April 27, 2009
Even though Tyrannosaurus has been a dinosaur celebrity for a century, we have only recently begun to understand how it evolved. For many years it seemed to be a larger and deadlier version of earlier carnivorous dinosaurs like Allosaurus, but recent discoveries have placed Tyrannosaurus and its relatives among the coelurosaurs. New finds made in Asia, in particular, have revealed that some of the earliest dinosaurs that would give rise to Tyrannosaurus were small, long-armed predators that were covered in feathers. Frustratingly, there has been a 50-million-year gap in our knowledge between these early types and the more familiar tyrannosaurids, which flourished between 80 and 65 million years ago, but a new discovery announced in the Proceedings of the Royal Society B helps to fill in the void.
Called Xiongguanlong baimoensis, the new tyrannosaurid comes from sediments in western China that are 125 million to 99 million years old. Even though the skull was somewhat crushed during fossilization, it is well enough preserved to show that this dinosaur had a long, low skull that was broad across the back. Interestingly, though, it was not the largest theropod dinosaur of its time. Other non-predatory theropods, like a recently-announced ornithomimosaur named Beishanlong, were considerably larger. Indeed, it was not the bone-crushing terror that its later relatives would be.
Xiongguanlong occupies an important place in tyrannosaurid evolution. Not only did it temporally exist between the earliest tyrannosauroid dinosaurs and the later, larger genera, but it is intermediate between the two in terms of form, as well. This does not mean it is necessarily the direct ancestor or descendant of any known dinosaurs, but it is useful in determining the general pattern of tyrannosauroid evolution. It also hints that there is still a lot left to be found: who knows how many other strange tyrannosauroids there once were?
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