A feathered dinosaur is any species of dinosaur possessing feathers, including all species of birds. In recent decades, evidence has accumulated that many non-avian dinosaur species also possessed feathers or feather-like structures in some shape or form. The extent to which feathers or feather-like structures were present in dinosaurs as a whole is a subject of ongoing debate and research.
It has been suggested that feathers had originally functioned as thermal insulation, as it remains their function in the down feathers of infant birds prior to their eventual modification in birds into structures that support flight.
Since scientific research began on dinosaurs in the early 1800s, they were generally thought to be closely related to modern reptiles such as lizards. The word dinosaur itself, coined in 1842 by paleontologist Richard Owen, comes from the Greek for 'terrible lizard'. That view began to shift during the so-called dinosaur renaissance in scientific research in the late 1960s; by the mid-1990s, significant evidence had emerged that dinosaurs were much more closely related to birds, which descended directly from an earlier group of theropod dinosaurs.
Knowledge of the origin of feathers developed as new fossils were discovered throughout the 2000s and the 2010s, and technology enabled scientists to study fossils more closely. Among non-avian dinosaurs, feathers or feather-like integument have been discovered in dozens of genera via direct and indirect fossil evidence. Although the vast majority of feather discoveries have been in coelurosaurian theropods, feather-like integument has also been discovered in at least three ornithischians, suggesting that feathers may have been present on the last common ancestor of the Ornithoscelida, a dinosaur group including both theropods and ornithischians. It is possible that feathers first developed in even earlier archosaurs, in light of the discovery of vaned feathers in pterosaurs.Michael Benton, A colourful view of the origin of dinosaur feathers. Nature 604, 630-631 (2022)Cincotta, A., Nicolaï, M., Campos, H.B.N. et al. Pterosaur melanosomes support signalling functions for early feathers. Nature 604, 684–688 (2022). https://doi.org/10.1038/s41586-022-04622-3 Fossil feathers from the dinosaur Sinosauropteryx contain traces of beta-proteins (formerly called beta-keratins), confirming that early feathers had a composition similar to that of feathers in modern birds. Crocodilians also possess beta keratin similar to those of birds, which suggests that they evolved from common ancestral genes.
History of research
Early
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). He showed that, apart from its hands and feathers, Archaeopteryx was quite similar to Compsognathus. Thus Archaeopteryx represents a transitional fossil. In 1868, he published On the Animals which are most nearly intermediate between Birds and Reptiles, which made that case.Foster, Michael; Lankester, E. Ray 1898–1903. The scientific memoirs of Thomas Henry Huxley. 4 vols and supplement. London: Macmillan.
The first restoration of a feathered dinosaur was Huxley's depiction in 1876 of a feathered Compsognathus, made to accompany a bird evolution lecture he delivered in New York, in which he speculated that the aforementioned dinosaur might have had feathers.
Dinosaur renaissance A century later, during the dinosaur renaissance, paleoartists began to create modern restorations of highly active dinosaurs. In 1969, Robert T. Bakker drew a running Deinonychus. His student Gregory S. Paul depicted non-avian maniraptoran dinosaurs with feathers and protofeathers, starting in the late 1970s.
Fossil discoveries
The first known specimen of Archaeopteryx, on the basis of which the genus was named, was an isolated feather, although whether or not it belongs to Archaeopteryx has been controversial. 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. Although some reviewers have raised questions about the naming and interpretation of this fossil, if correct, that early Jurassic fossil is the oldest known evidence of feathers, almost 30 million years older than the next-oldest-known evidence.Tetrapod Zoology, ScienceBlogs
The most important discoveries at Liaoning have been a host of 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. 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.
Behavioural evidence, in the form of an oviraptorosaur on its nest, showed another link with birds. Its forearms were folded, like those of a bird. Although no feathers were preserved, it is likely that these would have been present to insulate eggs and juveniles.
Not all of the Chinese fossil discoveries proved valid however. 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. His claim made National Geographic review their research and they too came to the same conclusion.
