The feathered dinosaurs
Discussions by Yann Oliver
The feathered dinosaurs
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Several true dinosaurs with true feathers, as well as dinosaurs with feather-like structures and some very bird-like creatures have recently been found. They almost unambiguously prove that birds are a kind of dinosaurs, and that at least some theropods had feathers and were warm-blooded.
Feathers versus scales. Discussion on the origin of feathers, and on which dinosaurs may or may not have had feathers.
Description of feathered and bird-like theropods. Sinosauropteryx, Protarchaeopteryx, Beipiaosaurus Caudipteryx, Sinornithosaurus, Microraptor, Rahonavis, Unenlagia, Shuvuuia, Archaeopteryx.
Feathers versus scales
Feathers are probably modified scales, as can be seen by the scaly legs of modern birds. They are rarely fossilised, only in extremely fine sediment. This means that no conclusions can be drawn only on negative evidence of feathers if the skin is not preserved.
There are only a few cases for which the skin is known:
A few ceratopian and hadrosaur samples consist of regular, non-overlapping scales, as well as some skin of sauropod embryos.
A piece of a Carnotaurus head skin show a pattern of small scales and bigger ones at prominent positions (snout, nostrils) which might have been more coloured, as can be seen in lots of lizards.
A bit of a tyrannosaurid tail may have been scaly, though the skin was found near the skeleton and may belong to another animal..
Pelecanimimus is a very well-preserved ostrich dinosaur. Some fibres were originally thought to be feathers, but are now recognised to be muscle fibres. There seems to be no remains of external skin.
What seems to be anchor points for feathers are known from the arms of Rahonavis and Avimimus.
Feathers or proto-feathers are known from the dinosaurs described below (Sinosauropteryx, Protarchaeopteryx, Caudipteryx, Beipiaosaurus, Sinornithosaurus, Microraptor, and of course Archaeopteryx).
In the dinosaur family tree, Sinosauropteryx is not particularly close to birds: the tyrannosaurs, ostrich dinosaurs and troodonts were roughly as close to birds as it, and deinonychosaurs and oviraptors were much closer. The proto-feathers of Sinosauropteryx indicate that all these dinosaurs had probably a kind of covering integument like Sinosauropteryx.
The tyrannosaurs are in fact the only evolved theropods to show scales (on a bit of the tail): this case may be due to their big size, as today elephants lack hair, presumably to avoid overheating.
As shown by the scales of the primitive theropod Carnotaurus, of ceratopians, of hadrosaurs and of sauropods, the other dinosaurs (ornithischians, sauropods, primitive theropods) probably had standard scales.
Feathers (or feather-like structures) are known to have preceded flight. They probably occurred for insulation, or perhaps as a trap for insects (though this is not the case of Sinosauropteryx).
Some kind of covering integument has also been found in pterosaurs. This might show that some type of covered skin was already present in the common ancestor of pterosaurs and dinosaurs, and that the \\\'hair\\\' would have disappeared later in big animals (like today elephants). In fact, no skin impression from a small dinosaur shows scales, neither do any ornithischian or sauropod skin show something different from scales.
There had been claims that the supposed feathers of the Chinese fossils were a preservation artifact. Actually they have roughly the same appearance as those of birds fossilized in the same locality, so there is no serious reason to think they are of different nature; moreover, no non-theropod fossil from the same site shows such an artifact, but sometimes show unambiguous hair (some mammals) or scales (some reptiles).
Description of the feathered theropods
Most of the feathered animals described below come from roughly the same area in China. Feathers are certainly not a peculiarity of a small Chinese group of dinosaurs, since these dinosaurs are not particularly close to each other; the fossils are simply exceptionnally preserved.
