Dilong paradoxus An Early Tyrannosauroid Dinosaur

Dilong – An Ancestor of Tyrannosaurus rex that is a bit of a Paradox

Dilong paradoxus, the “paradoxical emperor dragon” from the Yixian Formation of Liaoning Province of China is believed to be a primitive member of the Tyrannosauroidea. The evolution of tyrannosaurs is shrouded in mystery, most palaeontologists believe that they originated in the Northern Hemisphere sometime in the Late Jurassic and eventually displaced allosaurs as the apex predators in most parts of the world. Indeed, recent research into the later tyrannosaurids, the large carnivores of the Late Cretaceous (Campanian to Maastrichtian faunal stages), indicates that these animals did not originate in North America.

An animal such as Tyrannosaurus rex, the archetypal dinosaur, without whom no Jurassic Park movie would be the same, was probably descended from tyrannosaurids that migrated into North America from Asia.

T. rex is synonymous with the USA, however, its ancestors may have originated in China, another case of “made in China” I’m afraid.

The Evolution of the Tyrannosauroidea

There is a lot scientists still don’t know about the evolution of the tyrannosaurs. Fossils of predators are extremely rare in the fossil record, much rarer than the relatively more common herbivores for example. In any ecosystem the herbivores will tend to outnumber the meat-eaters, the ratio depends on the amount of food meat-eaters have to consume, the number of herbivores required to sustain them. Calculations have been made indicating that large predators such as a size-able tyrannosaurs,Tarbosaurus or Gorgosaurus for example, may have required large numbers of herbivores to sustain them with a ratio of meat-eaters to herbivores in the mega fauna population being in excess of 1:50.

The discovery of Eotyrannus (E. lengi) by the amateur fossil collector Gavin Leng has further complicated the evolutionary path of the tyrannosaur group. Eotyrannus is evidently another primitive tyrannosauroid.

The structure of the skull shows similarities with other tyrannosaurs and the dentition in the premaxilla is typical of a tyrannosaur,being broad, slightly re-curved and having the characteristic “D” shaped cross section. Trouble is Eotyrannus is associated with the Wessex Formation of the Isle of Wight. Dilong another primitive member of the tyrannosaurs is associated with Liaoning Province in China. From the fossils found so far, scientists have concluded that Dilong and Eotyrannus were very similar, in fact some palaeontologists have suggested that they are the same genus. The difficulty is, if both Dilong and Eotyrannus are basal forms of tyrannosaurs, in what part of the world did the tyrannosaurs evolve?

An Illustration of Dilong – A Primitive Tyrannosaur

Picture credit: Everything Dinosaur

Dilong

Dilong was a lightly built predator, with an estimated size of approximately 2 metres. The most complete fossil skeleton of this dinosaur found to date is of an individual with an estimated length of 1.5 metres, but this is believed to represent a sub-adult, so a fully grown animal was probably bigger. Although, Dilong is know from at least four fossil specimens, only one fossil shows evidence of protofeathers. This particular fossil, discovered in a layer of shale, had been extensively flattened and the bones were found in a disarticulated state and the specimen is far from complete.

However, surrounding the tail and in the area representing the back of the head indications of simple, filamentous feathers were found. Some of these simple protofeathers were up to 3 cm long. The presence of protofeathers is evidence that an ancestor of the big tyrannnosaurs such as T. rex had feathers.

Does this mean that dinosaurs such as Tyrannosaurus rex were also feathered?

Although, the feathered specimen is far from complete, the fine sediment that surrounded this particular fossil has permitted evidence of feathers to be preserved. Scientists have concluded that this little dinosaur was probably covered in a downy coat of simple feathers. These feathers would have helped insulate the animal and prevent too much heat loss, reinforcing the theory that at least some dinosaurs were endothermic (warm-blooded).

A Model of Dilong (Carnegie Collection Dilong Model)

Picture credit: Everything Dinosaur

To view models of Tyrannosaurus rex and other tyrannosaurids: Dinosaur and Prehistoric Animal Models.

Scientists believe that the larger, later tyrannosaurs were not feathered, not fully covered in feathers and not feathered as adults. Their reasoning is quite simple and in the absence of tangible fossil evidence, this is the best the scientists can do for the moment.

If feathers originally evolved as a form of insulation for small animals, this would make sense for smaller genera of tyrannosaur and even juvenile members of larger genera. However, the bigger an animal grows, the larger the mass the animal has in relation to its surface area. Large mammals like elephants for example, need to lose heat from their bodies rather than hold onto it.

Consequently, these animals are nearly hairless. An adult T. rex weighing something like 7,000 kilos would have great difficulty losing heat if it was covered in a thick, insulating coat of protofeathers. A hatchling T. rex perhaps weighing little more than a kilogramme on the other hand would find a downy coat a great benefit to help it regulate its temperature. A warm-blooded, young Tyrannosaurus rex may have had a coat of protofeathers, but as it grew it may have gradually shed its coat until it had lost it virtually entirely as an adult.

T. rex may have retained some protofeathers, perhaps on the back of the neck or head which could have had a display function in mature adults, but this is speculation for the moment. It is doubted whether this animal would have had feathers around the mouth and surrounding the neck as they would have been very difficult for this short-armed animal to clean and they would quickly have been covered in blood and gore as the animal fed. Many vultures for example, have bald heads and necks, an adaptation reflecting their habit of having to reach deep into carcases to feed.