A field guide to mesozoic birds and other winged dinosaurs

Hesperornis, the premaxillary nail and mandibular (lower jaw) nail were
the most heavily keratinized parts of the beak. These “nails,” which often
formed small hooks at the beak tips, are where the beaks would have been
most solid, like typical modern bird bills. The same authors note that the
simple presence of teeth in the maxilla and dentary of these species probably
means that they entirely lacked the latericorn and ramicorn beak plates
which normally cover the ‘lips’ of the jaws, and that the presence of hardened
rhamphotheca on the edges of the
jaws may be unique to modern birds.
However, as noted above, the tip of the
upper jaw (premaxilla) in Hesperornis
is also toothless and provides space for
an overhanging edge (tomia) of some
kind to be present. This would have
been somewhat softer tissue, like the
more pliable bills of ducks and geese.
Further support for the presence of a
beak on the premaxilla comes from
the presence of a “rhamphothecal
groove” on the upper part in front of
the naris (nasal opening in the skull),
which likely served as an anchor point
for the keratin on the skull.
So how far did the beak extend?
Heironymus & Witmer found
that the latericorn almost always extends
to the back of the subnarial bar
in birds. This is a process of the premaxilla
that extends back to separate
the naris from the maxilla. This means
that beaks will very rarely, if ever, extend
onto the maxilla itself. The maxilla
in Hesperornis even compensates
for this limitation by extending a bit
forward underneath the subnarial bar to extend the tooth row past the full
extent of the beak.
Based on the evidence above, in Hesperornis, the toothless, pointed
beak tips would have been made of solid, normal keratin, while the rest of
the beak would have been more like stiffened skin grading into normal skin
and feathers toward the back of the skull. At no point would the teeth have
occupied the same physical space as the rhamphotheca, though they may
40
Above: Evolution of the avian beak and
loss of teeth. From top to bottom: Archaeopteryx
lithgraphica, Yixianornis
grabaui, Hesperornis regalis, Polarornis
gregorii. Not to scale.