Origin of Alamosaurus and a possible Implication

A juvenile Alamosaurus attacked by a T. rex pack

The Cenomanian transgression was the nemesis of North American sauropods. Rising sea levels meant insufficient land and resources for Abydosaurus and other giants. After they succumbed, a long hiatus ensued. From the late Cenomanian to the mid Maastrichtian, sauropods were absent in America. Even when the environment was capable of supporting huge herbivores, hadrosaurs such as Magnapaulia filled that niche. By the late Maastrichtian, however, sauropods reappeared in Laramidia. Alamosaurus proliferated in the southern part of that landmass, and extended its range at least as far north as Utah.
Where did Alamosaurus come from? There are two possibilities. One is Asia, where there was no sauropod hiatus, and the big Nemegtosaurus existed in early Maastrichtian time, just prior to Alamosaurus. In view of ample evidence for migration of dinosaurs between Asia and America in the late Cretaceous, Asia appears a plausible source for Alamosaurus. South America, however, appears more likely.
First of all, Alamosaurus appears most common in southern areas, such as Texas and New Mexico, suggesting origination farther south. In addition, it had more in common with South American sauropods than Asian ones. Although the last sauropods on all three continents were titanosaurs, Gondwana taxa were more closely related than Asian ones. Phylogenetic analyses indicate Saltasaurus,  the South American titanosaur, was nearest to Alamosaurus. Moreover, Nemegtosaurus (probably=Opisthocoelicaudia) had opisthocoelous caudal vertebrae, whereas Alamosaurus did not. Lastly, like Saltasaurus, Alamosaurus was armored whereas there is no evidence for armor in Nemegtosaurus.
The probable South American origin of Alamosaurus has an interesting implication. Like the southward radiation of North American mammals and other taxa, it indicates a connection between North and South America around mid-late Maastrichtian time. Evidence for migration toward the close of the K suggests Bakker's theory of biogeographic chaos has some merit after all. Although the final extinction was undoubtedly due to an impact, a considerable amount of extinction may have occurred just prior to that, in the twilight of the Cretaceous. The youngest dinosaur bearing units in Laramidia show evidence of a decline in diversity. It is noteworthy that this decline affected the taxa undoubtedly hunted by tyrannosaurs--ornithiscians. Coinciding with T. rex, the drop in ceratopsid and hadrosaur diversity probably owed much to predator escalation. The last tyrannosaur represented a quantum leap in capability. Evolving the means to cope with an armored giant, T. rex could've overwhelmed weaker herbivores. Centrosaurines, nodosaurs and lambeosaurs may have succumbed to the archpredator. Only the biggest, toughest, best armed or fastest prey could survive.
Evidently, Alamosaurus caused T. rex to evolve quickly. The tyrannosaur's oldest remains occur in the Javelina, alongside the titanosaur. Had the tyrant king evolved soon, or even while evolving, it might've radiated southward via the same route the titanosaur took. Entering South America, the archpredator could've wreaked havoc. This is highly likely because the local fauna lacked coevolutionary preparation to deal with T. rex. Small titanosaurs and local hadrosaurs, particularly, would've been highly vulnerable. Depending on the extent of Gondwana land connections at the time, devastation might've spread far and wide.
It is difficult to test this theory, because outside Laramidia the terminal Maastrichtian record is poor. Studies have shown that dinosaur diversity appears unaffected in South America or elsewhere in Gondwana. This may be fallacious since the Gondwana record extends only to about middle Maastrichtian. Even in Laramidia there is little evidence of a loss of variety prior to the last 1-2 million years (i.e. the time of T. rex). This period may not be documented in southern continents. There is no known terrestrial K-Pg boundary in Gondwana.
At least one prediction is fulfilled: Migratian occurred, proving a north-south connection and faunal exchange similar to the later, Great American Interchange. Almost certainly, tyrannosaurs interacted with migrating titanosaurs and became coadapted to them. Able to hunt sauropods, the archpredator might've entered the original titanosaur habitats, via the same immigratian corridor,  c 68-67 MYA.