T. Rex: predator or scavenger?
Bill Waterson was always right on the money with his "Calvin and Hobbes" cartoons; it's to be regretted that he stopped doing the strip (though I fully understand and sympathize with his reasons, and do not blame him). In one sequence, Calvin has to write a science paper, and he takes my title above as his subject. Of course, he does no research and ultimately his reason for preferring predator over scavenger is because it just wouldn't be cool if Tyrannosaurus was a scavenger.
I agree with Calvin's conclusion, but I think I can make a better case. This is a hot topic in paleontology right now but all of the things I've read about it, on both sides, primarily are the result of studying extant skeletons of Tyrannosaurus and other theropods, trying to make deductions from them.
They seem to be trying to make their deductions based on study of things like bone cross-sections (exotherms and endotherms have much different bone growth patterns) or biomechanical analysis of skeletons, or measurements made of preserved footprints of the theropods.
So far, the evidence appears to be ambiguous. Different researchers have found justification for different conclusions. No consensus has emerged.
The theory of evolution implies that a species will evolve to maximize its chance of survival and breeding within the environment in which it lives. Part of that environment is climate and food, but part of it is also predators (or lack thereof) and that will strongly affect the way a herbivore develops over time. Would impalas be able to run as fast as they can if they were not being constantly chased by cheetahs? The impala usually gets away (the cheetah usually requires several attacks to make a kill) but slower impalas are less likely to escape, so survival favors the fast. On the other hand, we know of some species which developed in environments free of predators and they are far, far different. One example is certain flightless birds on certain islands in the South Pacific; they have no effective escape mechanisms and when humans introduced cats and/or dogs onto those islands, the flightless birds were decimated in fairly short order.
There's no reason to believe it would have been any different for the dinosaurs.
It seems to me that we can learn a great deal about Tyrannosaurus and its kin by studying what happened to the other dinosaurs once the large theropods became common, which is to say in the Cretaceous. The theropods would have been part of the environment for cretaceous herbivores, and theropod behavior and capabilities would have affected how the herbivores evolved. If theropods primarily scavenged, that would be much different than if the theropods were primarily active hunters. Note that the two are not necessarily mutually exclusive. While there do exist species which are exclusively scavengers, such as buzzards, most active predators will also scavenge if the opportunity arises. (Cheetahs are among the very few predators which do not do this, because cheetahs cannot eat carrion. They can only eat freshly killed meat.) So the question is not whether Tyrannosaurus was a scavenger; it's extremely likely that it was because nearly all modern predators are. The question is whether Tyrannosaurus was exclusively a scavenger, or whether it also actively hunted.
If you compare the dinosaurs from the Jurassic who were clearly not predators to the non-predators of the late Cretaceous, there is a qualitative change. The first really large theropods such as Allosaurus appear at the very end of the Jurassic.
Apatosaurus (formerly "Brontosaurus") and its relatives were big, slow and probably not very smart. They wouldn't be threatened by scavengers (by definition) but would be easy prey for a large active predator. They flourished in the Jurassic but die out about the time the theropods become common. No sauropods survived into the Cretaceous. Neither do several other similarly slow and unprotected genuses such as Stegosaurus.
The big theropods, however, flourish and spread going into the Cretaceous and appear to be very common right up until the end. If they were active predators, the main animals which potentially would be prey in the late Cretaceous were the hadrosaurs (duckbills), the ceratopsians (primarily the horned dinosaurs), and the ankylosaurs, and as the Cretaceous progresses every one of them shows adaptation which suggests fear of a big predator. In two cases it's arguable but in one it is in my opinion completely unambiguous.
The duckbills had long muscular hind-legs, a long tail for balance, and shorter fore-legs which may have been used while grazing. However, it's possible they were able to run on just their hind-legs if they needed a burst of speed. Their skulls and jaws are far different and unquestionably identify them as herbivores, but it looks suspiciously as if they were starting take a page out of the book of their main predators so that they could run as fast and potentially escape. They had no obvious way to fight back, so their only defense would have been speed, and they look as if they have evolved near the end to be fast runners. Parasaurolophus, for instance, lived right at the end of the Cretaceous at about the time of Tyrannosaurus. (See picture.)
