Alright, so I’m just going to come out and say it. I was wrong! Which is cool, because science just happened, in the form of me hypothesizing on available evidence, and new evidence coming to light supporting an alternative hypothesis. That’s science, and it’s awesome!
So I am of course talking about the case of the feathery T. rex. As pretty much everyone in the palaeo community knows, a new study just came out summarizing a series of skin impressions from various areas of the bodies of later tyrannosaurids that many (myself included) had thought to be feathery. Now, this paper has come under a lot of criticism, with some claiming that they do not even represent actual scales (and there has been talk of specimens that contradict these), and some of the authors (emphasis on some) are hesitant about this conclusion, but I’m not going to talk so much about whether or not it’s valid. I’m going to talk about the implications for what if it is valid, and what that means for our understanding of T. rex as a dinosaur.
Part 1: Large-scale secondary scale evolution in a coelurosaur!
This is the biggest thing to come from this paper. To our knowledge, every single species of coelurosaur thus far discovered has been found with an extensive body covering of feathers, and secondary scale evolution (if it has even happened, I have my doubts) has been mainly limited to just the feet. This means that these large tyrannosaurids represent the first coelurosaurs to fully ditch their feathery coating, as all basal members discovered with preserved integument show a distinct lack of body scales, and a plethora of feathers.
This is really cool, mainly because it represents on a massive scale the potential for feathers to evolve back into scales that very closely resemble those from other non-coelurosaurs. This has pretty big implications in regards to the genetics of keratinous integument, but the big question I have regarding this secondary scale evolution is why?
Part 2: Why?
This paper proposes a few hypotheses in regards to why this might have happened. One is that rhinoceroses and elephants in more forested environments have a thicker coat of hair than those in more sunny environments. This was cited to Dave Hone’s book The Tyrannosaur Chronicles: The Biology of the Tyrant Dinosaurs, and while the information I found was slightly different (rhinos were not mentioned to possess this trait, and it was stated that asian elephants had “the options of forest shade and water”), I decided to pursue this topic more. This crucial argument is missing quite a bit, namely the addition of another species of elephant that actually lives in Africa that inhabits forests the majority of the time. The African Forest Elephant inhabits the forests of the Congo basin, and has pretty much no difference in the covering of hair on its body (especially considering how the feathers of Yutyrannus were a massive, thick, full body covering of 30cm bristles, not a small bit of fuzz of the tops of their heads). So it seems that this idea of a loss of feathers being a response to less-forested environments isn’t that strong.
Another hypothesis presented was that this was an adaptation in response to more cursorial habits. This is a reasonable hypothesis, and I look forward to possible tests to determine whether or not this prompt such a feather loss as can be seen between Yutyrannus and Albertosaurus (thanks to Thomas Holtz for the heads up on this!)
Finally, the idea is put forward that gigantism is the driving factor, with larger tyrannosaurids having a greater need to dump excess heat. Again, though there’s a massive problem, which the authors actually highlight. The mass estimates of Yutyrannus and Albertosaurus are remarkably similar, despite the fact that this secondary feather loss supposedly occurred at the latter. Why would they lose feathers to cope with gigantism, if there was no change in mass?
So essentially, there is no clear reason presented in the paper for why this event occurred. However, if we assume that these impressions do represent a full-body covering of scales, there’s a much bigger problem then why.
Part 3: Winter is coming.
The fact that the authors compared tyranosaurids to large mammals is particularly interesting. So let’s take an African Bush Elephant, the largest species of elephant, almost completely bare-skinned, and the largest living land animal on the planet. The mass of an adult Loxodonta africana is between 6146 and 7464 kg. The estimated mass of an adult T. rex in this paper is 5014 to 8361 kg, so our range is about the same in terms of body mass (average of 6,805 kg in L. africanus, and 6,688 in T. rex) . Now, lets look at the environments that they lived in.
L. africana inhabits savannahs, deserts, and rainforests around sub-saharan Africa. The average temperature in which this large animal inhabits is about 20-30 degrees celsius, and this temperature is usually a constant in the area it lives in, not really varying between seasons.
By contrast, T. rex evolved to inhabit an area in which the mean annual air temperature (MAAT) was approximately 10 degrees celsius, as did all other tyrannosaurids that these impressions were found in. This is especially important considering that this was not a constant value, and this average was brought by a warm season and a cold season, with the warm season warm enough to support the warm-weather plants in the area, but cold enough in the winter to bring down the average to 10 degrees.
Now the big question is this: Can animals of a body mass of about 6,700 kg survive in such conditions? Elephants can, of course, survive in freezing temperatures, but only for a short amount of time. In fact this became a problem in Canadian zoos, such as that in my city, Toronto, which incidentally has a mean annual temperature of about 10 degrees celsius. It was found that once the temperature had dropped below a certain point (between 5 and 10 degrees celsius), they had to spend the majority of their time indoors, or freeze to death. As such, if tyrannosaurids of a similar body mass were to inhabit such conditions with only a layer of skin separating their bodies and the cold, they would freeze without the manmade structures that the Toronto elephants had. And while I do recognize that there are distinct behavioural and physiological differences between elephants and tyrannosaurs that could have allowed tyrannosaurs to maintain higher body temperatures (more active lifestyles for example), there does seem to be a trend in animals that live through winters in which herbivores such as caribou, being able to constantly metabolize a vast supply of food are able to keep warmer and keep more active lifestyles than carnivores such as wolves and coyotes, which must often take extended periods of inactivity to keep warm, often relying on fat reserves due to a dearth of food.
Now the authors of the paper do acknowledge that the temperatures of these areas are relatively cold. However, they do not realize the full impacts of this fact, merely suggesting that “[feather] loss is unrelated to palaeoclimate”. However, what they do not examine is that despite climate not playing a role in the secondary loss of feathers in tyrannosaurids, these animals still would have had to adapt to the climate! Feathers (while potentially acting as cool-down instruments) are great insulators, and their presence on Yutyrannus in a 10 degree MAAT environment means that this animal was being kept warm in its cold environment. If later animals lost this insulation, despite changes in body mass, they would have frozen without additional insulation.
Assuming tyrannosaurids were nonmigratory (there is no fossil evidence to suggest they were migratory), and that these impressions represent an actual scaly covering, they must have evolved a form of insulation to help them cope with the cold. This could have taken many different forms, perhaps a layer of insulating fat. But what is most interesting is that the drive to lose these feathers must have outweighed the drive for these animals to stay warm.
As such, the discovery of these impressions does not simplify the story of tyrannosaurid integument at all. Instead, it makes it all the more complex.
That’s all for now. See you soon