Feathered dinosaurs
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Feathered dinosaurs are regarded by many paleontologists as transitional fossils between birds and dinosaurs. It was already well known that ancient birds such as Archaeopteryx had many saurian characteristics, such as claws on their 'fingers' and teeth. For many years it had been theorized that birds evolved from theropod dinosaurs. In the late 1990s, discoveries of feathered dinosaurs provided conclusive evidence of the connection, though the genealogical details are still incomplete.
Early theories
Shortly after the 1859 publication of Charles Darwin's The Origin of Species, British biologist and evolution-defender Thomas Henry Huxley proposed that birds were descendants of dinosaurs. He cited skeletal similarities, particularly among some saurischian dinosaurs, fossils of the 'first bird' Archaeopteryx and modern birds. In 1868 he published On the Animals which are Most Nearly Intermediate between Birds and Reptiles, making the case. The leading dinosaur expert of the time, Richard Owen, disagreed, claiming Archaeopteryx as the first bird outside dinosaur lineage. For the next century, claims that birds were dinosaur descendants faded, with more popular bird-ancestry hypotheses including 'crocodylomorph' and 'thecodont"'ancestors, rather than dinosaurs or other archosaurs.Then, in 1964, John Ostrom discovered a fossilized dinosaur he called Deinonychus antirrhopus, a theropod whose skeletal resemblance to birds seemed unmistakable. Ostrom has since become a leading proponent of the theory that birds are direct descendants of dinosaurs. Further comparisons of bird and dinosaur skeletons, as well as cladistic analysis strengthened the case for the link, particularly for a branch of theropods called maniraptors. Skeletal similarities include the neck, the pubis, the wrists (semi-lunate carpal), the 'arms' and pectoral girdle, the shoulder blade, the clavicle and the breast bone. In all, over a hundred distinct anatomical features are shared by birds and theropod dinosaurs. By the 1990s, most paleontologists considered birds to be surviving dinosaurs and referred to 'non-avian dinosaurs' (those that went extinct), to distinguish them from birds (aves or avian dinosaurs).
Direct evidence to support the theory was missing, however. Some mainstream ornithologists including Smithsonian Institute curator Storrs L. Olson, disputed the links, citing the lack of fossil evidence for feathered dinosaurs.
Adding to the controversy, in 1999 a supposed 'missing link' fossil of an apparently feathered dinosaur named "Archaeoraptor liaoningensis", found in Liaoning Province, northeastern China, turned out to be a fake (the bottom portion being from a dromaeosaurid now known as Microraptor and the upper portion being from a primitive bird now known as Yanornis (formerly Archaeovolans).
Fossil evidence
After a century of hypotheses without hard evidence, beautifully preserved (and legitimate) fossils of feathered dinosaurs were discovered, during the 1990s and 2000s. The fossils were preserved in a Lagerstätte — a sedimentary deposit exhibiting remarkable richness and completeness in its fossils — in Liaoning, China. The area had repeatedly been smothered in volcanic ash produced by eruptions in Inner Mongolia, 124 million years ago, during the Early Cretaceous Period. The fine-grained ash preserved the living organisms, that it buried, in extraordinary detail. The area was teeming with life, with millions of leaves, the oldest known angiosperms, insects, fish, frogs, salamanders, mammals, turtles, lizards and crocodilians having been discovered, to date.The most important discoveries at Liaoning have been a host of spectacular feathered dinosaur fossils, with a steady stream of new finds filling in the picture of the dinosaur-bird connection and adding more to theories of the evolutionary development of feathers and flight.
Current knowledge
A number of non-avian dinosaurs are now known to have been feathered. Direct evidence of feathers exists for the following genera (listed in order of publication):- Sinosauropteryx
- Protarchaeopteryx
- Caudipteryx
- Shuvuuia
- Sinornithosaurus
- Beipiaosaurus
- Microraptor
- Epidendrosaurus
- Cryptovolans
- Scansoriopteryx
- Yixianosaurus
- Dilong
- Pedopenna
- Jinfengopteryx
The first dinosaurs found to have true flight-structured feathers (pennaceous feathers) were Protarchaeopteryx and Caudipteryx (135-121 mya). It is unlikely that these feathers were used for flight; they were more likely used for display.
