Feathered dinosaur
Since scientific research began on dinosaurs in the early 1800s, they were generally believed to be closely related to modern reptiles such as lizards.
Among non-avian dinosaurs, feathers or feather-like integument have been discovered in dozens of genera via direct and indirect fossil evidence.
[7][8] Shortly after the 1859 publication of Charles Darwin's On the Origin of Species, the British biologist Thomas Henry Huxley proposed that birds were descendants of dinosaurs.
He compared the skeletal structure of Compsognathus, a small theropod dinosaur, and the "first bird" Archaeopteryx lithographica (both of which were found in the Upper Jurassic Bavarian limestone of Solnhofen).
His student Gregory S. Paul depicted non-avian maniraptoran dinosaurs with feathers and protofeathers, starting in the late 1970s.
[13][14] One of the earliest discoveries of possible feather impressions by non-avian dinosaurs is a trace fossil (Fulicopus lyellii) of the 195–199 million year old Portland Formation in the northeastern United States.
Gierlinski (1996, 1997, 1998) and Kundrát (2004) have interpreted traces between two footprints in this fossil as feather impressions from the belly of a squatting dilophosaurid.
Turner et al. (2007) reported quill knobs from an ulna of Velociraptor mongoliensis, and these are strongly correlated with large and well-developed secondary feathers.
In 1999, a supposed fossil of an apparently feathered dinosaur named Archaeoraptor liaoningensis, also found in Liaoning, turned out to be a forgery.
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 fossil animals.
[21] In 2011, samples of amber were discovered to contain preserved feathers from 75 to 80 million years ago during the Cretaceous Period, with evidence that they were from both dinosaurs and birds.
[22][23] More complex feathers were revealed to have variations in coloration similar to modern birds, while simpler protofeathers were predominantly dark.
The specimens are too rare to be broken open to study their melanosomes (pigment-bearing organelles), but there are plans for using non-destructive high-resolution X-ray imaging.
Lida Xing, a researcher from the China University of Geosciences in Beijing, found the specimen at an amber market in Myanmar.
A similar stage in their evolution to the complex coats of birds and mammals can be observed in living reptiles such as iguanas and Gonocephalus agamids.
[33] According to Prum's (1999) proposed model, at stage I, the follicle originates with a cylindrical epidermal depression around the base of the feather papilla.
At stage IV, differentiated distal and proximal barbules produce a closed, pennaceous vane (a contour feather).
[48] Paleontological and evolutionary developmental studies show that feathers or feather-like structures were converting back to scales.
Feathers are largely made of the keratin protein complex, which has disulfide bonds between amino acids that give it stability and elasticity.
For an organism whose metabolism works at high internal temperatures of 40 °C (104 °F) or greater, it can be extremely important to prevent the excess production of hydrogen sulfide.
Filamentous structures are clearly present in pterosaurs,[55] and long, hollow quills have been reported in specimens of the ornithischian dinosaurs Psittacosaurus and Tianyulong[56][57] although there has been disagreement.
[61] More recently, findings in Russia of the basal neornithischian Kulindadromeus report that although the lower leg and tail seemed to be scaled, "varied integumentary structures were found directly associated with skeletal elements, supporting the hypothesis that simple filamentous feathers, as well as compound feather-like structures comparable to those in theropods, were widespread amongst the whole dinosaur clade.
"[62] In contrast, a 2016 study published in the Journal of Geology suggested that the integumentary structures found on Kulindadromeus and Psittacosaurus may be highly deformed scales rather than filamentous feathers.
