Delayed Radiation of Giant Tyrannosaurs

Even the largest tyrannosaurs were not able to radiate into all habitats initially. Spreading required the means to deal with all survival strategies of potential prey, which varied by region. An initial lack of some abilities excluded big tyrannosaurs from some environments. Their apparent presence in one, the Amur, may be misleading.

Tyrannosaurus and close relatives decimated herbivore communities and spurred escalation among surviving prey. Centrosaurines vanished when T. rex appeared. They were other casualties.

Lambeosaurs were adversely affected. They survived only in areas lacking Tarbosaurus or Tyrannosaurus. The Zhucheng, Nemegt and Hell Creek faunas appear bereft of lambeosaurs. Crested taxa were out of their depth when archpredators required emphasis on escaping. Elaborate crests evolved for intraspecific interaction, and limited what a lambeosaur could invest in means of evasion. Only where (or for as long as) super-tyrannosaurs were absent could lambeosaurs endure. Refugia included Laiyang, the upper Horseshoe Canyon, the upper Kirtland and New Jersey, western Europe and NW Africa.

There is, however, a putative exception. Dated to 70-68 Ma, the Amur localities have yielded abundant and diverse lambeosaurs. Olorotitan, Charonosaurus etc apparently thrived even though their region does not appear to have been a refugium. Researchers have found tyrannosaur remains, mostly teeth, in local strata. A few of these, notably a 130mm tooth crown, suggest a tyrannosaur the size of T. rex. Considered autochthonous, the material appears to falsify the proposed role of big tyrannosaurs in excluding or extirpating lambeosaurs.

The evidence may be misleading, however. Inasmuch as most of the tyrannosaur teeth are far smaller than 130mm, an Albertosarus--sized taxon may have been the actual top predator. Based on a tooth, the name Albertosaurus periculosis may be valid. The North American Albertosaurus was a contemporary. In this scenario, giant hunters lived in an adjacent region and only enterd the Amur occasionally, in times of stress.

The Amur may have replicated the older Laiyang community. In the Zhucheng (Xingezhuang) environment Zhuchengtyrannus, and a possible early Tarbosaurus, battled huge prey, notably Shantungosaurus and a sauropod. Local, giant tyrannosaurs specialized in prey large enough to stand its ground and too bulky for rapid escape. In contrast, the coeval Laiyang environment had no sauropods and only average sized hadrosaurs, including the crested Tsintaosaurus, which almost certainly fled. Interestingly, instead of Zhuchengtyrannus Laiyang had the not so impressive Chingkankousaurus. Like American counterparts, it probably sped after its quarry.

A similar situation could've prevailed in the Amur, where the big tyrannosaur may have been out of its element. Evidence for this is the proposed referral of the 130mm tooth to Zhuchengtyrannus. Unlike Tarbosaurus or Tyrannosaurus, Z. magnus, like its presumed prey, was probably noncursorial. As bitten hadrosaur tails clearly show, Tyrannosaurus pursued fleeing prey, as did the undisputed apex predator of the Nemegt, Tarbosaurus. Facing giants instead of fleeing taxa, Zhuchengtyrannus was different. This argues it usually avoided the Amur as well as Laiyang, where smaller, fleet tyrannosaurs filled the hadrosaur hunter niche.

Then why is Zhuchengtyrannus found in the Amur? Probably because of droughts. Sauropod (and giant hadrosaur) habitats tended to be drier than most hadrosaur environments. The latter were riparian or close to rivers. Naturally, in times of drought, the sauropod/big tyrannosaur habitats tended to dry up first. This caused Zhuchengtyrannus, and some sauropods, to enter the Amur region in search of food and water. The scent of water, and carcasses from mass mortality events, probably lured them. Their presence in the Amur, however, was only intermittent and brief. They were not adapted to the Amur hence not true denizens of it.

Initially, the super-tyrannosaurs of Asia were like Tyrannosaurus, arising in sauropod habitat. They spread to, and affected, the regions of medium-sized prey only gradually. To do that a giant tyrannosaur had to be cursorial. Evolving rapidity, however, was not easy for a bulky, giant predator. It took time, and environments where there was selection pressure for speed as well as prowess. The Nemegt and the Alamosaurus habitats had average-sized hadrosaurs as well as sauropods. Therefore, Tarbosaurus and Tyrannosaurus evolved to pursue as well as battle prey. The ability to chase facilitated radiation, or expansion of ranges. In contrast, the original habitat of Zhuchengtyrannus required just bulk and ferocity. Lacking cursoriality, it could not effectively spread--at least not in the period documented in Asia.

The inability of some giants to pursue was likely only temporary. Just as Tyrannosaurus spread to the ornithiscian dominated areas, leaving exctinction and escalation in its wake, the same scenario was likely repeated in Asia.

Even the stratigraphically highest Amur localities, such as Blagoveschensk, are probably separated from the K-Pg by a disconformity of 2 million years. The Amur record terminates roughly 68 Ma. In North America, the lambeosaur Hypacrosaurus survived to about the same time. Since the record of the last 2 million years of the Cretaceous is extant in North America, the radical alteration of lowland or ornithiscian communities is clearly observable. Since this is not the case in the Amur region, the survival of its lambeosaurs to the end is generally taken for granted. Since Asian tyrannosaurs ultimately could've replicated the trajectory of T. rex, lambeosaur survival to the K-Pg could be illusory.