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.
posted by starman at 3:00 AM
8 Comments:
There is not solid proof of Tyrannosaurus being outside of what is now South America. Well preserved fossil remains would provide good evidence, but those are currently lacking.
As far as South America goes, I noticed an ecological oddity. There are no ornithoschians in the tropics! Except ankylosaur tracks from Western South America. Whatever ecological constraint caused that, might have blocked T. rex prey from moving south, and therefore, impaired the dispersal south of their predators. Though, the GABI saw apex predators able to shift diet readily, and colonize South America, without most of their Holarctic prey accompanying them, for example, low ruminant diversity in the Neotropics.
Apparently rainforest was a shatter zone in the GABI, maybe Laurasian megafauna were facing a barrier to migration. Big cats remember, can make use of trees to ambush prey. Tyrannosaurids were strictly ground based, and in tropical rainforest biomes, the megafaunal prey is thin on the ground. Where were the latest Cretaceous rainforests? And what was their extent?
But that said, the Maastrichtian fauna are less well known for South America and Eastern North America, than from Western North America. I still think this could be a sampling issue.
South America had hadrosaurs such as Secernosaurus, and Kritosaurus the apparent immigrant from North America. But apparently tyrannosaur(oid)s were never able to follow.
Certainly South America had forests but I doubt they were rainforests as sauropods preferred fairly dry habitat (and perhaps mainly conifers).
We can't be sure what kept tyrannosaurs out of South America but I suspect they just weren't able until the opportunity was gone. The theoretically best opportunity to disperse southward--the lowstand c 69-70 Ma--preceded the evolution of Tyrannosaurus. Alamosaurus was able to move northward and compete with big hadrosaurs for tree foliage but tyrannosaurs weren't yet able to compete with abelisaurs for their niche. Tyrannosaurus probably COULD have, but it largely evolved in response to Alamosaurus. By the time it had evolved, the brief lowstand had ended and the "paleopanamanian" connection was gone, not to reappear for the rest of the Maastrichtian.
March 2, 2022
Well, why did kritosaurins successfully cross south, but not Edmontosaurus or Triceratops? What was special about them? Or for that matter, about Alamosaurus, the only titanosaur to cross north?
In North America, did kritosaurins show preference for dense forest, by any chance?
Then regarding titanosaur habitat preference, comparison with Africa, says such huge megafauna will be rarer in tropical rainforest, than in open woodland or scrubland habitats. But seeing titanosaur diversity in the Neotropics, surely some species can be assumed to have tolerated, or even preferred, atypical biomes for the clade. Like large mammals today.
To be honest, I don't think the South American ecosystems of the time, are known well enough, to say very much with certainty.
Ditto for the vegetation at the time, with some authorities doubting the existence of rainforest in the Late Cretaceous at all. But if it was present on the land bridges or just beyond, it would have been a selective filter back then, as during the Plio-Pleistocene. And that seems to be at least in the right ball park, really.
I have an idea why hadrosaurs were good at migrating--the subject of a upcoming blog.
True, we have much to learn about Gondwana environments. But the long dominance of sauropods suggests South America and other regions consisted mostly of semi-arid terrain with many conifers. Auracariacea was common in southern regions and in the Nemegt. Lack of ceratopsids in the latter--or all areas where auracariacea was dominant--suggests lack of suitable habitat prevented North American horned dinosaurs from radiating southward (Btw Triceratops, even Torosaurus came after the low stand migration "window"; Regaliceratops or Anchiceratops might've coincided with it, not that they could take advantage of it). Edmontosaurines were also absent in the Nemegt btw.
Apparent high titanosaur diversity may suggest varying habitats. On the other hand high diversity could be illusory given fairly rapid turnover of genera.
March 3, 2022
Is there anything comparing Asiamerican hadrosaurs, including kritosaurins, to divide them into guilds?
I repeat, as far as the Late Cretaceous interchange goes, hadrosaurs were not gifted migrators. They migrated throygh longitudes, not latitudes. Or am I unaware on non-kritosaurin hadrosaurids, from South America? (You know much more, I'm just a tour guide)
So again, I'll repeat, the taxonomic filter looks like climate and vegetation, selectively blocked migration. Timing might matter, only as regards paleoclimatology.
Soon I'll have a blog about hadrosaur migration near the end of the K.
March 3, 2022
I hope you do, but please Denmark hadrosaurids subclades, and mention relevant peculiarities. Feeding habits affect dispersal, look how Mammut the forest dweller, for all its adaptability, didn't get to South America.
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