The Maastrichtian lowstand and Biogeographic Chaos

In the 1980s Bakker proposed that mixing of faunas doomed the dinosaurs. In his view, mass migration exposed faunas everywhere to unfamiliar diseases, parasites and predators. Lacking coevolutionary preparation to deal with these, dinosaurs succumbed en masse. My view was similar except that I emphasized the role of predators, namely Tyrannosaurus, instead of disease. Generally Bakker's theory, and mine, is falsified by transgression near the K-Pg boundary. Higher sea levels c 66 Ma precluded migration at the relevant time. Connections between land masses had been inundated, or severed. Nevertheless, biogeographic chaos, even a predatory role, may have some merit.

Migration appears to have caused some Maastrichtian extinction. It occurred not at the end of the stage but around the middle. At the time regression enabled many dinosaurs to expand their ranges greatly. Invasive species proliferated, to the detriment of other taxa. What follows is my reconstruction of events:

-A profound but brief lowstand occurred c 70-69 Ma.

-Alamosaurus moved northward into Laramidia, where it competed successfully with large hadrosaurs for conifer foliage.

-Tarbosaurus or a close relation traversed the Bering bridge and entered North America.

-Lambeosaurs such as Blasisaurus entered western Europe from Scandinavia(?). At least one taxon, Ajnabia, migrated even farther south, into Africa. Even areas not fully connected by regression were accesible to hadrosaurs. Their preference for riparian habitat suggests excellent swimming skills, conferring an ability to traverse some sea barriers.

-The lowstand ended c 69 Ma, and with it further opportunities for dispersal.

-Tarbosaurus (or Zhuchengtyrannus?) evolved into Tyrannosaurus (imperator?) in order to hunt Alamosaurus. The evolution of the archpredator took time. When it finally appeared, the fleeting lowstand had ended. Ergo Tyrannosaurus could not enter South America, or even Appalachia. Nevertheless it had a profound impact in Laramidia.

-Predator escalation, in the form of Tyrannosaurus, eclipsed a number of herbivore taxa. Apparently, those least able to resist included centrosaurines, lambeosaurines and nodosaurs. All waned and died out in mid-late Maastrichtian time. It was then that the archpredator, after arising in the titanosaur dominated southern/intermontane areas, spread to the lowlands. Almost immediately, ornithischian diversity crashed. Centrosaurines were likely vulnerable because they had eschewed antipredator defense--a large nasal horn--in favor of structures more useful for display or intraspecific contests. The same basic issue may have affected others.

- The lambeosaur invasion of Europe has been correlated with stress and extinction in native taxa. Rhabdodonts and Struthiosaurus vanished, as did most sauropods. Unlike Alamosaurus, Lirainosaurus and other European sauropods were relatively small, hence unable to compete with hadrosaurs for local, lower foliage. Titanosaur diversity plunged c mid Maastrichtian. Pathological eggshells attest to the stress caused by the influx of competitors.

Blasisaurus, Arenysaurus and Canardia did not cause the demise of all dinosaurs in western Europe, as Bakker's theory may have predicted. But invasive taxa proved devastating.

The arrival of Alamosaurus and Tarbosaurus in North America led to predator escalation and extinction.

Tyrannosaurus--Endemic to Laramidia?

 

 I've long speculated that falling sea levels doomed many dinosaurs. Regression itself was no problem, but by late Maastrichtian time there were potential adverse effects. By connecting formerly separated regions, regression might've exposed the less escalated faunas to highly escalated enemies, leading to overpredation and mass extinction. In this scenario, T. rex played the greatest role. Spreading far beyond Laramidia, or western North America, the tyrant king wreaked havoc. Shrinking of the western interior sea granted the archpredator access to eastern North America, while the paleo-Panamanian emigration route--the same one Alamosaurus used to radiate northward--led to an invasion of South America. Sweeping into distant lands, where prey lacked coevolutionary preparation to survive (in sharp contrast to coadapted Laramidian prey such as Triceratops) the arch predator obliterated whole ecosystems, wiping our sauropods, hadrosaurs and a host of other taxa. This scenario resembles Bakker's theory of biogeographic chaos except that it emphasizes the role of a predator, not disease. Although direct evidence for the the spread of T. rex is lacking, the notion seemed plausible given evidence for regression and dispersal of other taxa. Some data, however, casts doubt on the scenario.

In both regions allegedly invaded by T. rex--eastern North America and South America--faunas dated at virtually the end of the  Maastrichtian age show no evidence of the archpredator or faunal disruption.

In eastern North America Dryptosaurus, from the New Egypt formation of New Jersey, is clearly a native predator, distinct from T. rex. Thought to be 67 million years old, or within a million years of the K-Pg, Dryptosaurus suggests the local fauna persisted to the end. Although T. rex  might, in theory, have entered in the last million years this appears unlikely. The Maastrichtian lowstand--the greatest opportunity for migration--occurred around the middle of the stage, whereas toward the end there was actually transgression. Ergo, if Tyrannosaurus  hadn't appeared in the east by 67 MYA it probably never did.

The same appears true in South America. Although the Mesozoic record there generally terminates long before the K-Pg, the uppermost part of the Marilia formation in Brazil is about 67 million years old. A large titanosaur, Uberabatitan, is known from this horizon, but not T. rex or a close relation. As far as is known, abelisaurs remanded the top predators in South America (and elsewhere in Gondwana e.g. Chenanisaurus). Not only does T. rex appear absent, it doesn't appear likely it ever arrived. Since the migration "window" was only open around mid Maastrichtian, by c 67 MYA it was too late. It now appears most probable T. rex remained more or less endemic to western North America to the end. While its initial radiation may have caused some extinction--centrosaurines, perhaps local lambeosaurs and nodosaurs--the archpredator played little if any role in the K-Pg mass extinction.