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. Initial analysis suggests that some of the feathers were used for insulation, and not flight. More complex feathers were revealed to have variations in coloration similar to modern birds, while simpler protofeathers were predominantly dark. Only 11 specimens are currently known. 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. Melanosomes produce colouration in feathers; as differently-shaped melanosomes produce different colours, subsequent research on melanosomes preserved in feathered dinosaur specimens has led to reconstructions of the life appearance of several dinosaur species. These include Anchiornis, Sinosauropteryx, Microraptor, and Archaeopteryx.
In 2016, the discovery was announced of a feathered dinosaur tail preserved in amber that is estimated to be 99 million years old. Lida Xing, a researcher from the China University of Geosciences in Beijing, found the specimen at an amber market in Myanmar. It is the first definitive discovery of dinosaur material in amber.
Current knowledge Non-avian dinosaur species preserved with evidence of feathers
Several non-avian dinosaurs are now known to have been feathered. Direct evidence of feathers exists for several species. In all examples, the evidence described consists of feather impressions, except those genera inferred to have had feathers based on skeletal or chemical evidence, such as the presence of quill knobs (the anchor points for wing feathers on the forelimb) or a pygostyle (the fused vertebrae at the tail tip which often supports large feathers).
Primitive feather types
Integumentary structures that gave rise to the feathers of birds are seen in the dorsal spines of reptiles and fish. 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. Feather structures are thought to have proceeded from simple hollow filaments through several stages of increasing complexity, ending with the large, deeply rooted feathers with strong pens (rachis), barbs and barbules that birds display today.
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. The first feather resulted when undifferentiated tubular follicle collar developed out of the old keratinocytes being pushed out. At stage II, the inner, basilar layer of the follicle collar differentiated into longitudinal barb ridges with unbranched keratin filaments, while the thin peripheral layer of the collar became the deciduous sheath, forming a tuft of unbranched barbs with a basal calamus. Stage III consists of two developmental novelties, IIIa and IIIb, as either could have occurred first. Stage IIIa involves helical displacement of barb ridges arising within the collar. The barb ridges on the anterior midline of the follicle fuse together, forming the rachis. The creation of a posterior barb locus follows, giving an indeterminate number of barbs. This resulted in a feather with a symmetrical, primarily branched structure with a rachis and unbranched barbs. In stage IIIb, barbules paired within the peripheral barbule plates of the barb ridges, create branched barbs with rami and barbules. This resulting feather is one with a tuft of branched barbs without a rachis. At stage IV, differentiated distal and proximal barbules produce a closed, pennaceous vane (a contour feather). A closed vane develops when pennulae on the distal barbules form a hooked shape to attach to the simpler proximal barbules of the adjacent barb. Stage V developmental novelties gave rise to additional structural diversity in the closed pennaceous feather. Here, asymmetrical flight feathers, bipinnate plumulaceous feathers, filoplumes, powder down, and bristles evolved.
Some evidence suggests that the original function of simple feathers was insulation. In particular, preserved patches of skin in large, derived, tyrannosauroids show scutes, while those in smaller, more primitive, forms show feathers. This may indicate that the larger forms had complex skins, with both scutes and filaments, or that tyrannosauroids may be like rhinos and elephants, having filaments at birth and then losing them as they developed to maturity. An adult Tyrannosaurus rex weighed about as much as an African elephant. If large tyrannosauroids were endotherms, they would have needed to radiate heat efficiently.Norell, M. Xu, X. (2005) "The Varieties of Tyrannosaurs", Natural History Magazine, May 2005. This is due to the different structural properties of feathers compared to fur.
Some evidence also suggests that more derived feather types may have served as insulation. For instance, a study of oviraptorid pennaceous wing feathers and nesting posture suggests that elongated wing feathers evidently may have served to fill gaps between brooding individuals' insulatory body chamber and the outside environment. This "wall" of wing feathers could have shielded eggs from temperature extremes.