XIONGGUANLONG

From an evolutionary point of view, tyrannosauroids are enigmatic critters. We know the middle Early Cretaceous forms, such as the small, fuzzy, long-armed Dilong, and we know the latest Late Cretaceous forms like the bone-crunching, short-armed, giant Tyrannosaurus. What we didn't know, as of 2008, is much of what went on in between. Fortunately, we now know Xiongguanlong from the middlest Middle Cretaceous of China, which is the best-represented tyrannosauroid from the entire Barremian-Campanian period, no less, and shed a little much-needed light on one of the missing chapters of "tyrant lizard" evolution.

Xiongguanlong is similarly slender, fuzzy and long-armed but somewhat bigger than the oldest (smallest) tyrannosauroids, but somewhat smaller than the large-skulled, robust-jawed and short-armed latest (largest) tyrannosauroids, though it does mark the earliest appearance of some of the latter's hallmarks. A short, broad braincase, broad bony struts at the temples and expanded areas for jaw muscle attachment all represent modifications for increased bite force, combined with a midline crest at the back of the skull and expanded structures on the neck vertebrae to support a large head. Funnily enough, Xinguanlong had a large head thanks mainly to a stupendously slender, long, smooth snout that accounted for two-thirds of its skull's length. And its teeth were blade-like rather than conical, so tyrannosauroids weren't crunching bones just yet.