Parsimony

If an animal preserves several impression from several parts of the body that all show the exact same pattern (as in T. rex), and close relatives also show the same pattern from different parts of the body (as in tyrannosauridae), then the most parsimonious conclusion is that this pattern covers the whole body. Suggesting that the other parts of the body lacking impressions (of which there are very few) might have had feathers struck us as a “god of the gaps” argument in relation to universal origins. Defaulting to the classic Occam’s razor heuristic, the most parsimonious explanation was simply that the whole body was covered in scales.

To quickly address the idea that the scales are actually textured skin: we haven’t ruled this option out, but we feel parsimony also comes into play here. These structures look very much like the reticulae and small scutella we see on the feet of birds and well, if it looks like a duck… We would be more concerned with the taphonomy argument if we were getting these isolated scale-like impression from other North American coelurosaurs, but we aren’t. In fact, in recent years we have obtained several beautiful ornithomimid specimens from similar sediments that preserve extensive impressions of feathers.

To put it simply: these arguments feel a little too much like wishful thinking for our liking, and in our opinion the most parsimonious explanation is that tyrannosaurids were covered in reticulae or small scutella.

2. Feathers and Gigantism

In addition to the physical evidence for reticulate scales, there is also a biomechanical argument to be made. One of our major consultants on this reconstruction, Scott Hartman, has been conducting physiological modelling on early dinosaurs and other reptiles, including quantifying thermal constraints (Hartman 2015, Hartman et al. 2016). He is not working specifically on T. rex, but his research has implications for its potential feathering. According to his research, depending on ambient temperature, animals stop receiving any benefit from dermal insulation at somewhere between 1 and 3 tonnes. Due to the costs of producing such integument, this may cause these traits to be selected against, as has occurred in many large mammals and fur.

Looking at the skin evidence from the rest of the dinosaur family tree, this shouldn’t be that large of a surprise. If we assume that filaments are basal to all dinosaurs, then they have already been lost at least 4 times prior to tyrannosauroids: thyreophorans, cerapods, sauropods, and ceratosaurs. This makes feather loss in one additional clade hardly a stretch of the imagination.

Occasionally, it has been suggested in the literature that juvenile tyrannosaurids may have had feathers which they shed in favour of scales as they grew. While not technically impossible, we have not opted to explore this idea as no animal, living or extinct, is known to do this.