A field guide to mesozoic birds and other winged dinosaurs

Above: Sinosauropteryx prima, a compsognathid exhibiting
Stage III down-like feathers.
have had Stage II down feathers, but the preservation in relevant fossils is
too poor to be certain. At the very least, we can assume that down feathers
emerged at or near the base of the advanced theropod group Coelurosauria,
and are present in all more advanced theropods including modern birds.
The next stage in feather evolution according to Prum is uncertain.
Stage III could have been basic down with the addition of smaller, microscopic
branching structures on their barbs (called barbules). Alternately,
the next stage may have involved the barbs beginning to grow in a helical
fashion up along a central filament, or rachis, in a type of advanced down
known as a semiplume. Either barbules or a semiplume structure could
have evolved first, or they could have evolved simultaneously. The fossil
evidence is uninformative on this point, as neither barbuled down feathers
nor semiplumes have been definitively identified in fossils of non-aviremigians.
However, some potential fossil feathers, classified in the species
Praeornis sharovi, appear to show a central rachis and thick barbs lacking
differentiated barbules, instead showing solid ridges on the barbs. This may
be an early form of, or derivation of, an unbarbed semiplume. However, the
identification of the Praeornis feathers is controversial and some researchers
have even proposed that they are not feathers at all, but cycad leaves,
though chemical testing seems to indicate that they are indeed animal in
origin.
Stage III feathers have been positively identified in the compsognathid
Sinosauropteryx prima. This species appears to have had both mod-
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