|Life restoration of the S. prima type specimen by M. Martyniuk|
"I will generalize at this point that they are modified stage I, based on the likely LESS basal Sinosauropteryx prima and general agreement on which stage it has."
Another case of general agreement about fossil feathers that (as with Foth's paper) was demonstrated to be wrong long ago in important research which was subsequently ignored by all researchers who simply accept the meme that Sinosauropteryx has stage 1 filaments without thinking about it too much.
Currie & Chen 2001 (which as far as I know is the only non-BAND paper to examine the S. prima integument in detail and actually publish their findings) concluded that "The mixture of thick and thin strands close to the body, the increased presence of thinner strands distally, the fact that the thicker strands are positioned close to the body and are normally oriented at higher angles from the body than more distal strands (Fig. 12g), the presence of areas where many of the finer strands lie adjacent and parallel to each other (even kinking together in a few places), and the tendency of finer filaments to angle away on both sides from thicker structures (Fig. 13a) all suggest a feather-like structure with central shafts and plumulaceous barbs."
So, the feathers of S. prima are at least Prum's stage 3! So why does everyone continue to refer to them as "protofeathers" or stage 1 feathers? It's not like this is an obscure paper. Sorry if I sound exasperated [but] the the state of the literature on fossil feathers is absolutely atrocious and seems to be plagued by lack of research and plain laziness. Not only do important specimens get barely a sentence mentioning the feathers after pages and pages of detailed osteology, but the papers that do bother to describe feather structure go almost completely ignored by subsequent authors.
Currie, P.J.; and Chen, P.-j. (2001). "Anatomy of Sinosauropteryx prima from Liaoning, northeastern China". Canadian Journal of Earth Sciences 38 (1): 705–727.
You can read the comment thread at Jaime's article for the full discussion. To summarize, Jaime replied that while he's aware of Foth and Currie/Chen's work, he still prefers to default to the stage 1 identification until completely unambiguous evidence is found. Jaime notes that Currie/Chen only demonstrate barbs on a single side of the rachis in the S. prima holotype (but unambiguously on both sides of the rachis in one of the adult referred specimens), which would be bizarre and out of line with Prum's developmental model, unless they were secondarily derived from true stage 3 feathers (IMO this apparent one-sided vane is probably best explained by taphonomy due to the severe level of overlap and impossibility of finding a completely isolated feather). Jaime also notes the possibility that there are in act two distinct types of stage 1 feather present, a short, thick, rachis-like filament and longer, thinner, barb-like filaments. But given the fact that, as Currie/Chen reported, the thinner filaments are almost always at an angle to the thicker ones, the simplest explanation is that these are standard, barbed, plumulaceous feathers similar in structure to modern down.
As I pointed out in a rather convoluted way in my initial comment, there are two main problems with the state of research into fossil feather types. One, feather anatomy often gets the short shrift in descriptions of new fossil specimens, even those that spend a lot of time detailing the intricacies of the bone anatomy. Second, there seems to be a systematic problem in the scientific literature, where researchers (for whatever reason) ignore research that has implications for the conclusions they are drawing about new feathered specimens. For example, the Sciurumimus description states that based on their examination, the feathers in the specimen appear to be monofilaments and therefore stage 1. but it would be important to note that this could possibly be an illusion created by taphonomy, and that in principle taphonomy can makes fossil feathers appear more primitive than they are. This is not to say that the feathers are more advanced than they seem, but that other research suggests they very well could be. This is a pretty major omission. Even more advanced feathers suffer from this problem. In a 2010 Bite Stuff post on the feathers of the microraptorian specimen "Dave" claimed that the body including arms were covered in branched filamentous feathers rather than vaned feathers, when the paper explicitly states the wing feathers were vaned and pennaceous, and infers the presence of barbules based on the arrangement of barbs.
I don't mean to pick on Jaime here (though I continue to see people citing his article to defend reconstructing Dave with fluffy arm feathers). His reasoning in the Sciurumimus article is fair, if a bit over-conservative in my opinion given the available data. My major complaint is the fact that these issues are never even raised in the literature, even just to shoot them down as not providing strong enough evidence. Instead, these potentially important papers continue to be ignored by relevant research. Whether or not Currie and Chen convincingly demonstrated that the feathers of S. prima are branched, or Foth convincingly showed that taphonomy tends to make feathers look more primitive... Well, to not even acknowledge this stuff in a paper about primitive feather fossils is just poor science.