The Nemesis of Lambeosaurs

The dinosaurs from the Amur localities (Udurchukan and Yuliangze formations) appear highly unusual, considering their middle Maastrichtian age. They are geologically younger than Nemegt taxa, yet appear older, as if they were a revival of the Campanian. Unlike the Nemegt and the Hell Creek (and equivalents) Amur units yield numerous lambeosaurs, and lack derived tyrannosaurs. In view of this the Maastrichtian age given for the Udurchukan and Yuliangze once seemed dubious, yet it is well established. How can the Amur faunas, so unusual for their place and time, be explained?

They may be explained by delays in the radiation of derived tyrannosaurs, prior to the latest Maastrichtian. The geological record indicates tyrannosaur giants were initially confined to certain areas. Their impact was profound but it was long limited geographically. Around middle Maastrichtian time certain regions, including the Amur, were unaffected. This meant a remarkable disconnect between the big tyrannosaur environments and the others. A comparison of the Zhucheng and Laiyang localities of the Wangshi series, and the Javelina and upper Horseshoe Canyon formations, reveals striking faunal differences, even among close coeval units, reflecting the presence or absence of advanced tyrannosaurs.

The archpredators appear to have evolved in continental interior habitats with titanosaurs or the giant Shantungosaurus. Initially, they eschewed the wetter environments preferred by most hadrosaurs. Eventually, however, the super hunters spread, to the detriment of taxa which, long unexposed to them, lacked coevolutionary preparation to cope. Lambeosaurs and primitive hadrosaurs faced annihilation.

Essentially, the crested taxa could not survive alongside the later, giant tyrannosaurs. No lambeosaurs existed in the Nemegt paleoenvironment where Tarbosaurus was top predator. Likewise Tyrannosaurus habitats appear devoid of lambeosaurs. The latter are, however, known from the mid Maastrichtian upper Horseshoe Canyon, where the top predator was the less derived Albertosaurus. It is noteworthy that Tyrannosaurus existed by this time but only in the Alamosaurus habitats. Wangshi localities provide another example. The habitat of Zhuchengtyrannus, like that of Tyrannosaurus, included an edmontosaurine, Shantungosaurus, but no lambeosaurs. In sharp contrast the Laiyang exposures, where no big tyrannosaurs are known, have yielded the lambeosaur Tsintaosaurus and the primitive Tanius. Apparently, presence or lack of derived predators explains the vast difference in faunas despite the proximity of the two coeval habitats (Laiyang has the anklosaur Pinacosaurus also known from Mongolian strata dated c 73 ma, about the same age as Zhucheng strata--which also have the similar Sinankylosaurus).

A big tyrannosaur from the late Maastrichtian Dalangshan formation of southern China and a possible edmontosaur, Microhadrosaurus, suggest the same pattern.

Where huge tyrannosaurs were not yet present, lambeosaurs still held out. The Amur region, therefore, can be compared to the coeval, upper Horseshoe Canyon. In both paleoenvironements, lambeosaurs were at least temporarily safe.

Like Hypacrosaurus, Olorotitan, Amurosaurus and other crested forms thrived in the absence of Tyrannosaurus or a comparable taxon. Some lasted into early late Maastrichtian time, the age of the Blagoveschensk beds yielding Amurosaurus. It's possible, though, they succumbed in the last million years or so of the Maastrichtian. By then, the spreading Tyrannosaurus extirpated Hypacrosaurus, the last of the North American lambeosaurs, and other taxa. As the Dalangshang teeth indicate, large tyrannosaurs still existed in Asia toward the end. Like their North American counterparts, they may have extended their ranges into lambeosaur refugia, dooming the crested taxa.

Teeth of a large tyrannosaur from the late Maastrichtian of southern China.