Hello! My name is Klara Widrig, and I am a PhD student in the department of Earth Sciences as well as a tour guide for the Bridging Binaries program here at the Sedgwick Museum. 

The display we are looking at here is a cast of the skull of a Tyrannosaurus rex from South Dakota in the United States. The fossil was nicknamed “Stan” after the person who first found it, and it’s especially famous because the skull is almost perfectly preserved, and most of the bones of the body were also intact. 

Because we have so much of Stan’s skeleton, lots of paleontologists have compared Stan to other very complete Tyrannosaurus skeletons, like Sue the T. rex at the Field Museum in Chicago. By making these comparisons, they realized there are two different types of T. rex skeletons: heavy “robust” morphs, and more delicate “gracile” morphs. Stan here is an example of a gracile morph. The two morphs might represent differences in age, individuals from different geographic areas, or males and females, with evidence to suggest females were the robust morph and males the gracile. If true, this would be an example of what is called sexual dimorphism.

If T. rex really was sexually dimorphic, it could have implications for their social behavior. 

However, when we make speculations about how extinct animals might have behaved, it is important that we don’t let our own cultural biases about gender roles get in the way. For example, when a fossil of an adult dinosaur called Citipati was found sitting on a nest of eggs, it was nicknamed “Big Mamma”.  But was Big Mamma actually a female? Birds are directly descended from predatory dinosaurs, and in many birds it is common for both parents to take care of the nest. So, based on the behaviour of dinosaurs’ living relatives, that means it’s entirely possible that Big Mamma was actually the father! If the robust morph Tyrannosaurus skeletons were females, they might have been more territorial and attack larger prey than the smaller males. This is actually the same pattern we see in many modern birds of prey, with females of some species being up to 50% larger than the males.

This is still an exciting area of ongoing research, and hopefully one day we’ll have more information about robust and gracile morphs and how T. rex really behaved.