And You Thought Your Grandparents Were Dinosaurs: Raptorex the O.G. of Tyrannosaurids

Had you asked anyone before Speilberg’s 1993 thriller Jurassic Park what they believed to be the scariest dinosaur, many would have probably answered Tyrannosaurus Rex.  But while that box office hit elevated interest in paleontology to more than just the passing fancy of every ten year old, it certainly diminished the reputation of the mighty carnivore, as people turned their attention to a relatively unknown character from the Cretaceous, the Velociraptor.  We may just be blessed with another terrific installment of the Jurassic Park franchise as a recent fossil discovery in Mongolia shows the existence of a new dinosaur, physiologically similar to the mighty T. Rex, but about the same size as Velociraptor.

Tyrannosauroidea, the clade incorporating T. Rex and his closest relatives, truly ruled the landscape of the late Cretaceous.  These multi-ton monsters comprise nearly all large carnivore skeletons collected dating to that time.  T. Rex alone has left us over thirty specimens, many with near complete skeletons, allowing for a rich understanding of the body plan for these creatures.

What is this body plan - what makes a Tyrannosaurid?  Of course all Tyrannosaurids carry the notable T. Rex attributes, namely the proportionately large head and comically tiny front legs.  But there is a long laundry list of other attributes that define this class of predator including things like the size of the olfactory bulbs.  The new species, named Raptorex kriegsteini, fits all the physiological criteria of Tyrannosaurids to a T, except for the size.  Being a 1/100 miniature clone of the great predators, the question worth examining isn’t what Raptorex is, but rather how Raptorex fits into the evolution of the larger Tyrannosaurids.

Large Tyrannosaurids ruled the land from about 90 million years ago to the great mass extinction 65 million years ago.  But smaller versions of these dinosaurs, albeit none nearly as similar as Raptorex, have been discovered in Northern China in the last ten years.  These fossils, like Raptorex date from about the mid Jurassic to early Cretaceous periods - approximately 125 million years ago.

It appears the Tyrannosaurid body plan came first in trial-size, and then rapidly grew to the fierce predator we know and love.  The evolutionary development as we currently understand it can be separated into three major morphological stages.  The first stage is comprised of small to medium sized dinosaurs that shared some physiological similarities to their larger descendants.  The Guanlong and Dilong dinosaurs of northern China characterize this stage. Raptorex introduces an entirely new stage in the evolutionary development where almost all of the morphological attributes of the later specimens are present in a smaller ancestor.  The third stage is defined by the rapid growth of the organisms that took place sometime in the early Cretaceous.

Questions and comments: andrewhaynie2009@u.northwestern.edu