Tyrannosaurus: Dispersal and Deescalation

Haq's data (2014) raises an interesting possibility: Tyrannosaurus may have spread far beyond Laramidia (and originally, Asia) after all. But GSP's work on speciation suggests the archpredator deescalated as it moved into new environments.

Haq mentioned three Maastrichtian lowstands, dated 69.4, 68.8 and 66.8 Ma. All are considered "medium amplitude" but the last event, occurring less than a million years before the K-Pg, was the most profound. This may have major implications for dinosaur biogeography. Presumably, the lowstands of c 69 Ma--those of 69.4 and 68.8 Ma--led to the dispersal events of middle Maastrichtian time. The ancestor of Tyrannosaurus (if not Tyrannosaurus itself) entered Laramidia from Asia. Around the same time, Alamosaurus migrated northward and lambeosaurs spread into western Europe and Africa.

Considering the magnitude of the lowstand of 66.8 Ma, it probably meant even greater opportunities for migration. Presumably the lowstand of 66.8 Ma was the one that severely shrank the western interior sea, and caused Laramidia and Appalachia to reconnect in the dakotas. The lowstand was followed by a highstand just before the K-Pg, but for perhaps 400,000 years dispersal throughout the Americas was possible. In addition to Appalachia, South America may have been accessible to the archpredator. If the earlier lowstands of c 69 Ma enabled Alamosaurus to move northward, the more profound event of 66.8 Ma may have seen Tyrannosaurus radiate southward.

Previously, scenarios of Tyrannosaurus spreading from Laramidia appeared dubious because of the presence of Dryptosaurus in Appalachia and Uberabatitan in South America. Both taxa lived about 67 million years ago, or just 1 million years before the K-Pg, and therefore suggest the original faunas remained intact to the end. It appears possible, though, that Dryptosaurus and Uberabatitan predate, albeit slightly, the advent of Tyrannosaurus in their regions. Although present dating is too imprecise to say that the archpredator could've come 200,000 years after the native taxa lived, it does appear possible.

By about 66.6 Ma, Tyrannosaurus might've ranged far and wide, in the process extirpating many taxa unable to deal with it. Non-Laramidian faunas tended to be unescalated, and lacked coevolutionary preparation for Tyrannosaurus. Together with other invasive taxa, the archpredator caused considerable extinction before the asteroid struck. It is noteworthy that a loss of diversity in Laramidia coincided more or less with the advent of Tyrannosaurus. Centrosaurines, nodosaurs and lambeosaurs declined or vanished. It may be significant that centrosaurines, which had abandoned horns since the late Campanian, proved vulnerable while chasmosaurines, which retained large horns, survived. But Tyrannosaurus itself was affected as it moved into different environments.

Inasmuch as the oldest Tyrannosaurus material is from the Alamosaurus environents, the archpredator probably evolved its extraordinary power to overcome titanosaurs. Although some earlier tyrannosaurs, in ornithiscian dominated environments, were formidable, none equaled the power of Tyrannosaurus. This suggests that the incredible jaw power and robust teeth of the archpredator were superfluous in ornithiscian environments. It might be predicted, therefore, that Tyrannosaurus deescalated after entering them. This might be borne out by GSP's thesis.

GSP perceived three species of Tyrannosaurus.The first, T. imperator, was the earliest and most robust. Presumably the original titanosaur hunter, T. imperator may have been somewhat out of its depth in the ornithiscian dominated Hell Creek. It occurs low in the unit, replaced by newer species. By about middle Hell Creek time, the not quite as robust T. rex was taking over the top predator niche. In late Hell Creek time, the least robust species, T. regina, had appeared.

GSP's thesis may not be generally accepted. But it appears that the spread of the titanosaur-hunter, related extinction, and speciation, are likely.