Microraptor, on the other hand, was covered with flight-feathers, both on its arms and legs, making it a four-winged theropod. Analysis indicates that this animal was a glider, not a flier, which has led to the speculation that Microraptor was arboreal (tree-dwelling).
Feathered dinosaur fossil finds to date, together with cladistic analysis, provide convincing evidence that birds are in fact descendants of dinosaurs. They also suggest that many types of theropod may have had feathers, not just those that are especially similar to birds. In particular, the smaller theropod species may all have had feathers and possibly even the larger theropods (for instance T. rex) may have had feathers, in their early stages of development after hatching. Large adult theropods are unlikely to have had feathers, however, as the need for insulation would be less important, since inertial heat retention would likely be sufficient to manage heat. Retention of internal heat may even have become a problem, had these very large creatures been feathered.
Shared features of birds and dinosaurs
Over a hundred distinct anatomical features are shared by birds and theropod dinosaurs. A recent case in point: "Archaeopteryx, therefore, is closely related to the theropods. This in turn means that theropod dinosaurs are the ancestors of the modern birds that followed Archaeopteryx. The find, according to G. Mayr, 'not only provides further evidence for the theropod ancestry of birds, but it also blurs the distinction between basal (the earliest) birds and basal deinonychosaurs, their fearsome-clawed ancestors. 'I do think that the question of a theropod ancestry of birds can now be considered settled once and forever,' Mayr said." [link] According to palaeontologist John Hutchinson, "For those that have actually seen the relevant specimens and considered all of the relevant data (which is a basic procedure for any scientist), it is becoming increasingly difficult to draw the line between 'bird' and 'non-avian dinosaur'." [link]Feathers
The first good specimen of a 'feathered dinosaur' was the 1861 discovery of Archaeopteryx, in Germany, in the Solnhofen limestone, which is a lagerstätte; one of the rare and remarkable geological formations known for their superbly detailed fossils. Coming just two years after Darwin's seminal The Origin of Species, the evidence of a transitional fossil between reptiles and birds spurred the debate between evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, the specimens are commonly mistaken for Compsognathus.Since the 1990s, a number of feathered dinosaurs have been found, providing clear evidence of the close relationship between dinosaurs and birds. Most of these specimens were local to Liaoning province in northeastern China, which was part of an island continent in the Cretaceous Period. However, the feathers were only preserved by the lagerstätte of the Yixian Formation; it is therefore possible that dinosaurs elsewhere in the world may also have been feathered, even though the feathers have not been preserved.
The feathered dinosaurs discovered so far include Beipiaosaurus, Caudipteryx, Dilong, Microraptor, Protarchaeopteryx, Shuvuuia, Sinornithosaurus, Sinosauropteryx and Jinfengopteryx, along with dinosaur-like birds, like Confuciusornis. All of them have been found in the same area and formation, in northern China. The dromaeosauridae family, in particular, seems to have been heavily feathered and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.
Skeleton
Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the association of multiple skeletal features also shared by the two groups is the more important link for paleontologists. Furthermore, it is increasingly clear that the relationship between birds, dinosaurs and the evolution of flight is more complex than has been previously realized. For example, while it was once believed that birds simply evolved from dinosaurs and went their separate way, some scientists now believe that some dinosaurs, such as the dromaeosaurs, may have actually evolved from birds, losing the power of flight while keeping the feathers in a manner similar to the Ostrich and other ratites.Comparisons of bird and dinosaur skeletons, as well as cladistic analysis, strengthens the case for the link, particularly for a branch of theropods called maniraptors. Specific similarities have already been listed.
