A field guide to mesozoic birds and other winged dinosaurs

carnivorous. However, many deinonychosaurian fossils which preserve
feathers show a small portion of the tip of the snout that is unfeathered.
This featherless snout tip is also seen in some toothed, beakless enantiornitheans.
It is possible this could be evidence of “rhamphotheca” in its loosest
sense--the very lightly cornified, flexible bill skin found toward the back
of the beaks in some modern birds, where the horn-like, keratinous portion
thins out into normal skin.
Around the base of Avialae, the reduction of teeth becomes commonplace
in several independent lineages, probably due to a shift to more
omnivorous diets. Almost all known basal avialans have very few, very small
teeth in the upper jaws, and several lost teeth altogether. This trend appears
to culminate with the confuciusornithids, which are not only toothless but
have sharply pointed jaws that, in some very rare specimens, preserve the
actual keratin of a beak. These impressions show that in early beaked birds,
the rhamphotheca was thin and delicate and probably not as heavily keratinized
as in modern birds.
In most enantiornitheans, the jaws are fully toothed, with no evidence
of beaks. It may be tempting to think that this could unite the enantiornitheans
with the toothy, beakless deinonychosaurians in a “Sauriurae”
to the exclusion of the beaked euornitheans (“true birds”). However,
given the numerous times beaks have evolved independently in vertebrates,
it’s more likely that each of the examples of basal avialans with reduced
or absent teeth arose independently of one another, or that some reversal
occurred at the base of ornithothoraces to return birds to a state of fullytoothed
jaws. While many enantiornitheans preserve jaw material, only one
species exhibits the kind of toothlessness at the front of the jaws that could
imply a beak: Gobipteryx minuta, which, like confuciusornithids, were
beaked and completely toothless.
All known Mesozoic euornitheans (the fan-tailed birds, including
modern birds), unlike the typically beakless enantiornitheans, had small
beaks restricted to the jaw tips, with teeth further back in the jaw. While
hongshanornithids were originally reported to have beaks and to be completely
toothless, O’Connor and colleagues later showed that they had tooth
sockets preserved in the upper and possibly lower jaws. However, the jaw
tips were toothless and probably beaked. In some (perhaps most) Mesozoic
euornitheans, an additional bone was present forward of the dentary: this
predentary bone was always toothless and likely evolved specifically to accommodate
a jaw-tip beak. Predentaries are known from the most primitive
euornitheans like Hongshanornis longicresta up to the most advanced
non-avian species like Hesperornis regalis; however, they seem to have been
lost or incorporated into a solid, single lower jaw bone shortly before the
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