The ceratopsians take a different approach. Triceratops (picture) couldn't possibly outrun a big predator, but it was superbly equipped to fight one. With all four legs being extremely strong, with a very strong neck, and with massive horns, it could have charged a predator and gored it badly. The long skull shield possibly served many purposes including that of counterbalancing the skull around the neck, but it also protected the neck which may been a favorite bite-to-kill target for a big predator. With jaws like a Tyrannosaurus had and as strong as it appears to have been, it could conceivably have severed the spine in the neck with a bite and a shake. But that wouldn't work with Triceratops or any of its relatives. If Tyrannosaurus tried for that, it would be blocked and it would have to venture close enough to permit Triceratops to charge and skewer one of the predator's legs. Triceratops would then have fought like a bull: once its horns were in, it would toss its head up or sideways to rip and tear. The resulting horrible damage would have been enough to make the predator fall, at which point its body would be within reach of further attacks by Triceratops. Triceratops looks as if it was completely capable of killing a Tyrannosaurus, and for a prey animal the death of a predator is the ideal outcome.
However, it's been argued that those features may have instead been sexual displays. Caribou have horns, but they fight wolves only when they can't outrun them. I don't find that argument convincing, but it's at least plausible. Triceratops isn't built like a runner to flee; it's built more like a musk ox or water buffalo who are slow but strong, and when they are faced with a predator, they fight. What's lacking for the "sexual display" idea is evidence. If we ever find a ceratopsian skull which shows clear wounds from what would seem to be a horn of the same species, then it will become more acceptable. In the meantime it is simply speculation.
But so is my argument, although I think it is more plausible. Sexual display headgear (such as are worn by elk and deer) are designed differently than weapons (such as those of a water buffalo) and the horns of the ceratopsians much more resemble the latter. While there is enormous variation in the horn patterns of the various ceratopsians and some approach the baroque, every one of the later ones had at least one horn which could have been used as a weapon against a predator. Consider Torosaurus (picture), Pentaceratops (picture), or Chasmosaurus (picture). They may have served as sexual display devices, but I think there can be little doubt that they also served as weapons.
The most important of the three groups for my purpose is the ankylosaurs, because it's the one which I think represents unambiguous proof. Ankylosaurus (picture) is sometimes called the "four legged tank". Its back was covered with armor plates topped by spikes; it had a row of spikes all the way round it on the edge of its body, and it had a flexible tail with a big bony ball on the end which looks suspiciously like a weapon. In fact, it looks like a mace. Ankylosaurus' preferred orientation would have been to face away from a predator, to bring its tail into play. With sufficient strength behind it, the tail weapon should have been quite capable of breaking a leg bone in a large biped careless enough to get within range. For such a predator, that injury would have lead to death through starvation or by being killed while defenseless by some other predator, but in any case it would have slowed it down enough so that the Ankylosaurus could escape. The threat was real and obvious and may have been sufficient to deter attack, which from the point of view of Ankylosaurus would be sufficient. (Thus the lack of broken legs on Tyrannosaurus skeletons doesn't disprove this theory.)
Ankylosaurus' armor makes no sense except as defense against something big, fast, ferocious and hungry. There isn't any way that an Ankylosaurus could outrun a Tyrannosaurus, but it had adequate defense built in so that it didn't really need to. If attacked by two or more theropods, it would have crouched down and let its armor protect its legs. In such a position it would have been nearly invulnerable.
The real question is this: if there was no big, fast, ferocious and hungry predator, why would Ankylosaurus have evolved such elaborate defenses? All that stuff is a massive metabolic investment; if it served no purpose it would be a disadvantage and natural selection would have selected against it. No dinosaur of the mid-Jurassic has anything remotely like this. On ankylosaurus there isn't just a little bit of it; there's as much armor plate and spikes as can possibly be packed in. We're seeing the end product of a long process of selection which clearly favored as much armor as possible. The only way that could happen is if that armor represented a substantial survival advantage, which means there must have been a big, fast, ferocious and hungry predator. The only candidate which we've found which fits that bill is Tyrannosaurus and the other theropods.
Were the theropods exclusively scavengers, Ankylosaurus wouldn't have needed all that protection. Were the theropods active predators, Ankylosaur would have needed every bit that it had.
I consider the characteristics of Ankylosaurus to be conclusive evidence that Tyrannosaurus was an active predator. (Besides, it wouldn't be cool if they were exclusively scavengers.)
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