Our beloved Tyrannosaurus , the great carnivorous dinosaur, king of the tyrannical reptiles, may have been covered in feathers . And there is a lot of debate around this. There are reconstructions that show it scaly, others fully feathered, like a kind of 9-ton chicken… Where does this come from? What is most likely?
Feathers, like scales, hair, or soft tissue, are rarely preserved in a fossil organism. If it is already a truly exceptional case for an animal to fossilize, for this fossil to contain information about soft tissues is even rarer. However, sometimes that happens, and we speak of “exceptional conservation sites”. Does that mean Tyrannosaurus remains have been found associated with feather impressions? Not exactly, it is a phylogenetic inference.
Let’s go by parts. The phylogeny of an organism is nothing more than its evolutionary history and its position in the “family tree” of its group. So when we talk about “phylogenetic inferences”, they are nothing more than inferences made from what we know about their close relatives. Can we ever make these inferences? We can, since we know that nature is very conservative: the energy resources of an organism are very expensive, and that is why throughout evolution new structures only appear if the carriers are favored by having them. And to be favored in this context is nothing more than being able to reproduce successfully. The consequence of all this is that in the same group it is normal for the characteristics to be similar, because introducing extra changes is energetically and evolutionarily “expensive”. Thanks to this, when we find an isolated dinosaur bone we can classify it and even propose a reconstruction of this animal “ roughly” . Because these remains belong to an animal with many unknown new characteristics is something “unlikely”, “expensive”. This is what we refer to in paleontology and evolutionary biology when we speak of “maximum parsimony”, the minimum change.
In the early 2000s, ornithologists Richard O. Prum and Alan H. Brush proposed that the origin of feathers might be hidden in their development. We call this discipline, that of relating embryonic development to the evolution of a lineage, Evolutionary Developmental Biology or “Evo-Devo” , and it has its roots in the works of the German naturalist Ernst Haeckel. Well, Prum and Brush proposed that the evolution of feathers had followed steps or stages similar to those of their development in bird embryos.
In itself, this hypothesis is very interesting, but what was really impressive was that these hypothetical primitive feathers or “protofeathers” were very similar to the structures observed in many dinosaurs found in the special conservation sites of Liaoning , in China. In these impressive deposits, dinosaurs have appeared showing feathers in different stages.
There is a methodology in paleobiology called “Extant Phylogenetic Bracket” or EPB that would be translated as “phylogenetic bracket of current relatives”. This method, introduced by paleontologists such as Witmer or Bryant and Seymour, establishes that, in order to infer what a non-conserved structure would be like in a fossil living being – for example, the presence of feathers or scales in a dinosaur, but also the position of a muscle or determinate ligament-, we must see the two current groups with which it is related: these two groups would form the “brackets”. In the case of dinosaurs, birds and crocodiles are used. And thanks to the existence of deposits of exceptional conservation, other fossils that do show the characteristic we are wondering about can be used for this “parenthesis”.
Thus, we can ask ourselves the question : which dinosaurs would be feathered? And answer it according to current knowledge. Specimens with fully developed feathers on the arms and tail (remiges and wheel feathers), cover feathers (those that cover the trunk of birds) and “protofeathers” as the down of some chicks have been recovered from exceptional deposits. These specimens belong to groups of theropod (carnivorous) dinosaurs of various groups, such as the dromaeosaurs (the dromis or raptors), the oviraptorosaurs … and the tyrannosauroids (yes, there is a “cousin” of Tyrannosaurus with protofeathers). Based on a “cladogram” -the tree that shows their kinship relationships- and including this information, a structure is observed in the acquisition of feathers from the so-called ” coelurosaurs ” (in the image marked as C), which reaches its highest expression from the ” maniraptores ” (labeled as M in the image), in which we would have all types of feathers.
Applying the EPB methodology and following the “least change” criterion, all tyrannosauroids should have protofeathers . And that is why, you can read in many places that the T.rex could have been covered in feathers. Because, simply, “it touches him” as a family inheritance.
But recently a study of skin impressions associated with Tyrannosaurus fossils was published, all of them scaly, so that it could give the impression of being in the middle of a tennis match, or a tug-of-war. But the truth is that there is an intermediate situation.
For the feather impressions to be perfectly preserved , fossilization conditions are required that are not those of the sites where T. rex fossils abound. And from time to time these “patches” of scales have appeared. Well, in the same way that not all mammals are completely hairy, something must have happened in the dinosaurs, since it is very likely that, like mammals, they were warm-blooded. Thus, theropod dinosaurs that came from feathered families, if they grew large and/or lived in temperate zones, could reduce these feathers . In the same way that elephants or rhinoceroses have reduced their hair. Or ourselves (ok, some more than others).
This debate has been restricted to the academic world and paleoart for the last few years. But he recently made the leap into the world of film and popular culture with the Tyrannosaurus from the Jurassic World Dominion prologue, in which we see both primitive feathers and scales.
And it is that life makes its way. And scientific knowledge, when they let him, too.
References:
Prum R.O.; Brush A.H. 2002. The evolutionary origin and diversification of feathers. Quarterly Review of Biology, 77: 261-295
Xu, X. et al. 2012. A gigantic feathered dinosaur from the Lower Cretaceous of China. Nature, 484: 92-95.
Xu, X. et al. 2004. Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids. Nature 431 (7009): 680-684.
Bell, P. R. et al. 2017. Tyrannosauroid integument reveals conflicting patterns of gigantism and feather evolution. Biology Letters. 13 (6): 20170092.