Sinosauropteryx is a primitive coelurosaur very close to Compsognathus. It is exceptionally well-preserved: for example, two symmetrical oviducts can be seen (contrary to most bird species), bearing two unlaid eggs. The most striking feature is the presence of a kind of fur, consisting of hollow ridges with small branching filaments. This dermal structure is different from modern feathers, but probably analogous to true feathers. Sinosauropteryx does not show any adaptation to flight, and is not closely related to birds. It has the (relatively) longest known tail among dinosaurs.
Protarchaeopteryx is an Early Cretaceous Chinese dinosaur about turkey-sized, with long limbs, and arms longer than those of other theropods but shorter than Archaeopteryx\\\'. There are feather impressions known from the forelimbs, body and tail. The feathers are similar to modern feathers (rachis, barbs and barbicles), but they are symmetrical, hence Protarchaeopteryx could probably not fly, though this is not certain. Contrary to Archaeopteryx, Deinonychus etc, it lacks a hyperextendable second toe at the foot. Unlike all known birds, it had serrated teeth. It had no opposable first toe, hence it could not perch.
Beipiaosaurus is another Early Cretaceous Chinese dinosaur. It is a primitive therizinosaur (with some features close to oviraptorosaurs), and shows a kind of fur similar to that of Sinosauropteryx, with filaments 5cm long in average (up to 7cm).
Caudipteryx is yet another Early Cretaceous Chinese dinosaur from the same sediments as Beipiaosaurus and Protarchaeopteryx, about turkey-sized. It had long limbs, and very short arms. It also had complex but symmetrical feathers. It had elongate and slender teeth, but only on the front of the jaws, which is different from any known dinosaur. It may have been able to perch. Anyway, its arms were much too short to allow flying. See in-depth discussion at Dinodata.
Sinornithosaurus is a deinonychosaur from the Early Cretaceous of China. Its body was covered by down (roughly some 4cm long), some of which is similar to avian down, some of which is closer to classical feathers, with a central shaft and branching filaments, but with a simpler sutrcture. The articulation of the shoulder is in such a position that Sinornithosaurus could flap with its arms.
Microraptor is a very small (50cm) arboreal Chinese theropod, perhaps the most birdlike non-avian dinosaur known so far. It was probably flightless, nevertheless some features of the foot (distant first finger, elongate phalanges) suggest it was a climber. The feathers are partly simple filaments, partly more complex fuzz with a branching structure. The shape of teeth varies with position. Details at Dinodata.
Rahonavis, a raven-sized dinosaur from the Late Cretaceous of Madagascar, had small, wings and a sickle-claw (though smaller than that of deinonychosaurs). It could probably fly. There are anchor points for feathers in the bones, though no feathers are directly known
Unenlagia, from the Late Cretaceous of South America, had long legs, wing-like arms, and a strong shoulder joint allowing enhanced arm mobility, especially flapping (though it was too big to fly). No skin impressions were found, hence no feathers are known. This animal may be the juvenile form of Megaraptor, a more deinonychosaur-like dinosaur (they are from the same time and place).
Shuvuuia is an alvarezsaur from Mongolia. It has a long neck, long tail, long limbs and very short, single-clawed arms. The fossil shows hollow shafts around it. They have been analyzed and have been shown to contain the decay products of beta keratin, and not of alpha keratin. Among the epiderm of extant tetrapods, beta keratin is only produced by reptiles and birds; and feathers are the only integument not containing alpha keratin.
An extraordinary well-preserved as-yet-unnamed juvenile dinosaur (could be a juvenile of Sinornithosaurus or Microraptor) shows proto-feathers looking like fur, up to the tip of the snout... Learn more (and see marvellous photos) at the American Museum of Natural History.
Archaeopteryx is from the Late Jurassic of Germany. The presence of feathers is certain. These are asymmetrical, an adaptation for flying. It had large, wing-like arms able to flap, and long legs. It also had a deinonychosaur-like hyperextendable toe. The first toe is opposable, meaning that it could perch. Archaeopteryx could probably fly and flap, but more clumsily than modern birds.
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