Since its discovery more than 100 years ago, Tyrannosaurus rex has reigned as king of the predatory dinosaurs. Popularized by countless books, television shows, and a certain 1993 movie, the "tyrant king" is well known to nearly everyone on Earth. It was a burly bear-faced monster with long banana-shaped teeth lining a mouth that could swallow a person whole. Tyrannosaurus stood on two strong legs, but had arms shorter than you or me. From snout to tail tip, its skin was covered in scales.
About that last bit though, are we sure it was scaly?
Tyrannosaurus and its closest kin, collectively known as tyrannosaurs, belong to a group of dinosaurs called coelurosaurs. They are known for their extremely birdlike anatomy and feathered fossils. These fossils show that dinosaurs' earliest feathers were hair-like. Over time, they evolved fluffy branched forms and eventually the flat-vaned feathers sported by birds we see today. The large tyrannosaurs from the latest Cretaceous, such as Tyrannosaurus, Tarbosaurus, Albertosaurus, and Gorgosaurus, left few skin impressions for us to study. These impressions were small, only about the size of a coin. From these impressions we can see that the feet and underside of the tail were covered in small scales.
Other impressions from other parts of the body are described as looking completely naked, or like plucked chicken skin. Unfortunately, none of these impressions have been formally published and illustrated in a scientific journal by researchers.
But is this information enough for paleontologists to claim that Tyrannosaurus was fluffy?
To determine the presence or absence of features not preserved in the fossil record, scientists use a method called phylogenetic bracketing. Phylogenetic bracketing looks at the closest relatives of an organism and uses their anatomy to infer what was probably present but currently unknown.
For instance, the extinct saber-toothed cat, Smilodon, is currently known from just bones and teeth. With phylogenetic bracketing, we can deduce that like other cats, it was covered in fur. What's more, phylogenetic bracketing predicts that its fur was probably tan colored with darker ring-shaped spots. Of course, finding a Smilodon fossil that certainly showed that it had naked skin or black fur would take priority over phylogenetic bracketing. But in the absence of such fossils, the most conservative approach is to trust the predictions of phylogenetic bracketing.
This technique can be applied to all sorts of extinct organisms, including, of course, Tyrannosaurus.
Tyrannosaurus may have not been found with feathers. But two earlier members of the tyrannosaur group — Dilong and Yutyrannus — were found with long, hair-like covering. Although we have discovered impressions showing scales on the feet and underside of the tail, the distribution of scales is similar to Juravenator, a coelurosaur known to have had both feathers and scales.
Additionally, the skin impressions from other parts of tyrannosaurs' body are described as naked or "plucked." This is consistent with animals that may have had feathers in life, but lost them before fossilization.
Based on our current knowledge, we can tell that Tyrannosaurus was quite unlike the pop culture image of a reptilian monster. While future fossil finds may answer this question with certainty, our current knowledge suggests that the king of the dinosaurs was fuzzier than we remember.