A field guide to mesozoic birds and other winged dinosaurs

fined Aves the way Mammalia came to be defined--as a “crown group”, that
is, the group containing all modern bird groups and any species closer to
them than to more distantly related prehistoric lineages. This use of Aves
preserves Linnaeus’ usage (he didn’t know of any prehistoric birds), but
also excludes fossil bird groups such as Archaeopteryx and even those very
similar to modern birds like Ichthyornis. Subsequently, attempts were made
to re-define Aves to include Archaeopteryx for historical reasons; after all,
it has always been considered the “first bird”. Critics called these attempts
arbitrary: why include Archaeopteryx and not their closest relatives (such
as Microraptor), which were more like modern birds in some respects? As
of this writing, a body known as the International Society for Phylogenetic
Nomenclature is being organized to govern the naming of clades, and it
seems likely that it will adopt the crown group usage as official.
When this usage is officially adopted, it will leave most Mesozoic
birds outside of Aves. This will only require a semantic change; true “birds”
will be restricted to the group of modern-style birds only. Non-avian members
of the lineage leading to modern birds such as Ichthyornis will be considered
“stem-birds” (and, somewhat ironically, also a “non-avian dinosaur”
genus, as all birds are included in the clade Dinosauria).
So what is a “bird”? As a vernacular rather than scientific term, this is
a matter of loose convention rather than scientific precision. Most common
definitions of the word in English hinge on certain key aspects: egg-laying,
feathers, and flight. If this usage is adopted, Archaeopteryx may still be considered
birds, depending on their controversial flight abilities (though they
were almost certainly at least glissant, i.e. capable of passive gliding). Microraptor
are probably “birds” as well under this definition. Of course, not
all birds fly, and just as there are ground birds and flightless birds today,
many Mesozoic lineages lost flight, or evolved before flight. Therefore, any
dinosaurs which would now be included by one of the first definitions proposed
for the clade Aves, by Allan Charig in 1985, are featured in this book.
Charig’s definition of Aves linked it to the presence of feathers. While he did
not specify what he meant by feathers, for the purposes of this book I am
using a conservative interpretation: feathers with a fully modern anatomy,
consisting of a rachis (central “quill”), and a vane comprised of barbs linked
together by barbules (see diagram on p. 39). This is also the same characteristic
that was used to classify Archaeopteryx as birds in the first place, so it
seems most appropriate for use in this work. This definition is equivalent to
the modern clade Aviremigia, that is, all winged dinosaurs.
By their nature, feathers do not fossilize well, nor often, so there
may certainly be some species or even entire lineages which were “birds”
under the above definition, but are excluded here because we do not have
11