A field guide to mesozoic birds and other winged dinosaurs

muscles. Finally, in the concestor of all modern birds, the teeth were lost,
resulting in the characteristic toothless bill of Aves.
The Origin of Feathers
The details regarding exactly how feathers first evolved have been
historically contentious and are still not entirely clear. However, hypotheses
based on the developmental stages of modern feathers (such as those
proposed by Richard Prum, e.g. Prum 1999) seem to match the fossil record
fairly well at this time. According to Prum, the first feathers (Stage I
of feather evolution) would have been simple quill-like structures emerging
from a follicle in the skin, similar to the way scales and hairs emerge from
skin follicles. These simple quills, often referred to as “protofeathers”, would
have been monofilaments with no branching structure, equivalent to the
rachides that form the central structure of modern feathers. The presence
of Stage I feathers in the fossil record is uncertain. The earliest probable
examples come from the Late Jurassic coelurosaurian or megalosaurian genus
Sciurumimus. The long, somewhat flattened “elongate broad filamentous
feathers” (EBFFs) of some primitive feathered dinosaurs including the
segnosaurian species Beipiaosaurus inexpectus may represent either Stage
I feathers, novel structures derived from more advanced feather types, or
simply vaned feathers distorted by crushing. Similarly, the unusual monofilament
quills of ceratopsians Psittacosaurus and heterodontosaurians
Tianyulong may represent Stage I feathers. If this is the case, the first feathers
must have evolved at or before the origin of Dinosauria. It is even possible
that “pycnofibres”, down-like filaments found in pterosaurs, evolved
from Stage I feathers. If this is the case, feathers may have their origins in
the early Triassic period at the origin of the group Ornithosuchia, shortly
after the ancestors of birds split from the ancestors of crocodiles.
Stage II feathers evolved when the feather follicle collar (equivalent
to the calamus of modern feathers, used as the nib in quill pens) gave rise to
several filaments instead of forming a single long quill. This type of feather
still exists today in the form of down feathers, the simplest form of which is
made up of a short calamus and numerous long, soft filaments that form a
tuft rather than a vane. All of the filaments (called barbs) in a down feather
are anchored to the central calamus rather than to a central quill or rachis.
Down feathers have been identified in compsognathids like Sinosauropteryx
prima. More primitive theropods, like Dilong and Yutyrannus, may
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