Contents Updated: Tuesday, August 24, 1999
Did dinosaurs have feathers? Cold blooded reptiles need to take in heat from their surroundings to function and are hairless and featherless to facilitate it. But warm blooded creatures like birds and mammals regulate their temperature from internal resources and generally evolve external insulation so as to retain their heat.
Traditionally dinosaurs, considered to be reptiles, were assumed to have had scales like snakes. The basis of this assumption is not however too reasonable. Who would have thought that mammoths had shaggy hair if they were considered to be like modern elephants? Mammoth skin has been found preserved in the permafrost and we know they were hairy. The question of the downiness of dinosaurs is similar.
In the mid 70's the first suggestions were made that the ancestors of birds, the theropods, were feathered and probably warm blooded. The counter arguments were loudly voiced by the orthodox. They reasoned: first, that dinosaur mummies showed that dinosaurs had scales, like cold blooded reptiles, not feathers like warm blooded birds. Second, no feathers had been found on small dinosaurs. So Occam's razor favored scales in dinosaurs rather than feathers.
However the argument requires that both the dinosaurs' ancestors and their descendants were scaly. Now we know that birds are descended from dinosaurs. So dinosaurs' ancestors were scaly and their descendants, the birds, are feathery. Birds are closer relatives to dinosaurs than reptiles; nowadays birds are classified as dinosaurs. It would be safer to assume that dinosaurs were also feathered or downy.
The answer could only come from direct fossil evidence.
No feathers had been clearly preserved on any small dinosaur. Compsognathus apparently lacked feathers but in fact the specimen gave no indication of scales either, whatever sort of skin it had not having been preserved. Feathers are not even found on all Archaeopteryx specimens from the same sediments, yet some specimens show them clearly. Compsognathus and Archaeopteryx are so similar that it seems unlikely that one would have been feathery and the other bald!
Yet orthodoxy always presumed that scales were more certain than feathers.
Pterosaurs, the great flying "dinosaurs", were neither reptile nor bird. Their anatomy and physiology set them apart from other reptiles. They were so different that they should be in a class of their own, of equal rank to both birds and reptiles. The information comes from some of the best preserved fossils in the world, unearthed in Kazakhstan and western Mongolia, and collected by Russian and Chinese palaeontologists.
Most are of a pigeon-sized pterosaur called Sordes pilosus. Among the fossilised skeletons the researchers found fossils of the animals' soft parts, the wings and the pelt. The pelt was plainly hairy-from head to tail, with the hair petering out at the base of the wings. The hairs, black filaments about half a centimetre long, were probably made of keratin.
So, pterosaurs were hairy. Reptiles are not supposed to be hairy, but, even 120 years ago, when palaeontologists saw the first fossil pterosaurs, some said pterosaurs ought to have been insulated to be able to fly—a pterosaur would have to stay warm.
The Sordes remains also help to settle the extent of the wings and where they attached to the body. A small membrane stretched from the wrist to the neck, and the main wing was attached to the arm and the body. The disputed point is whether the large wing membrane was also joined to the legs.
In these specimens, the main wing joined the legs and another membrane stretched between the legs, though it was not attached to the tail. If pterosaurs were as diverse as it seems, then the arrangement might have varied, but the main wing was probably always attached to the legs. The fossils of the wings show that they were naked and leathery, and strengthened with more fibres, like the battens in a sail.
Fossils from the Tatar region of western Mongolia, provide yet more information on pterosaur anatomy. Usually pterosaur skeletons are so crushed that they are two dimensional. The Tatar specimens are almost perfect, like bones your dog has just brought in from the garden. Some are completely undistorted and provide a true picture of the pterosaur's anatomy.
The remains of a large pterosaur called Phobetor indicated it was a fish eater, with large back teeth for cracking open tough-skinned fish. More important than the teeth are the fossils of its wrist bones. The animal's wrist action was subtle. Like a bird, Phobetor could fold its wings backward and downward, a movement important, not only at rest but also in flight, allowing the pterosaur to arch its wings and stabilise its flight.
