Tyrannosaurus: Escalation and Dispersal

Haq's data (2014) raises an interesting possibility: Tyrannosaurus may have spread far beyond Laramidia (and originally, Asia) after all. Near the end of the Cretaceous, the archpredator reached its acme not only in terms of prowess. Receding seas could've maximized its geographical range, with devastating consequences.

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. Extirpation of gondwana sauropods etc was also possible, as Tyrannosaurus retained its prowess to the end.

GSP was wrong to suggest the last Tyrannosaurus individuals were less robust and powerful than their predecessors. T. regina may not be valid. The biggest known Tyrannosaurus specimen, RSM P2523.8 ("Scotty") is from the Frenchman. Like the Scollard, the Frenchman is equivalent to the upper third of the Hell Creek, and therefore documents the last of the "tyrant lizards." Inasmuch as the archpredator was at least as formidable in the latest Maastrichtian as it was in the middle, and its opportunities to spread had been as good or better, it might've wreaked havoc on an even larger scale.