The extraordinary power of
T. rex suggests initial reliance on more massive (and armored) prey than those from the Hell Creek and equivalents.
Prior to the tyrant king, tyrannosaurs lived alongside prey similar to that of the Hell Creek--ceratopsids and hadrosaurs--for millions of years, and presumably hunted them successfully, without becoming as impressive as T. rex. Why was the last tyrannosaur so big and deadly? History suggests it was initially coadapted to big sauropods.
Some material suggests Alamosaurus grew as large as Argentinosaurus. That made A. sanjuanensis among the largest land animals that ever lived. Not surprising, the behemoth is often considered invulnerable. Many believe attacking it would've been suicidal. No doubt a full grown Alamosaurus would've been a difficult proposition for any theropod. But the largest theropods probably evolved to take down sauropod giants. At least occasionally, they preyed on adults.
At various times in the Jurassic and Cretaceous, giant theropods coincided with huge sauropods. Examples are Saurophaganax and Brachiosaurus, Mapusaurus and Argentinosaurus, Giganotosaurus and Andesaurus, Tarbosaurus and Nemegtosaurus and Tyrannosaurus and Alamosaurus.
Noting that big predators had long arisen in response to giant sauropods, Starkov considered Tyrannosaurus no exception. The arch-predator evolved large size to better deal with A. sanjuanensis. Since the oldest Tyrannosaurus material is from the Alamosaurus bearing units, notably the Javelina, it is reasonable to conclude that T. rex originally evolved to overcome the big titanosaur.
This view is supported not only by the size of Tyrannosaurus but its extraordinary hunting assets. The arch predator had extremely powerful jaws, and robust teeth. Had Tyrannosaurus preyed entirely on hadrosaurs and ceratopsids, never daring to battle the huge titanosaur, its extraordinary fighting power would seem superfluous.
Prior to the arrival of Alamosaurus in North America no tyrannosaur had evolved such incredible jaw power and such robust teeth. Not even giant Campanian hadrosaurs, and large ceratopsians such as Pentaceratops caused such assets to evolve. Campanian tyrannosaurs made do with bladelike teeth and weaker jaw musculature--their rear areas for muscle attachment were not expanded like in T. rex.
The tough dentition of Tyrannosaurus--which Bakker compared to railroad spikes--probably evolved to penetrate armor and thick hide. Like other titanosaurs, Alamosaurus had armor. Little of it is known but as the previous post noted, that was probably due to taphonomy. The behemoth probably had a fair number of scutes and ossicles, requiring very powerful jaws and teeth to smash through. By ripping out a chunk of flesh from the belly, neck or flank, Tyrannosaurus could've caused catastrophic hemorrhaging. In only a short while the supposedly invincible titanosaur was groggy and ripe for a coup de grace. The deadliness of Tyrannosaurus made killing a sauropod giant--even an armored one--possible. It was a dangerous undertaking, to be sure, but not suicidal, especially if the power of more than one T. rex was brought to bear.
Bakker noted evidence for packs of Allosaurus attacking sauropods, and Tyrannosaurus probably also hunted in packs.
Tyrannosaurs appear to have been gregarious. Phil Currie noted Tarbosaurus skeletons found in close proximity to others of their kind. An Albertosaurus bonebed is known. Tracks in British Columbia show tyrannosaurs walked together. Pack hunting enabled predators to overcome prey too tough for an individual to overcome. Faced with a pack, even an adult Alamosaurus was vulnerable.
Some time ago, a Tyrannosaurus tooth was found in association with an Alamosaurus cervical vertebra. Most likely, the tyrannosaur was just scavenging. It is possible, though, that it brought its quarry down with a bite to the neck--not a great source of meat for a scavenger but a good target for an attacker. By surrounding the prey, a pack enabled at least one individual to strike a vulnerable area.
A recent study bolsters the view of Tyrannosaurus as an Alamosaurus hunter. Tooth enamel of Tarbosaurus suggests the tyrannosaur devoured herbivores which ate araucarian conifers. That certainly suggests Tarbosaurus ate high feeders such as the sauropod Nemegtosaurus (probably=Opisthocoelicaudia). Saurolophus was undoubtedly another prime target but the study appears to confirm that big, advanced tyrannosaurs included sauropods in their diet, hence evolved to hunt them.
Interestingly, track evidence indicates Nemegtosaurus grew considerably larger than skeletal remains appear to suggest. If Tarbosaurus regularly preyed on a big sauropod, Tyrannosaurus could've done the same. In both the Nemegt and the American southwest, sauropods made up a large part of the potential prey, so there must've been strong selection pressure for tyrannosaurs capable of overcoming them.
