The simple answer to this, as every dinosaur enthusiast knows by now, is yes. Non-avian maniraptors did have feathers. There's no question about it. To say that there's more than enough evidence for this is an understatement. The most direct evidence, of course, is fossil evidence, and we have mountains of fossil evidence for feathers in non-avian maniraptors. It's a wonder that we do, given that soft tissues rarely fossilize. However, some formations, such as the Early Cretaceous Yixian Formation and Jiufotang Formation and the Late Jurassic Tiaojishan Formation in China, often do preserve soft tissues, including integumentary structures, in exquisite detail, making them veritable treasure troves for paleontologists. Several maniraptor taxa have been found in such deposits, many of them preserving feathers. In some cases it is also possible to infer the presence of feathers through skeletal features. To date, at least fifteen non-avialian maniraptors have been preserved with direct evidence of feathers. (I'm not including avialians simply because no one ever has any problem with them being feathered for... reasons, I guess. Even though many of them are preserved in the exact same deposits as non-avialian maniraptors. I don't understand it either.)
That's right, there was direct evidence of feathered non-avialian maniraptors before the 1990s! In 1987, Kuzanov (the original describer of Avimimus) reported the presence of a ridge on the ulna of this oviraptorosaur, indicating the presence of secondary feathers (wing feathers on the ulna). Evidently that wasn't convincing enough at the time and this has since been often overlooked.
Though first thought to be an archaeopterygid avialian, this has since turned out to be a basal oviraptorosaur. Though the holotype isn't quite as beautifully preserved as other Yixian fossils, it does preserve large symmetrical pennaceous feathers forming a tail fan on this maniraptor. (There are several main types of feathers. Pennaceous feathers are the vaned feathers we typically see in modern birds. Plumaceous feathers are essentially down. Protofeathers are primitive filaments that can either be single filaments or multiple filaments stemming from the same follicle. All three types are known in non-avian maniraptors.)
|Fossil of Caudipteryx zoui photographed by Laikayiu, from Wikipedia.|
Unlike Protarchaeopteryx, this basal oviraptorosaur is known from several good specimens. It also preserves a tail fan, as well as protofeathers on the body and primary feathers (wing feathers attaching to the second finger). Interestingly, it appears that Caudipteryx zoui didn't have secondary feathers, while Caudipteryx dongi did.
Rahonavis gets the prize for the first dromaeosaurid found to show evidence of feathers. It's another non-avian maniraptor that was, at first, mistaken for an archaeopterygid avialian. Rahonavis preserved quill knobs on its ulna. Quill knobs anchor the secondary feathers in many modern birds (particularly in strong fliers), and likely served the same purpose in Rahonavis.
One specimen of the alvarezsaurid Shuvuuia came with the remnants of preserved feathers. Unlike other extinct maniraptors for which feathers have been found, it came from the Djadochta Formation in Mongolia, a formation that, although the origin of many excellent fossils, is not known for preservation of soft tissues, and as a consequence only some poorly-preserved protofeathers are present. However, this specimen is still significant in that its feathers have been subject to chemical analysis that show they contain (and lack) the same proteins as modern bird feathers do.
|Fossil of Sinornithosaurus millenii photographed by Laikayiu, from Wikipedia.|
Rahonavis may have gotten the prize for the first dromaeosaurid to show evidence of feathers, but Sinornithosaurus gets the honor for the first dromaeosaurid to actually have its feathers preserved. Specimens of Sinornithosaurus have plumaceous feathers all over the body, but truth be told I'm a little fuzzy on the details of the plumage of Sinornithosaurus proper and someone will have to fill me in, particularly as most descriptions of Sinornithosaurus plumage also include information from from the specimen NGMC 91 "Dave", which was considered a possible juvenile Sinornithosaurus at one point but is probably a distinct taxon.
For those keeping track, at this point we have four major groups of maniraptors with evidence of feathers: avialians (of course), oviraptorosaurs (Avimimus, Protarchaeopteryx, and Caudipteryx), dromaeosaurids (Rahonavis and Sinornithosaurus), and alvarezsaurids (Shuvuuia). With the discovery of Beipiaosaurus, the therizinosaurs join their ranks. The first specimen of Beipiaosaurus found had thick, shaggy protofeathers on its body, but subsequent, better preserved specimens showed something else surprising: long bristle-shaped feathers on at least the neck and tail forming a second coat underneath the protofeathers. These bristle-shaped protofeathers were given the name Elongated Broad Filamentous Feathers (EBFFs). They may be unique to therizinosaurs, but possible EBFFs have been identified in some yet-to-be-described basal coelurosaur taxa.
In 1999 a supposed new feathered dinosaur known as "Archaeoraptor" garnered some media attention, but was soon revealed to be a hoax after it was examined by professionals. "Archaeoraptor" was a chimera comprising the upper half of the Mesozoic bird Yanornis and the lower half of a then-undescribed dromaeosaurid. About a year later, that dromaeosaurid became known as Microraptor. Newer specimens of Microraptor preserve a surprising feature: Microraptor had four wings. It had feathers all over its body, including very long wing feathers on its arms and hands and a tail fan on the tip of its tail. This pattern was not unlike other feathered maniraptors known at this point, but Microraptor also had long pennaceous feathers on its legs going down onto the feet. These large wings suggest that Microraptor was capable of some form of aerial locomotion, but exactly how it flew is still debated. Long leg feathers are now also known in several Mesozoic avialians (including Archaeopteryx) and troodonts, but none of theirs are quite as extensive as those of Microraptor.