Most pterosaurs discovered so far have been fish eaters. They skimmed their long jaws through the water, snagging fish on their plentiful teeth. The head of one was was more like that of a frog-mouth, wide and deep, with huge eyes and a gaping mouth. This pterosaur perhaps lived the same sort of life as today's frog mouth—the bird-feeding on insects at dusk, swooping about with its mouth agape. Like the frogmouth and other night-jars, it had hairs around its mouth that might have helped to trap insects.
Another dramatic find was a giant pterosaur named Azhdarcho, the Uzbek word for dragon, appropriate for an animal with a wingspan of 5 metres. Most people who have studied the giant pterosaurs believed that they were the vultures of their time, soaring the thermals looking for dead dinosaurs. A close examination of the bones in Azdharcho's neck suggests that this is unlikely. The pterosaur's long neck was relatively stiff and immobile, more suited to skimming for fish than for dismembering dinosaurs.
Pterosaurs were diverse and successful animals that occupied the same niche as birds do today.
Now that several examples have been found of dinosaurs and pterosaurs with down, feathers or fur, it is beginning to look more assured that some at least of them had furry or feathered skins, the small ones at least. Even earlier in the century, the Thescelosaurus was described as having a none scaly, punctured surface which might have been feathered. These findings hint that feathers might have evolved in tetanuran theropods well before the evolution of birds, at least by the late Jurassic. Triassic and earlier Jurassic theropods might also have been insulated. Coelophysis might have had feathers.
A fossil dinosaur which has been called Sinosauropteryx prima and of the compsognathid group has revived the controversy. It seemed to have feathers.
The discovery was broadcast with the headline: China says fossil is oldest bird found. The fossil was found in Liaoning Province by a fossil-hunter, farmer Li Yumin and it might be the earliest ancestor of modern birds. He took it to the Chinese Geology Museum in Beijing, believing it to be a dragon. Ji Qiang, director of the museum, determined that the fossil was that of an ancestral bird. The skull is crushed and the forelimbs might be slightly disarticulated. There is also another fossil which did not seem to have any feathers although a closer inspection could reveal otherwise.
Initially it was described as more than 200 million years old. In fact from radiometric dating of associated tuffs, the fossil is within narrow limits from the lowermost Cretaceous or the uppermost Jurassic period of about 121 to 142 million years ago. The beds that produce these fossils are a lacustrine Lagerstatte deposits that preserves soft tissues very well, and have a high content of volcanic ash allowing accurate dating. In fact the fossil has traces of soft tissue in the orbit and in the gastral area. The beds have produced all of the Confuciusornis specimens of extinct primitive perching birds with horny beaks and feathers and the more modern Liaoningornis, an essentially modern type of bird but having teeth not a beak.
In appearance it is closer to a small terrestrial dinosaur about half a metre in length, but it has a number of characteristics that prove the evolutionary link with birds. The forelegs show a definite tendency towards developing wings, Mr Ji said. The key factor, however, was the discovery of feathery imprints in the fossil. Phillip Currie of the Tyrell Museum of Palaeontology in Alberta, Canada the first Westerner to examine the find says it is "one of the most exciting discoveries in decades. It is a little feathered dinosaur". He totally discounts the possibility that the impressions were dendrites, feathery mineral crystals common to many fossils.
Many modern dinosaur experts consider birds to be a branch of the dinosaur family tree while others believe they evolved separately from a common ancestor with the dinosaurs. Currie asserts that Sinosauropteryx is a dinosaur showing that the first idea is the correct one. Down evolved in some small dinosaurs in the juvenile form as a protective layer and insulator on the backs of little creatures which curled up for warmth and safety. Sexual selection then led to it being adapted into adult forms by the process of neoteny. Some of the downy dinosaurs then found the down on their forelimbs helped them in catching flying insects as prey and flight feathers and flight ensued.
