent studies suggest the T-Rex was not that fast:

http://news.nationalgeographic.com/2017/07/tyrannosaur-trex-running-speed/

Hard to see how a carrion-eater would evolve a body plan that raises his jaws high above the “big, dead plant eater corpses littering the landscape”. I’d expect evolution to reward a four-legged stance for being as close as possible to where the carrion falls: On the ground.

And nose not on top, but at the front or downward-facing, to smell what’s on the ground…I could go on and on.

Or if this really was a bipedal carrion eater, then I’d expect the front legs to not atrophy, but grow long and strong to haul the carrion way up in the air. Or pull apart carcasses and pull pieces into the mouth (poor T Rex’s arms couldn’t reach his mouth).

You did specify “big” carcasses, like the apatosaur, but that would be a pretty narrow specialization. T Rex would stand up on his hand legs to dig into a big carcass, I guess. The big jaws wouldn’t be good for delicate work nibbling on smaller carcasses, but I guess would be OK on the big ones (almost as good as they’d be bringing down prey for a predator). Maybe. But OTOH, quadrupedalism seems like it would work for a wide range of carcass sizes. In the case of the big ones, they’re not going anywhere, munch from the bottom and let it collapse into your gullet.

I’m not arguing utilitarianism because of some “Creator” having a goal to make “good” dinosaurs, but just evolutionary efficiency and fitness. If a posited carrion-eater is an awkward carrion-eater, I’d look for an ecological niche for which it’s a better fit. T Rex was an apex whatever-it-was, and it didn’t get that way by being sloppy.

The analogy with modern two-legged predators, that their prey is much smaller, seems a bit weak. For one thing, we’re talking animals much smaller than a T Rex, and thus at risk for having their prey snatched by a bigger predator. Speed is of the essence for these smaller bipeds, they’d go for lots of small prey they can gulp down quickly and keep.

T Rex could probably afford to take his time to eat a big herbivore. And who says he couldn’t sneak up on his prey? I’d say my posited ability to burst through foliage might’ve given him some element of surprise. And if he could burst through foliage, he could hide behind it, waiting for prey to come closer. Big doesn’t mean he couldn’t be quiet.

The forest verge habitat might have favored the ability to burst suddenly from hiding, and run down a plains-dweller that strayed too close to the forest. T Rex could’ve been an ambush predator many ways.

One last tiny point to gnaw on…the habitat might or might not have been a true “jungle”, but if T Rex liked the taste of large herbivores, he wouldn’t find those on a grassy plain, he’d find them where there was enough vegetation to feed the behemoths. Some place with trees for which those long necks evolved. Forest, jungle, tomato, tomatoe.

Anything but a carrion-eater.

And we don’t even know for sure if he was a predator. There is good evidence to show that he ate carrion, which would require little in the way of speed or maneuverability in broken terrain. There must have been a lot of big, dead plant eater corpses littering the landscape. Indeed, some recent research seems to show his leg musculature and bones could not support an agile, mobile predator; certainly not in forested terrain. If he was a predator on live flesh, perhaps he was an ambush predator, haunting waterholes and other such places where his prey had to come to him.

Two legged predators generally today (such as most birds) feed on prey much smaller than themselves, and they sneak up on on it, such as herons and egrets. The only large bipedal predator I can think of today were the big extinct birds like the New Zealand moas, or some of the big predatory land birds of the post-dinosaur era.

We don’t really know the answer to my question, but I think we’re missing something. All we know for sure is that T. Rex ate huge mouthfuls of meat, as evidenced by his jaws and dentition. Other than that, we just don’t know.

But that just pushes it back to a more basic question: Why bipedalism? More specifically, what’s the advantage to a carnivore?

Carnivores catch things, so speed is a pretty good guess. But a cheetah flies along on four legs…on a flat plain.

The habitat of these carnivorous dinosaurs was often jungle-like, lots of vegetation and few flat open areas. The kind of the terrain where a four-legged sprinter has twice the chance of tangling a leg, crashing, even breaking limbs and becoming another predator’s meal. Switching to half as many legs, and concentrating lots of muscle power in them, is a winning strategy.

As is having a battering-ram shaped head with a heavy skull, to break through the vegetation.

And tiny vestigial forelimbs that present a minimal profile so as to not get caught in the vines and broken.

Notwithstanding that illustrators love to show T Rex with his mouth open and teeth bared, I’ll bet he kept his mouth shut while running through the jungle. (Do illustrators think T Rex fed like a baleen whale?)

The T Rex had such tiny front legs to make it a more streamlined jungle battering ram.

https://www.youtube.com/watch?v=eOgwuZE_jp0

One of the biggest mysteries about T. rex has nagged palaeontologists for more than a century: what use did the giant have for arms so stubby that they could not even have reached its mouth? Early ideas, later discarded, suggested that the two-clawed arms helped T. rex to grip a partner during mating or to rise from repose. Later palaeontologists argued that the arms were vestigial — an idea beloved by cartoonists, who never tire of showing T. rex embarrassed by its useless, puny guns.

But research by palaeobiologist Sara Burch at Ohio University suggests that such jokes are unfair. She has studied the musculature of crocodylians as well as that of the only living members of the dinosaur line — birds. If the arms of T. rex had been vestigial, they would have lost the various anatomical landmarks that indicate muscle attachments, but the fossils “retain evidence of substantial musculature,” she says.

But knowing that T. rex used its arms doesn’t reveal what they were used for. To Carr, the arms were part of the dinosaur’s arsenal. “Tyrannosaurids used their arms in the same way all theropods used their arms, for grasping and stabilizing objects” — namely prey, he says.

Holtz visualizes a less rigorous role for the forelimbs. On the basis of previous estimates of muscle strength, he argues that T. rex had weak arms. And because many tyrannosaurs have arms with healed fractures, he says, “their life habits could not require constant use of these arms”. Holtz suggests that they were used primarily for display, perhaps during mating or competition— a possibility that seems more likely if these limbs were cloaked in feathers.