Many recent discoveries of Theropod Dinosaurs have revealed traces of feather coverings, leading to speculation that all members of this group may have been feathered. The discovery of the feathered Tyrannosaurids, Dilong paradoxus and Yutyrannus huali, from the Early Cretaceous of China, has extended this speculation to the very largest Tyrannosaurid Dinosaurs, particularly as Yutyrannus itself was a large Tyrannosaurid (about nine meters) with an extensive feather coat, raising the possibility that the largest known Therapod Dinosaur, Tyrannosaurus rex, might have had feathers.
In a paper published in the journal Biology Letters on 7 June 2017, Phill Bell of the University of New England, Nicolás Campione of the Palaeobiology Programme at Uppsala University, Scott Persons and Philip Currie of the University of Alberta, Peter Larson of the Black Hills Institute of Geological Research, Darren Tanke of the Royal Tyrrell Museum of Palaeontology and Robert Bakker of the Houston Museum of Natural Science, describe the results of a study of skin traces from Tyrannosaurus rex and several other large Tyrannosaurids, and draw conclusions about the evolution of skin covering of Tyrannosaurid Dinosaurs from this study.
Bell et al. examined a number of skin traces associated with a Tyrannosaurus specimen collected from the Hell Creek Formation near Baker in Montana (specimen HMNS 2006.1743.01, sometimes known as ‘Wyrex’). This specimen has numerous skin patches from different parts of its body, none of which show any trace of feathers, leading Bell et al. to conclude it was featherless. The scales of HMNS 2006.1743.01 are small in size and highly variable in shape, with no larger feature scales (known from other Dinosaur species) and no scales showing polarity (i.e. having a distinct anterior-posterior axis, presumably the first step in developing towards a feather-like form). Interestingly, in several places where large patches of skin are preserved, these scales show signs of being arranged into a leaf-like pattern, with scales arranged into trapezoidal or triangular clusters, separated by bands of incised interstitial skin, which follow a branching arrangement similar to the midrib and lateral veins on a plant leaf.
Integument of Tyrannosaurus rex (HMNS 2006.1743.01). (a) Proximal caudal vertebrae 6–8 in right lateral view. Integument from the neck (b,c), the ilium (d,e) and caudal vertebrae (f–h). Green lines indicate ‘vein and midrib’ patterns. Scale in (a) is 10 cm; (b)–(e) are 5 mm and (f)–(h) are 10 mm. Bell et al. (2017).
Next Bell et al. examined preserved skin specimens from four other large, Late Cretaceous Tyrannosaurs, Daspletosaurus, Tarbosaurus, Albertosaurus, and Gorgosaurus. Unlike the Early Cretaceous Chinese specimens, none of these Late Cretaceous North American and Mongolian Tyrannosaurs show any signs of feathers.
The distribution of feathers among Theropod Dinosaurs suggests that at least primitive feathers (i.e. hollow tubes) were present in the earliest Coelurosaurs, the group to which the Tyrannosaurs belong, with more advanced features such as rachis, barbs and barbules and eventually fully formed flight feathers appearing sequentially within Maniraptoran Theropods and eventually Birds. Some Ornithischian Dinosaurs also had primitive feathers, suggesting that either simple feathers appeared before the two groups diverged, or that they evolved separately in the two groups. No trace of feathers or feather-like structures has ever been found in any Sauropod Dinosaur.
Like other Coelurosaurs, the earliest Tyrannosaurs appear to have had a full coat of simple, fibrous feathers, and as in other non-avian Theropods, no Tyrannosaur has been found with evidence of a partial-feather covering. Indeed, the earliest Birds, such as Archaeopteryx, appear to have been completely covered in feathers, with the scaly feet and legs of modern Birds apparently secondarily derived. This strongly suggests that the scaly skin of later Tyrannosaurs is also secondarily derived from a feathery covering.
The earliest Tyrannosaurs were relatively small animals, with gigantism achieved twice within the group, once rapidly, in the Early Cretaceous lineage that led to Yutyrannus, and once more gradually over the course of the Cretaceous in the lineage that culminated in Daspletosaurus, Tarbosaurus, Albertosaurus, Gorgosaurus and Tyrannosaurus. This is important, as it shows that Yutyrannus is not the ancestor of the Late Cretaceous giant Tyrannosaurs, and that it therefore presumably evolved under different ecological and evolutionary pressures to these animals, and that understanding these pressures may be the key to understanding the distribution of feathers among Tyrannosaurids.
Some earlier studies have suggested that temperatures in Early Cretaceous western Liaoning (where Dilong paradoxus and Yutyrannus huali lived) may have been significantly cooler than the North American and Mongolian environments that produced the Late Cretaceous giant Tyrannosaurs, leading these Early Cretaceous forms to need an insulating feather coat that the Late Cretaceous forms lacked. However recent palaeoenvironmental studies have not supported this, implying a similar temperature range. One difference that may be important is tree-cover, with the Liaoning deposits thought to have been laid down in a dense forest environment, while the later Cretaceous Tyrannosaurids are believed to have inhabited more open environments. This reflects the case in modern Asian Elephants and Javan Rhinoceroses, which are live in dense forest environments and are hairier than their African relatives that live on open Savanah. While all of these animals live in hot climates, and are more concerned with losing excess heat that keeping warm (always more of a problem for large animals anyway), the Asian forms, which live beneath the shade of large trees, are apparently subject to less heat-stress than the African forms.
However the Late Cretaceous Tyrannosaurids are known to have lived alongside feathered Dinosaurs, with Albertosaurus in particular having been found alongside feathered Ornithomimids. Bell et al. suggest that the feather-loss in Tyrannosaurids compared to Maniraptorans (such as the Ornithomimids) may be related to a more active lifestyle, i.e. expending more energy and therefore generating more body-heat. Tyrannosaurs had relatively longer legs than other large Theropods, and arctometatarsalian feet (expand), which has been interpreted as being indicative of a more active lifestyle, possibly even engaging in bouts of rapid or lengthy pursuit (generally the only ways for a large predator in an open environment to catch prey, since opportunities for ambush are relatively limited).
Finally, the Late Cretaceous Tyrannosaurs grew considerably larger than the Early Cretaceous forms, with only the smallest, Albertosaurus, at an estimated 1325–2210 kg is thought to be comparable in size to Yutyrannus, at about 1414 kg, while the other species were much larger; Tarbosaurus at 1744–2945 kg, Gorgosaurus at 2145–3577 kg, and Tyrannosaurus at 5014–8361 kg, suggesting that these species would have been subject to more heat-stress.
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