A field guide to mesozoic birds and other winged dinosaurs

Beaks & Teeth
In modern birds, the snout is covered in a keratinous beak (known
scientifically as a rhamphotheca), and therefore is bare of feathers. However,
many Mesozoic birds lacked beaks and thus the extent of the snout
feathering was highly variable. Few fossils preserve the delicate and usually
short feathers present on the snout, however, we know of some species,
such as Eoenantiornis buhleri and Sinornithosaurus millennii, in which the
feathers extend three-quarters of the way or more toward the tip of the jaws,
resulting in a snub-billed appearance. In other species, such as Microraptor
zhaoianus and some enantiornitheans (“opposite birds”) with specialized
and elongated jaws, the feathers did not extend much forward of the eye. As
in modern birds, it is likely that some had bare heads and/or necks. Featherless
portions of the head in modern birds are usually related to display, and
in some examples, heat loss or ease of preening.
It is well known that many dinosaurs had beaks, but also that, famously,
many of these also possessed teeth. However, the exact arrangement
of beak and teeth in the jaw is commonly misunderstood. Almost any
life restoration of a Hesperornis, for example, will show a keratinous beak
covering the entire extent of the upper and lower jaws. Some of these clearly
show teeth erupting directly from the tomia (edge) of a continuous keratin
beak. The continuous appearance of this beak is likely incorrect in itself,
since non-avian birds probably all had “compound rhamphotheca,” beaks
made up of several distinct plates that are often visible in life.
In bird species like Hesperornis regalis, the lower jaw (dentary) teeth
continue almost all the way to the jaw tip, though the very tip (and the small
predentary bone that was probably present) was toothless. On the underside
of the upper jaw (premaxilla), there were indentations where the lower
teeth would have locked into the bone. If there was a hard beak present, it
would have had to have been pitted to accommodate the lower teeth. However,
these indentations are inset to the edge of the jaw. The edges of the
upper jaw slightly overhang the lower jaw, which would have allowed for
the beak edge, if it was there, to not come into contact with the lower teeth,
which would have caused tooth wear any time the mouth closed. Upper
teeth in this species are restricted to the very back of the mouth (maxilla).
This can also be seen in the ‘dental grooves’ on the underside of the skull.
According to Heironymus & Witmer 2010, in both Ichthyornis and
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