Reproduction biology
A recent discovery, in a Tyrannosaurus rex skeleton, provided more evidence that dinosaurs and birds evolved from a common ancestor and, for the first time, allowed palentologists to sex a dinosaur. When laying eggs, female birds have a special type of bone structure, called medullary bone, that grows in their limbs, forming a layer inside the hard outer bone. It is rich in calcium and used for making eggshells. The presence of endosteally derived bone tissues, lining the interior marrow cavities of portions of the Tyrannosaurus rex specimen's hindlimb elements, suggested similar reproductive strategies and revealed the specimen to be female (Schweitzer et al., 2005).Lungs
Big meat-eating dinosaurs had a complex system of air sacs similar to the setup in today's birds, according to an investigation led by Patrick O'Connor of Ohio University. The lungs of theropod dinosaurs, carnivores that walked on two legs and had birdlike feet, likely pumped air into hollow sacs in their skeletons, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said. The study, funded in part by the National Science Foundation, is detailed in the July 14, 2005 issue of the journal Nature.Heart
Modern computerized tomography (CT) scans of dinosaur chest cavities, performed five years ago, found the apparent remnants of complex, four-chambered hearts, more like mammals and birds.Care of young
The embryo had no teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth (see altricial). This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth [link].Gizzard
Both birds and dinosaurs have used gizzard stones. When found in association with fossils, making a certain connection, such stones are called gastroliths.Behaviour
A recently discovered troodont fossil, Mei long, demonstrates that the animals slept like birds, with their heads tucked under their arms [link].Taxonomy
While fossil feather impressions are extremely rare, therefore only a few feathered dinosaurs have been identified so far, scientists can infer the presence of feathers on poorly-preserved specimens, through a process called phylogenetic bracketing. Almost all scientists agree that, due to similarities in all fossil feather specimens and through developmental research, feathers could only have evolved once in dinosaurs and then have been passed down to all later, more derived species (although it is possible that some lineages lost feathers secondarily). If a dinosaur falls at a point on an evolutionary tree within the known feather-bearing lineages, scientists assume it too had feathers, unless conflicting evidence is found. This technique can also be used to infer the type of feathers a species may have had, since the developmental history of feathers is now reasonbly well-known (Prum & Brush, 2002).The evolutionary tree below shows the relationships of the various groups of feathered dinosaurs, noting which species have been discovered with feather impressions and which scientists infer had feathers via 'bracketing'.
Coelurosauria ├─Tugulusaurus (may or may not have been feathered) │ └───Tyrannosauroidea (primitive feathers) │ ├──?Dilong paradoxus (feather impressions) │ └───Tyrannosaurus (scale impressions, may have lost some feathers) │ ├─Compsognathidae (primitive, two-branched feathers) │ ├─?Sinosauropteryx (feather impressions) │ ├─Juravenator (scale impressions, may have lost some feathers) │ └─Compsognathus (feathers possible) │ └─Maniraptoriformes ├─Ornithomimosauria (feathers likely) ├─Alvarezsauridae (feathers with a central vane) │ └───Shuvuuia (feathers preserved in three dimensions) │ └───Maniraptora (flight feathers and down-like contour feathers) ├─Yixianosaurus (feather impressions) │ ├─Oviraptoriformes (vaned, plumaceous feathers) │ ├─'Protarchaeopteryx'' (feather impressions) │ │ │ ├─Oviraptorosauria │ │ ├─'Caudipteryx'' (feather impressions) │ │ └───Oviraptoridae (feathers extremely likely, skeletal feather adaptations) │ │ │ └─Therizinosauria │ └─Beipiaosaurus (feather impressions) │ └───Paraves (flight feathers and down-like contour feathers) ├─Pedopenna (feather impressions) │ ├─Deinonychosauria (symmetrical pennaceous wing feathers) │ ├─Troodontidae │ │ └─Jinfengopteryx (feather impressions) │ │ │ └─Dromaeosauridae │ ├─Rahonavis (skeletal feather/flight adaptations) │ ├─Cryptovolans (feather impressions) │ ├─Microraptor (feather impressions) │ ├─Sinornithosaurus (feather impressions) │ └───Velociraptor (feathers extremely likely) │ ├─Epidendrosaurus (feather impressions) │ └───Aves (asymmetrical flight feathers)
References
See also
External links
- [Downy Dinos] on EvoWiki
- [DinoBuzz], dinosaur-bird controversy explained, by UC Berkley.
- [Journal of Dinosaur Paleontology], with many articles on dinosaur-bird links.
- [images of feathered dinosaurs]
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