Only known from the posterior half of its body, the oviraptorosaur Nomingia had a mass of fused vertebrae on the tip of its tail. This feature, known as a pygostyle, is also known in modern birds (as well as in Beipiaosaurus), and typically supports a fan of feathers. It likely served the same function in Nomingia.
NGMC 91 (2001)
|NGMC 91 photographed by Matt Martyniuk, from Wikipedia.|
Not yet officially named, this well-preserved dromaeosaurid specimen was described in 2001 and was nicknamed "Dave". Though initially thought to be a juvenile Sinornithosaurus, newer phylogenetic analyses find it a closer relative of Microraptor. It preserves plumaceous feathers all over the body (except for the toes, which have scales, similar to modern birds), and its wing feathers are probably badly preserved pennaceous feathers. It also preserves a tail fan on the tip of the tail.
Only known from arms and some ribs, Yixianosaurus preserves some feathers, but their exact structure is difficult to tell. It is also hard to say what kind of maniraptor Yixianosaurus was, though its long hands might suggest an affinity with scansoriopterygids, small climbing possible avialians from the Jurassic.
|Fossil of Jinfengopteryx elegans photographed by Laikayiu, from Wikipedia.|
Once thought to be an archaeopterygid (again; this is turning into a sort of running gag, which, considering the similarities between archaeopterygids and deinonychosaurs, shouldn't be surprising), this is really the first troodont to preserve feathers. As the wings are tightly folded against the body, the wing feathers are impossible to decipher, but it does preserve body feathers and retrices (long tail feathers) along the length of its tail (as opposed to a tail fan at the tip as in known dromaeosaurids and oviraptorosaurs).
In 2007 the famous Velociraptor also entered the ranks of non-avian maniraptors with direct evidence of feathers, this time due to quill knobs on the ulna (similarly to Rahonavis). This was also evidence that medium-sized flightless dromaeosaurids retained large wings, and some have used it to support the idea that deinonychosaurs were secondarily flightless (as quill knobs are found mostly in strong-flying birds). Those who have examined other Velociraptor specimens personally have confirmed that these quill knobs are not aberrations or artifacts of preservation in one particular specimen, and are present on multiple other Velociraptor specimens.
When first described in 2008, the presence of a pygostyle in this oviraptorosaur suggested that it had a tail fan. In fact, this was a triumph of sorts for skeletal inference of feathers because when, in 2010, new specimens were found that did indeed preserve actual feathers, lo and behold, they had tail fans (as well as primary and secondary feathers and body feathers). The other interesting thing about these specimens was that they represented different growth stages, and the younger specimen had ribbon-shaped wing feathers (with barbules only at the tips of the feathers) instead of regular pennaceous feathers as in adults. (On the other hand, the "ribbon-shaped feathers" are also similar to the feathers of young birds that are molting, and this might have been the case with the young Similicaudipteryx.) The younger specimen also lacked secondary feathers.
This is another troodont that was once thought to be an archaeopterygid. (What did I tell you? It's a running gag.) Although the original specimen preserved feathers, new specimens revealed even more information (as usual). Anchiornis was more heavily feathered than even most modern birds! It was completely feathered down to the toes, the only naked region being the very, very tip of the snout (which unfortunately rarely ever gets taken into account in many reconstructions of this taxon). Anchiornis also showed that, like basal dromaeosaurids (such as Microraptor) and many Mesozoic avialians, troodonts started out with long leg feathers. (Jinfengopteryx doesn't preserve long leg feathers, although allegedly there are some undescribed Yixian troodonts that do.) Furthermore, it was also important in that it was a Jurassic non-avialian maniraptor, showing that there were indeed feathered non-avialian dinosaurs before the existence of Archaeopteryx. (Not that it actually mattered before, but it was good to get confirmation.) Finally, Anchiornis also gets the honor of being the first Mesozoic dinosaur for which we can be quite certain how it looked like in life, as it was the first to get the color pigments preserved in its feathers completely analyzed!
In addition to these, several oviraptorids have been preserved brooding on their nests in postures that suggest they had large wing feathers to cover their eggs. A flange on the second finger of Deinonychus (also found in Sinornithosaurus) has also been suggested online to be an anchor for primary feathers.
Note that even the BAND (Birds Are Not Dinosaurs fringe group) do not deny that maniraptors have feathers, in spite of their great skill in ignoring all evidence and the fact that they've tried (and failed) to discredit the presence of feathers in other coelurosaur groups, so anyone who denies all this fossil evidence is essentially out of luck. (They instead proclaim that all maniraptors are "birds and not dinosaurs". Still not correct, but it's something.)
Some people out there desperately write off the growing evidence as being "fake", but this is flat out wrong on so many levels. Nearly all of these fossils have been studied by many different professionals (including BANDits), and many of these taxa (such as Microraptor, Caudipteryx, and Anchiornis) are known from multiple (sometimes hundreds) of specimens that all preserve feathers. Even the "Archaeoraptor" hoax was quickly exposed after actual scientists studied it, and note that the feathers in neither specimen that was used to composite the hoax were faked. The specimen itself was a hoax, but the feathers were not. Funnily enough, no one has the slightest problem with the mammals that preserve fur and avialian dinosaurs that preserve feathers from the same formations. Nice double standard there.