November 23, 2011
From museum displays to comic books and feature films, Tyrannosaurus rex has been celebrated as one of the biggest, meanest and ugliest predatory dinosaurs of all time. The image of this long-extinct carnivore as the apex of the apex predators has a nearly unstoppable amount of cultural inertia. Maybe that’s why people get upset when paleontologists and artists suggest that the tyrant dinosaur was at least partly covered in a coat of feathers. (Cracked.com even listed an illustration of a feathered Tyrannosaurus as one of “17 Images That Will Ruin Your Childhood.”) Such images make it seem as if the old “prize-fighter of antiquity” has gone soft—how could such an imposing predator go in for such a silly look? Tyrannosaurus was no turkey, right?
To date, no one has found the fossilized remnants of feathers with a Tyrannosaurus skeleton. A few patches of scaly skin are known from some big tyrannosaur specimens, and those scraps represent about all we know for sure about the body covering of the largest tyrants. So why is Tyrannosaurus so often depicted with a coat of dino-fuzz these days? That has everything to do with the evolutionary relationships of the great tyrannosaur lineage.
Until the early 1990s, paleontologists often placed tyrannosaurus with Allosaurus, Spinosaurus, Torvosaurus and others inside a group called the Carnosauria. These were the biggest of the carnivorous dinosaurs. But the group didn’t make evolutionary sense. As new discoveries were made and old finds were analyzed, paleontologists found that the dinosaurs within the Carnosauria actually belonged to several different and distinct lineages that had branched off from one another relatively early in dinosaur history. The tyrannosaurs were placed within the Coelurosauria, a large and varied group of theropod dinosaurs which includes dromaeosaurs, therizinosaurs, ornithomimosaurs, oviraptorosaurs and others. Almost every single coelurosaur lineage has been found to have feather-covered representatives, including the tyrannosaurs.
In 2004, paleontologist Xing Xu and colleagues described Dilong paradoxus, a small, roughly 130-million-year-old theropod which may be one of the earliest tyrannosauroid dinosaurs known. (The Tyrannosauroidea contains all the big, famous tyrannosaurids, such as Tyrannosaurus and Albertosaurus, as well as their closest relatives.) Small patches of filamentous protofeathers were found along the dinosaur’s neck and tail, indicating that—at least during their early evolutionary history—tyrannosaurs may have been covered in feathers, too. But the relevance of Dilong to the question of feathered tyrannosaurs partially rests on what Dilong turns out to be. The initial description cast the dinosaur as a tyrannosauroid, but subsequent analyses have differed as to whether Dilong is an early tyrannosauroid (as in Carr and Williamson, 2010) or belongs to some other coelurosaur group (as in Turner et al., 2011).
For the sake of argument, though, let’s say that Dilong was not a tyrannosauroid and actually belonged to a different coelurosaurian lineage. Would this mean that tyrannosaurs didn’t have feathers? Certainly not. Feathers were a widespread trait within the coelurosaurs, and simple, fuzzy protofeathers may go back to the last common ancestor of the group. Otherwise feathers would have to have evolved near the base of every lineage, and there is no indication that feathers evolved so many times. The spread of feathers among almost all coelurosaur groups hints at a shared origin.
Since so many other coelurosaurs had feathers, it is fair to infer that tyrannosaurs also did. This hypothesis is no more unreasonable than saying that close relatives of the earliest mammals such as Morganucodon were covered in fur on the basis of their evolutionary relationships. And, to pick another dinosaurian example, no one has yet described an ornithomimid dinosaur with evidence of feathers, yet we are comfortable attributing feathers to them because they are coelurosaurs. (Maybe their vaguely ostrich-like appearance helps a bit in this regard.) If feathers can reasonably be inferred for ornithomimosaurs on the basis of their family tree, then we can do so for tyrannosaurs.
So, within this evolutionary bracket, what kind of feathers might have clothed Tyrannosaurus and kin? The simple dino-fuzz of Dilong is a fair bet. Perhaps such a body covering would have served for insulation, but then again, the patchy distribution of filaments on Dilong and other coelurosaurs has raised the suggestion that some dinosaurs were only partly coated in feathers. Whatever their distribution on tyrannosaur bodies, though, the feathers probably didn’t look like those which allowed other coelurosaurs to eventually take to the air. After all, feathers were probably used for display and the regulation of body temperature first, and since no tyrannosauroid even came close to flying we should expect for them to have relatively simple feathers related to these functions.
Regarding Tyrannosaurus specifically, the tyrant king may have had feathers only during the early years of life. A fuzzy coat may have helped hatchling and juvenile Tyrannosaurus regulate their body temperature, but as the animals grew, the benefits provided by insulation may have disappeared. (Retaining heat is a problem often faced by small animals, while shedding excess heat is a problem faced by larger animals due to changes in surface-to-volume ratios as animals grow.) Maybe an adult Tyrannosaurus would have patches of protofeathers here and there, as in Peter Schouten’s illustration of the dinosaur, But given the evidence at hand, it is likely that baby Tyrannosaurus would have been fuzzier than their parents.
Frustratingly, though, we may never know for sure what sort of feathers Tyrannosaurus might have had, or during what part of life. Circumstances of fine preservation are required to detect feathers, and even then, sometimes only patches are preserved. The types of environments Tyrannosaurus lived in were not exactly amenable to the kind of rapid, fine-detail preservation required to detect feathers. Even in cases where skin patches are preserved, it is difficult to know whether there might have been protofeathers on other parts of the body, or whether some of those feathers fell off or otherwise eluded preservation. Delicate structures require delicate preservation to detect.
What we can say is that the idea of a feather-covered Tyrannosaurus is a reasonable hypothesis. We still know so little about the body covering of this dinosaur that artists can reasonably restore the dinosaur with scaly skin, a coat of feathers, or a patchwork of both (I would especially like to see more renditions of that third possibility). Perhaps future fossil discoveries will provide us with a clearer picture of what Tyrannosaurus looked like, but the current unknowns are fascinating. Asking what Tyrannosaurus looked like is not just a matter of speculation—obtaining an answer requires that we consider the patterns and processes of evolution, as well as the methods we use to restore creatures that have been dead for millions upon millions of years. Feather-covered or not, though, I wouldn’t want to call Tyrannosaurus a turkey to its face. If I did, I don’t think I could run away fast enough to avoid becoming the dinosaur’s Thanksgiving dinner.
From everyone here at Dinosaur Tracking, we hope that you enjoy your holiday dinosaur and have a warm Thanksgiving.
Carr, T., & Williamson, T. (2010). Bistahieversor sealeyi, gen. et sp. nov., a new tyrannosauroid from New Mexico and the origin of deep snouts in Tyrannosauroidea
Journal of Vertebrate Paleontology, 30 (1), 1-16 DOI: 10.1080/02724630903413032
Turner, A., Pol, D., & Norell, M. (2011). Anatomy of Mahakala omnogovae(Theropoda: Dromaeosauridae), Tögrögiin Shiree, Mongolia American Museum Novitates, 3722 (3722), 1-66 DOI: 10.1206/3722.2
Xu, X., Norell, M., Kuang, X., Wang, X., Zhao, Q., & Jia, C. (2004). Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids Nature, 431 (7009), 680-684 DOI: 10.1038/nature02855
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