Extinction and Escalation c 69 Ma

Previous posts mentioned the demise of centrosaurines and survival of chasmosaurines. This post examines the actual transition, around middle Maastrichtian time, and the specific taxa involved. It is interesting that well-armed chasmosaurines--forerunners of Triceratops--appeared concurrently, more or less, with the disappearance of the last centrosaurine. And both coincided with the advent of the archpredator.

Known from a high latitude paleoenvironment (in Alaska) Pachyrhinosaurus perotorum was the last species of its genus, and the last of the centrosaurines. P. perotorum vanished around 69 m.y.a. It's noteworthy that Tyrannosaurus first appeared around this time. Tyrannosaurus replaced Albertosaurus, which apparently disappeared c 69 m.y.a. Clearly, this represented predator escalation; a quantum leap in the threat level facing herbivores. Chasmosaurine escalation was in response to it. Apparently unable to adapt in time, P. perotorum succumbed.

At least four chasmosaurine taxa lived right after P. perotorum. Presumably the four evolved as P. perotorum vanished, and outlasted it. Discovered in the uppermost Horseshoe Canyon, Eotriceratops clearly anticipated Triceratops. So did the roughly coeval SW taxa Ojoceratatops and Torosaurus utahensis. Another close relation, Regaliceratops, differed from the other three in that it resurrected the prominent nasal horn of Styracosaurus. In other words Regaliceratops converged with centrosaurines--not the deescalated later ones but the well-armed taxa preceding them. Although atypical for a chasmosaurine, a prominent nasal horn was a good antipredator weapon. No doubt, Regaliceratops was better able to withstand the archpredator than the virtually hornless P. perotorum. It does not, however, appear widespread or numerous, hence not as successful as the lineage with large orbital horns, culminating in Triceratops.

This intepretation has a potential problem: Apparently known only from the SW or Alamosaurus bearing units at first, Tyrannosaurus may not have impacted mid Maastrichtian taxa in northern areas. At least two of the chasmosaurines, however, Ojoceratops and Torosaurus, existed in the SW. The similarity of Eotriceratops to these genera suggests interaction with the more southerly faunas (which as the North Horn indicates, extended northward, perhaps even farther than Utah). Like Torosaurus, Eotriceratops may have ranged into both northern and southern biomes. Ceratopsians probably migrated regularly in search of food, hence were exposed to the archpredator in some areas even if they weren't affected in all of them, initially. As for P. perotorum, its far northern location didn't ensure safety from Tyrannosaurus. Like other ceratopsians, P. perotorum probably migrated southward. In addition, the archpredator is thought to have entered Laramidia via the Bering area, in which case the Alaskan habitat of P. perotorum may have been the first to be affected.

Eotriceratops. A forerunner of Triceratops, Eotriceratops lived around 68.8 m.y.a.--essentially at the same time, or right after, the last centrosaurine disappeared (c 69 m.y.a.). As was noted in previous posts, the centrosaurines were at a disadavantage because they had evolved a nasal boss, from which it was apparently impossible to re evolve a nasal horn. In contrast chasmosaurines, retaining horns since the Campanian, simply improved upon their defense.

Ojoceratops a taxon of about 68 m.y.a.

Holotype skull of Regaliceratops, showing the prominent nasal horn. It may have lived 68.5 m.y.a. or soon after the faunal turnover spurred by Tyrannosaurus.