Feathered dinosaurs

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Sinornithosaurus by Jim Robins
Sinornithosaurus by Jim Robins
A Life Restoration of Deinonychus by John Conway
A Life Restoration of Deinonychus by John Conway

Feathered dinosaurs are regarded by many paleontologists as transitional fossils between birds and dinosaurs (see Dinosaur-bird connection). 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.

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[edit] 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). Some dinosaur restorations began to picture dinosaurs with a downy or feathery cover.

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.

[edit] 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 and the oldest known angiosperms, insects, fish, frogs, salamanders, mammals, turtles, lizards and crocodilians 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.

[edit] Genuine feathers?

There had been claims that the supposed feathers of the Chinese fossils were a preservation artifact.[1] Despite doubts the fossil feathers 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).

[edit] The Archaeoraptor fake

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. Comparing the photograph of the specimen with another find, Chinese paleontologist Xu Xing came to the conclusion that it was composed of two portions of different animals. His claim made Natural Geographic review their research and they too came to the same conclusion. The bottom portion is from a dromaeosaurid now known as Microraptor and the upper portion from a primitive bird now known as Yanornis (formerly Archaeovolans).

[edit] Current knowledge

[edit] List of dinosaur genera preserved with feathers

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):

  1. Sinosauropteryx (1996)
  2. Protarchaeopteryx (1997)
  3. Caudipteryx (1998)
  4. Shuvuuia (1998)
  5. Sinornithosaurus (1999)
  6. Beipiaosaurus (1999)
  7. Microraptor (2000)
  8. Epidendrosaurus (2002)
  9. Cryptovolans (2002)
  10. Scansoriopteryx (2002)
  11. Yixianosaurus (2003)
  12. Dilong (2004)
  13. Pedopenna (2005)
  14. Jinfengopteryx (2005)
  15. Sinocalliopteryx (2007)

[edit] Primitive feather types

At present, the most primitive (known) feathered dinosaur is Sinosauropteryx (Jurassic/Cretaceous, 150-120 mya), whose body was covered with feather-like structures that look like hollow tubes, or hairs. They may or may not have had barbs, like downy (plumulaceous) feathers. Another early fossil, Dilong paradoxus (Early Cretaceous), an ancestor of Tyrannosaurus rex, also had similar feather structures. These early fossils suggest that feathers originally developed as insulators, to maintain body temperatures (thus also providing evidence for warm-blooded dinosaurs). Flight would have been a later evolutionary adaptation (or exaptation) of feathers.

A model of Microraptor at the American Museum of Natural History
A model of Microraptor at the American Museum of Natural History

The first dinosaur fossils from the region found to have true flight-structured feathers (pennaceous feathers) were Protarchaeopteryx and Caudipteryx (135-121 mya). It is more likely that their feathers were used for display rather than for flight in these dinosaurs. Subsequent dinosaurs found with pennaceous feathers include Pedopenna and Jinfengopteryx. Several specimens of Microraptor, described by Xu et al. in 2003, show not only pennaceous feathers but also true asymmetrical flight feathers, present on the fore and hind limbs and tail. Asymmetrical feathers are considered important for flight in birds. Before the discovery of Microraptor gui, Archaeopteryx was the most primitive known animal with asymmetrical flight feathers.

[edit] Taxonomy and the inference of feathers in other dinosaurs

Feathered dinosaur fossil finds to date, together with cladistic analysis, 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.

Fossil feather impressions are extremely rare; therefore only a few feathered dinosaurs have been identified so far. However, through a process called phylogenetic bracketing, scientists can infer the presence of feathers on poorly-preserved specimens. All fossil feather specimens have been found to show certain similarities. Due to these similarities and through developmental research almost all scientists agree that feathers could only have evolved once in dinosaurs. Feathers would 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 (asymmetrical 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)

[edit] References

  1. ^ Feduccia, A. Lingham-Soliar, T., and Hinchliffe, J.R. (2005). "Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence." Journal of Morphology, 266: 125-166.

[edit] See also

[edit] External links

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