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An outline of the tyrannosauroid Kileskus, showing known parts in dark grey. Art by Conty, image from Wikipedia.

K is for Kileskus

Tyrannosaurus rex was the ultimate carnivorous, non-avian dinosaur. Even though lesser-known theropods – such as Spinosaurus and Carcharodontosaurus – were supposedly a little longer or heavier, they simply can’t compete with the celebrity of T. rex. For over a century, the giant tyrannosaur has represented the biggest, most powerful, and last of its lineage.

But tyrannosaurs were not always so imposing. Over the past few decades, paleontologists have been able to trace tyrannosaurs back into the Jurassic, when the predators were small, fuzzy carnivores that skittered around landscapes dominated by a cast of different carnivores. The recently-named Kileskus aristotocus was one of these meek tyrannosaurs.

Much like Stokesosaurus from Utah, England’s Proceratosaurus, and archaic tyrannosaurs from elsewhere, relatively little of Kileskus has been found. Paleontologist Alexander Averianov and coauthors described the dinosaur in 2010 on the basis of a few skull elements and possible bones from the theropod’s hands and feet. The fossils had been found in the roughly 165 million year old rock of West Siberia, Russia.

Despite the paucity of the known remains, however, Averianov and colleagues picked out characteristics that ally Kileskus with Proceratosaurus and the crested Guanlong from China’s Jurassic fossil beds. Specifically, within the greater tyrannosauroid family, Kileskus fell within a group of early tyrannosaurs that had different forms of cranial ornamentation called proceratosaurids. No sign of such adornments can be seen on the available bones of Kileskus, but, given its relationships, it wouldn’t be surprising if the dinosaur had some sort of nasal horn or flashy crest.

Kileskus was not a big dinosaur. Although it’s impossible to say exactly how long the tyrannosaur was, the dinosaur’s maxilla – the major tooth-bearing part of the upper jaw – is just under a foot long. The same bone in adult Tyrannosaurus is two feet long or longer, so Kileskus was a pipsqueak compared to its later relatives. And even during the time Kileskus roamed prehistoric Russia, the megalosaurs, ceratosaurs, and allosaurs were the large-bodied top predators. Tyrannosaurs would not rise to dominance and expand their range of body sizes until tens of millions of years later. Thanks to geological hindsight, we can see Kileskus at the humble beginnings of one of the most voracious dinosaur lineages of all time.

Previous Entries in the Dinosaur Alphabet series:

J is for Juravenator


Averianov, A., Krasnolutskii, S., Ivantsov, S. 2010. A new basal coelurosaur (Dinosauria: Theropoda) from the Middle Jurassic of Siberia. Proceedings of the Zoological Institute RAS 314, 1: 42–57.