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The Evolution Of Alioramus

August 25 , 2016

by starman

In 1976 Kurzanov described an unusual tyrannosaur from Nogoon Tsav. It appeared closely related to Tarbosaurus yet had features which set it apart, such as conspicuous bumps atop its snout, and a larger tooth count. On that basis, Kurzanov established a new taxon, Alioramus remotus.

In recent years, the discovery of similar specimens, named Alioramus altai and Qianzhousaurus, confirmed the existence of a separate group of tyrannosaurs, the Alioramini. Qianzhousaurus in particular, demonstrated that the differences between alioramins and other tyrannosaurs were not merely due to early ontogeny.

The existence of a sizeable tyrannosaur alongside Tarbosaurus raises the question of how the different taxa partitioned resources. It is reasonable to assume Alioramus hunted relatively small, light prey, while Tarbosaurus battled Nemegtosaurus, Saichania, Saurolophus, Therizinosaurus and Deinocheirus. One fact, though, suggests Alioramus did not coexist with Tarbosaurus, but supplanted it, at least to an extent.

It is interesting that Alioramus remotus and A. altai (which may be the same) were found in strata representing the uppermost part of the Nemegt formation. Nogoon Tsav, where A. remotus was found, and Tsagaan Khushu, which yielded A. altai, date from the end of the Nemegtian deposition. No alioramin is known from localities documenting the start or early phase of Nemegt time. This is true despite the fact that these localities, including Nemegt, have been as heavily prospected as stratigraphically higher beds. Although available statistics hardly permit definite conclusions, it does appear that Alioramus was a late arrival in the Nemegt.

How might that be explained? Possibly by a changing environment, and paleoecology.

Alioramins were long snouted and gracile tyrannosaurs. They almost certainly evolved to pursue light, fleet quarry. A long snout better enabled them to catch their prey. And there was no shortage of small, fast theropods in that environment. Ornithomimids appear to have been quite abundant. The Nemegt was less heavily forested than the Hell Creek. Much of it consisted of savannah. It was optimal habitat for fleet creatures.

There is evidence that, by upper or late Nemegtian time, ornithomimids were becoming more diverse and hence increasing in ecological importance. Like Alioramus, the ornithomimid Anserimimus (and a closely related taxon from Tsagaan Khushu) is known only from upper Nemegtian beds, specifically Bugin Tsav. Also known from that locality (and also from upper strata at Naran Bulak), Gallimimus was no longer the only ornithomimid in the Nemegt.

The highest level of the Nemegt formation tends to be poor in fossils. This level has sometimes been called the nogoontsavsvita. Consisting entirely of sandstone, it may hint at a drying environment. Even before its deposition, at Bugin Tsav, researchers noted caliches, suggesting “a hot and dry climate during the Nemegt age.” I’m not certain exactly what sort of paleoenvironment the nogoontsavsvita represents, but if a drying trend was underway-and it had happened before, between the Baynshirenian and Nemegtian periods–that might explain the evolution of new ornithomimids and tyrannosaurs. Less water meant an environment less able to support the giant prey of Tarbosaurus, yet still able to support a big population of smaller quarry–which diversified as new niches appeared, or were inherited. The late Nemegtian environment may have given rise to abundant, fleet prey.

How did Alioramus appear to hunt such prey? It is noteworthy that Alioramus and Tarbosaurus have features in common, such as a locking mechanism between the dentary and angular. In addition, some adult Tarbosaurus have nasal bumps. In view of these similarities, it is tempting to theorize that Alioramus branched off from Tarbosaurus just before the nogoontsavian, to radiate into its new niche. Some work, however, suggests the two tyrannosaurs weren’t closely enough related to support such a scenario.

Ornithomimids btw do appear to have been at least partly herbivorous. Alioramus probably evolved, or perhaps moved in, to exploit the new fauna.



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