[NOTE: Post has been updated to include a section on scale size]
This has certainly been an interesting year. Two papers dropped in the past three months that have put the brakes on a recent trend in paleo-art. That trend? Why the feather-coated T. rex of course.
First, in March, we saw the release of a paper detailing a new species of Daspletosaurus and its relationship to D. torosus.
Carr, T.D., Varricchio, D.J., Sedlmayr, J.C., Roberts, E.M., Moore, J.R. 2017. New Tyrannosaur with Evidence for Anagenesis and Crocodile-Like Facial Sensory System. Scientific Reports. 7(44942):1–11.
In this paper, Carr et al. argue for the designation of a new Daspletosaurus species, D. horneri. The authors argue, based on skull shape and chronostratigraphic position, that D. horneri was the direct ancestor to D. torosus. I thought that the authors put forth a compelling argument for this anagenic event and backed up their position well. Interestingly, this part of the paper should have been the most controversial. As anyone who has read anything from Horner and Scanella over the past eight years can attest, arguing for a direct ancestor-descendant relationship for dinosaurs is difficult to do and even harder to win over others in the field. So it is somewhat surprising to see a case for anagenesis in Daspletosaurus taken so well by the palontological community. All the more so given that it involves a tyrannosaur, the poster children for “cool guy” dinosaurs.
Instead, the most controversial part of the paper wound up being their soft-tissue reconstruction of the face for D. horneri. The author responsible for the soft-tissue reconstruction was Jayc Sedlmayr of Louisiana State University. Sedlmayr did his doctorate on osteological correlates for vasculature in extant archosaurs (birds & crocs). He is the seminal alumnus of the WitmerLab and thus is well within his wheelhouse for this type of soft-tissue reconstruction. Sedlmayr borrowed heavily from the work of another WitmerLab alumnus, Tobin Hieronymus, whose PhD work involved osteological correlates for integument on the skulls of animals. Although the skin is often well away from the underlying bones on most of the body, there are exceptions when it comes to the skull. There, areas that are not heavily muscled, tend to show intimate connections between the skin and the underlying bone. Hieronymus used these connections to determine how different integumentary appendages (scales, hair, feathers) affect the underlying bone (Hieronymus & Witmer 2007; Hieronymus et al. 2009). The authors found that the surface texture along the skull of D. horneri was “hummocky”. That is, it was covered in lots of closely packed ridges. According to Hieronymus & Witmer (2007), this texture correlates to scales as the overlying integumentary appendage. Thus, according to the authors, D. horneri had a scaly face (this is grossly oversimplified as the authors were able to piece together a variety of different integument variants along the skull, but you get the idea).
Scaly tyrannosaur cannonball one had been shot.
Then two weeks ago, we saw the release of another paper on tyrannosaur integument. However, unlike the previous paper, this one was specifically dedicated to integumentary types in tyrannosaurids.
Bell, P.R., Campione, N.E., Persons, W.S., Currie, P.J., Larson, P.L., Tanke, D.H., Bakker, R.T. 2017. Tyrannosauroid Integument Reveals Conflicting Patterns of Gigantism and Feather Evolution. Biology Letters. 13:20170092
In this paper, the authors set out to survey all known instances of “skin” impressions for tyrannosaurids. Their list of taxa included Albertosaurus, Tarbosaurus, Daspletosaurus, and Gorgosaurus. Their results pretty definitively indicated that scales were the predominant integumentary appendage on tyrannosaurids. The authors then went on to speculate why that would be if earlier tyrannosauroids had filamentous integument. They performed an ancestral character state reconstruction based on Parsimony and Bayesian-based trees from Brussatte and Carr 2016. Their results found that filaments came out strongly as the ancestral character for tyrannosauroids, but by no later than Tyrannosauridae proper, a reversion to scales had taken effect. The authors attributed this to body size evolution. Namely, larger tyrannosauroids reverted to scales over protofeathers.
Cannonball number 2 had just been shot.