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Tim Donovan wrote-
T-rex evolved great size, super-powerful jaws and robust teeth capable of penetrating armor. These aspects suggest the archpredator coped with large, armored prey such as Alamosaurus; otherwise they seem superfluous. Bladelike teeth probably would have always sufficed against unarmored hadrosaurs and even ceratopsids. Ankylosaurs alone were unlikely to have spurred the advent of T-rex, or such theropods would have appeared much earlier. In contrast, titanosaurs predated T-rex by only a stage or so, in Cordillera. First of all, there is no proof Alamosaurus was
armored (even assuming it is a valid genus), although I suppose the evidence is
mounting that all titanosaurians were armored. Still, this armor is very
sparse and I might expect a predator to go for an unarmored section, like the
neck. Also, the robust teeth could have many other explanations besides
penetrating armor, such as crushing bones, so that more flesh could be eaten or
carcasses could be eaten easier. After all, you don't see hyaenas evolving
their robust teeth to bust armor. Perhaps the appearence of large
ankylosaurs significantly earlier in the fossil record (Barremian, Early
Cretaceous) would be evidence suggesting that Tyrannosaurus did not evolve
robust teeth for armor penetration, as you would expect robust-toothed theropods
to appear alongside large ankylosaurs. However, only narrow toothed large
theropods such as Utahraptor and Acrocanthosaurus are known from such
deposits. Similarly, other environments with armored titanosaurs
(Maevarano Formation, Madagascar; Lecho Formation, Allen Formation, Argentina)
don't show any tendancy to evolve robust-toothed predators, as slender-toothed
abelisaurids and carcharodontosaurids are the top predators in these
locales.
As Starkov noted, ceratopsids had to become larger to survive T-rex. The replacement of Euoplocephalus with Ankylosaurus, which he neglected to mention, is another example of T-rex-driven size increase. Perhaps this happened the other way around, with
Tyrannosaurus evolving it's large size and robust skull to kill the larger
ornithischian species that were evolving. After all, large ornithischians
are absent in Gondwanan deposits populated by abelisaurids, while both the
Nemegt and Hell Creek had large ornithischians. For instance, the Nemegt
was home to Tarchia, the largest Asian ankylosaur, and Shantungosaurus, one of
the largest hadrosaurids. Also, the large segnosaur Therizinosaurus, which
is for all intents and purposes a large unarmored herbivore, inhabited the
Nemegt. Tyrannosaurus might have actually been limited to environments
with large ornithischians, so that it was never able to invade South America
after the land bridge formed.
If T-rex was well-adapted to hunt
titanosaurs, the results could have been devastating if or when it gained access to the titanosaur-dominated faunas of Gondwana. The sauropods and small ankylosaurs, etc. of southern regions had long confronted only the relatively small and weak abelisaurs. Lacking co-evolutionary preparation to withstand T-rex, Gondawana prey could have succumbed very quickly, just as avian quarry did when the efficient brown tree snake entered Guam. This lacks any material evidence. No tyrannosaurids
have ever been found in South America, Cretaceous Europe, Africa or India.
The abelisaurids and carcharodontosaurids of Gondwana, while having weaker jaws,
were oftentimes quite large. Also, the many small-medium ornithischians of
Hell Creek and Nemegt seemed to have survived fine alongside Tyrannosaurus, so I
don't see why their Gongwanan equivalents would be wiped out by invading
tyrannosaurs. Then there's the fact large titanosaurs lived in both the
Hell Creek and Nemegt (Opisthocoelicaudia) alongside Tyrannosaurus
anyway...
Mickey
Mortimer
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