FEATHERED DINOSAURS

A new range of fossil finds, mostly from China, are revealing that a large number of small theropod dinosaurs were covered by feathers. The three photos on this page are drawn from a recent article on the subject (Sloan, C.P. 1999. Feathers for T. Rex. National Geographic 196 (5): 98-107).

Fine feathers appear as a halo around the skeleton of Sinornithosaurus, a small, 120 million-year-old theropod dinosaur recently discovered in China.

Sinornithosaurus was a small dinosaur that probably fed on animals such as reptiles and mammals. Like larger raptors, to which it is closely related, the animal may have attacked with a jumping stance, as shown here. Although quite bird-like, this dinosaur could not glide or fly, which raises the issue as to why it evolved feathers.

The ever-more obvious link between dinosaurs and birds might suggest that feathers were linked to the evolution of flight. Unfortunately, this possibility has to be rejected for at least two reasons:

• Feathered dinosaurs appear to predate the appearance of the first birds
• Archaeopteryx, the earliest known bird, has feathers but, according to John Ostrom, was not capable of flight

Sexual Display. One possibility is that feathers may have served to enhance sexual display as part of mating behaviour. Such elaborate courtship rituals are common in birds and even mammals, but are generally absent in extant (living) reptiles.

Insulation. One important role for feathers may well have been insulation. If some dinosaurs were homeothermic ("warm blooded"), small individuals would face a problem of significant heat loss due to an unfavorable (high) surface to volume ratio. Modern birds and moderate-sized to small mammals solve this problem with insulation in the form of feathers or hair. If an animal (like a reptile) controls its temperature via behaviour, insulation causes a lag in thermal response that works against tight control of body temperature. A dense covering of feathers in small dinosaurs would only be adaptive in the case of homeothermic metabolism.

Large vs. Small Bodies. While insulation is a virtual necessity for small-bodied homeothermic animals, very large animals (such as many dinosaurs) would face the problem of getting rid of metabolic heat, since the surface area of the body increases more slowly than body volume as size increases. Large dinosaurs would be severely restricted in terms of environments if they possessed body insulation as adults. Here is an artists rendering of an adult Tyrannosaurus rex and a hatchling, showing how this problem might have been solved via natural selection. The hatchling has a covering of feathers to assist in retaining body heat, but the feathers are lost as the animal grows to adult size. This adaptation would be comparable to that of modern elephants, where the young have far more body hair than their parents.