Centrosaurine and Lambeosaur Extinction

Compared to lambeosaurs such as Parasaurolophus (right) Edmontosaurus had more economical means of intraspecific interaction, such as mating calls, as it did not rely on extensive, bony structures for this.

Likewise, compared to earlier ceratopsians, pictured above, Triceratops (pictured below) made relatively little investment in means of intraspecific interaction such as sexual advertizing or display (the likely purpose of elongated frills or frill spikes, some bent inward alongside the frill hence useless as weapons). Note the nasal boss of Achelousaurus (above right).

Previous posts attributed the demise of North American centrosaurines and lambeosaurs to the advent of Tyrannosaurus. But what made centrosaurines and lambeosaurs vulnerable while chasmosaurines and edmontosaurs survived? The failure of centrosaurines appears easy to explain. Lambeosaurs were also at a disadvantage even if their specific weakness is not as easy to discern.

In late Campanian time, centrosaurines underwent a process of deescalation. This resulted from the bearpaw transgression. By diminishing habitat and prey populations, the bearpaw seriously affected the most sensitive, upper part of the food pyramid, the predators (tyrannosaurs). Whereas Styracosaurus faced Daspletosaurus wilsoni, the acme of daspletosaur prowess, later centrosaurines knew the less impressive Daspletosaurus horneri and the smaller Albertosaurus. As the tyrannosaur threat waned, the means of battling them became less important. Centrosaurine evolution began to emphasize intraspecific interaction. The deadly nasal horn of Styracosaurus became thhe bent horn of Einiosaurus. The horn disappeared altogether in the later taxa Achelousaurus and Pachyrhinosaurus. Both sported a nasal boss, fine for pushing and shoving matches with others of their kind but of limited use against tyrannosaurs.

Deescalation was permissable in early Maastrichtian time but soon proved fatal. Confronted with a quantum leap in predatory danger, in the form of Tyrannosaurus, centrosaurines could not cope. Retaining horns throughout Campano-Maastrichtian time, chasmosaurines found it easier to adapt. They and their horns just became larger. In contrast, centrosaurines could not easily re--evolve horns from a nasal boss.

It's fallacious to assume centrosaurines were eliminated overnight, or in a few years. They just lost the competition for available niches--niches taken by the better-armed chasmosaurines. Pachyrhinosaurus was not helpless, just at a disadvantage. Regaliceratops and Eotriceratops--forerunners of Triceratops--were more likely to repel a derived tyrannosaur. Even if chasmosaurines were just 10% more likely to survive an attack, it would've meant a (geologically) rapid replacement of centrosaurines.

No doubt, the same applied to hadrosaurs. Given their substantial investment in cranial crests, for sound production (and species recognition) lambeosaurs could not focus on surviving predators to the same degree as Edmontosaurus. It's not so obvious, though, what their specific vulnerability was. Perhaps the crest meant a larger auditory area of the brain at the expense of other means of detecting predators, such as olfaction. But expanded premaxillary and nasal bones (i.e. crests) were unlikely to have diminished a lambeosaur's sense of smell (even if olfaction wasn't the crest's primary function). Perhaps vision was the achilles heel. Lambeosaurs might not have seen an approaching enemy as well as edmontosaurs, even if they could hear and smell a threat just as well.

The lambeosaur disadvantage may not seem significant. Perhaps lambeosaurs were only about 5% more likely to be surprised, and caught, than an edmontosaurine. But again, even a small disadvantage could mean the geologically rapid replacement of one group by another. Few if any lambeosaurs existed in the lastest Maastrichtian of Laramidia, documented by the Hell Creek and equivalents.

It's noteworthy that the last known lambeosaur in North America, Hypacrosaurus, had a smaller crest than Campanian taxa. Lambeosaurus magnicristatus and Parasaurolophus had the biggest crests, but did not last very long. Lambeosaur diversity soon waned in Laramidia. Hypacrosaurus is the only known post Campanian taxon, yet it was not well-suited to survive the archpredator. Compounding the drawback of a crest, Hypacrosaurus had elevated neural spines, probably for display. Not surprisingly, Tyrannosaurus inflicted the coup de grace.

It may not be entirely clear why certain taxa vanished with the advent of the archpredator. But the drawbacks of certain features, coupled with the prowess of T.rex, points to the latter as the probable cause.