The fossil Sinosauropteryx seems to be covered with short, hairlike impressions that must be of down or rudimentary feathers, similar to those of the birds from the same sediments. The feathers are relatively short, preserved mostly along the neck and back on the midline of the vertebral column, starting at the atlas and finishing at about half way along the tail, with very short ones, looking to be intermediate between scales and feathers near the vent, along the underside of the tail base. They are distributed in a patchy manner, on the top and bottom of the tail in clumps. None appear on the limbs. They are typical of bird fossils. They are small and about 1 cm long showing little detail to confirm whether they are feathers or scales, though some do seem to have quills.
Dr Paul Davies reported in December 1996 that another, even better, specimen of Sinosauropteryx had been found. It was a full meter long and totally covered in 2mm long feathers. Apparently it was an adult while the earlier specimen was a juvenile, the reason why it was only 56cm long and only covered with feathers along its back. For the sceptics, Larry Martin of the University of Kansas says he is unconvinced that the features are feathers if the fossil is a dinosaur.
The Academy of Natural Sciences of Philadelphia sent John Ostrom of Yale University with three other experts, Martin, Peter Wellnhofer and Alan Brush, to China to examine the fossils. They reported that the structures are not "true feathers" but what they are remains a mystery. Ostrom describes them as long parallel arrays of fibres which lack the branching pattern of feathers.
Despite the Chinese assertion, this early Cretaceous theropod is not a bird. It is similar to the late Jurassic Compsognathus, with short forelimbs and a long tail of over half of its body length, and is preserved in the same attitude as the Solnhofen Compsognathus. It is surely a compsognathid but since it seems to have a booted pelvis, it is a tetanuran, much less advanced than more bird-like theropods which show flight adaptations. These sediments might settle the question of feathered dinosaurs.
Indeed Ji Qiang has announced the discovery of a turkey sized fossil from the same sediments, named Protarchaeopteryx robusta, which has feather impressions on its tail. The animal has more powerful legs than the Archaeopteryx and weaker forelimbs so it is unlikely that it could fly even as well (or badly) as Archaeopteryx. Ji believes the new fossil lies between the proto bird, the Archaeopteryx, and the bird-like dinosaur, the Sinosauropteryx, with its curious bristle-like filaments on its back. The four experts however question whether the feathers on the animal's tail, which they cannot deny, belong to the fossil. They are not in the expected position and have either been displaced after death or have come from another animal.
The probable presence of feathers in at least some small theropods has important metabolic and thermoregulatory implications. The orthodox have recently shifted ground somewhat, claiming the presence of feathers does not necessarily indicate endothermy, contrary to long held opinion. They say that the flight muscles of early birds could only have been small reptile-like muscles because their small sternal plates were too weak to support the stronger flight muscles of a warm blooded animal. But some modern birds have flight muscles much smaller than the avian norm but still fly, and the forelimbs and shoulder girdles of early birds were large enough for flight. They also argue that the bones of early birds show they grew slowly like reptiles. But growth rings in bones only indicate that the birds grew slowly as adults. Juvenile growth might have been rapid but the thin walled bones of the fossil birds did not preserve any signs of it.
Dong Zhiming of the Institute of Vertebrate Palaeontolgy and Palaeoanthropology in Beijing states categorically: "This provides evidence that some small dinosaurs were warm blooded".
Which brings us to the point that many if not all of the thousands of furry tetrapods were warm blooded. Even furry insects tend to be warmer than their surroundings; they are endotherms. As we saw above down or fur prevents cold blooded ectotherms from rapidly absorbing from the environment the heat they need. Even reptiles adapted to hot sandy deserts have not evolved insulation as a solar screen. Only endotherms that need to retain body heat are insulated and so insulation is always good evidence for a higher metabolic rate and therefore for endothermy. If dinosaurs were furry or downy they most probably were warm blooded. However if all small dinosaurs were dovered in down then finding it is no evidence that a fossil is in the direct line of birds. The popular idea was that the dromaeosaurs were the ancestors of birds but the chinese therapod is a compsognathid.
Material from news items in New Scientist, an article also in NS by Pat Shipman of Pennsylvania State University and Jeff Proling's Dinosauria On-line a website which you must visit.