Paleontology has underlying assumptions about how the past is imagined. Hypotheses about the behavior, locomotion, and ecology of extinct animals depend on scientists’ perception of modern animals along with their intuitive interpretation of fossil specimens (and it is to some degree intuitive, whether they admit or not). In turn, those hypotheses, once published for other scientists and the public to digest, feed the collective imagination and discourse around paleontology.
Perhaps no other subject illustrates this principle more than the ongoing popular and academic controversy over dinosaur integument, or body coverings. In fact, we can go further and center that debate on one genus and species – Tyrannosaurus rex.
A favorite of fanatics and professors, T. rex, alongside it’s toothy relatives, draws intense scientific and media interest. This has been the case since the first mounted skeleton debuted in New York in the early twentieth century, and only escalated after that same skeleton’s silhouette graced the cover of Michael Crichton’s Jurassic Park in the late 1980s. Just one Tyrannosaurus specimen (Stan) has been featured in over fifty scientific publications, and that same specimen sold for over thirty million to an undisclosed buyer in October of 2020. The scientific significance fed the public’s appetite for this real-life monster, and the allure of studying this toothy critter in turn fueled generations of researchers, fossil hunters, and paleo-artists.
What did T. rex look like when it was alive? One early discovery supposed the animal might have had crocodilian armored scutes, but it was later found the armor was from a different, herbivorous dinosaur. Regardless, up until the 2000s, illustrations ubiquitously painted the animal as generically scaly, with an integument that couldn’t be readily distinguished as anything other than vaguely reptilian. Other dinosaurs had yielded skin fossils, particularly the duck-billed hadrosaurs. In general, these specimens exhibited small, wrinkly scales, more like the skin seen on the feet of tortoises and birds than the overlapping scales of snakes and lizards.
The discovery of small meat-eating dinosaurs from China preserved with primitive, simple, feathers changed this paradigm. The gradation from dinosaur to bird became blurred, not just in the evolutionary sense, but also in the lens dinosaur devotees used to look at bygone Mesozoic era. This stream of discoveries culminated in the discovery of Yutyrannus, a two ton meat-eating cousin of Tyrannosaurus. Suddenly, a feathered T. rex wasn’t such a crazy idea. As the distribution of feathers in dinosauria became wider and wider, some proposed that the group was ancestrally fluffy and that the scales recovered previously were outliers, or at least weren’t telling the complete story.
In fact, to people invested in paleontology, the subject became something of shibboleth. Did you favor the old, scaly depictions that spoke of a primitive world on the savage path to modernity, ascending Aristotle’s “great chain of being?” Or were you committed to a much fuzzier breed of saurian, one that attested to a tightly interconnected, predictable world that easily slotted into the modern paleontological synthesis?
Since Yutyrannus, some direct fossil evidence has come to light. Small skin impressions from T. rex and other tyrant dinosaurs – described in detail by researcher Phil Bell and others in 2017 – have shown finely grained, pebbly skin – not quite feathers, but also not the rugged, draconian look that decades past have shown. Nor are these skin samples concrete evidence of T. rex’s fashion sense – the recovered skin only covers a small area, with the largest segment measuring only several square centimeters. Extant dinosaurs in the form of birds are covered in feathers, but they often have naked skin on the neck and head. Ostriches even have bare skin on their thigh and lower torso, and this epidermis sometimes resembles the pebbly texture seen in T. rex. And if the geologically older and related Yutyrannus possessed feathers, why not T. rex?
On the other hand, large animals often have trouble disposing of excess warmth. For the heaviest land animals today, overheating is a very real problem. This is why elephants, rhinos, hippos, and even water buffalo have only a sparse covering of fur, despite the likelihood that their smaller ancestors were shaggy like most other mammals. Because those animals live in hot climates, their high mass means that they retain more heat than they shed. From what we know, T. rex weighed up to eight and half tons and lived in a climate much like that of Holocene (our current Geological epoch) Florida, so it probably faced similar physiological challenges. It’s possible T. rex might have evolved from feathered ancestors but became secondarily naked to cope with the stresses of gigantism. Embryological studies have shown that the ‘scales’ on bird feet are actually developmentally stunted feathers, and there is no biological law preventing that process from occurring all over a theropod’s body.
All of this also glosses over the important fact of taphonomy – the processes and stresses an organism undergoes as it passes from the world of the living to the world of rock and stone. The fossil record already has a bias against soft-tissue, so any scales or feathers we find must not necessarily be taken at face value. After all, sixty six-plus million years in the ground can facilitate all kinds of distortion.
Perhaps, given the data accumulated so far, paleontologists and dinosaur enthusiasts should allow for a certain degree of flexibility in dinosaurs and their kin. The ‘scales’ some clearly possessed were not those of snakes, nor were they really like the armor of crocodilians. Instead, they were more often than not a kind of pebbly, cornified, epidermis, allowing for protection and flexibility like that of modern animals. On the other hand, as avian feathers evolved we should expect to see a variety of filamentous body coverings, some perhaps suspiciously similar to each other. The development of different integument across different lineages over nearly two hundred million years should give those angling for easy answers pause. Convergence and atavism exist, even and especially among closely related clades. This messiness is the rule in nature, and not the exception.
In the end, whether or not T. rex appeared reptilian or avian (or some combination of the two) is of little relevance to the big paleontological questions of evolution and extinction. It does not help us solve our climate crisis. There is nothing we could learn about thermoregulation in large animals from this fossil species that we could not learn from living ones. However, what it does shed light on are different visions of our shared natural history. If you’ve ever been concerned about the accuracy of dinosaurs in Jurassic World or complained that “Science ruined dinosaurs,” you’re a part of this imaginative landscape. Whether you’ve adopted a retrograde or progessive stance, you’ve invested energy in bringing prehistory to life. To paraphrase famed paleontologist Dr. Robert T. Bakker, “the best and only time machine we will ever have is the one between our ears.”
By Alex Ruger, Earth.com Staff Writer