A field guide to mesozoic birds and other winged dinosaurs

advent of modern birds. Interestingly, the only other group of vertebrates
to have evolved predentaries are the ornithischian dinosaurs, which had
a similar arrangement of beaked and toothless jaw tips in front of fully
toothed jaws.
The earliest fully beaked and toothless euornitheans are also among
the most primitive: Archaeorhynchus, which lacked teeth and had flattened,
spoonbill-like beaks. Because more advanced birds retained teeth in both
jaws, this is almost certainly an independently-acquired condition unrelated
to the toothlessness of modern birds. The songlingornithids and the later
hesperornitheans and Ichthyornis all had toothless premaxillae and predentary
bones with toothy maxilla and dentaries. Evidence from bone texture
shows that they likely had keratinous beaks at the tips of their jaws, and
either feathery toothed jaws or pliable, skin-like rhamphothecae posterior
to the beak. Since both major lineages of modern birds (avians) lack teeth,
it’s probable that their common ancestor was also fully beaked, so teeth
must have been lost for good in the bird lineage shortly after ichthyornitheans
evolved. Interestingly, studies of ichthyornithean and hesperornithean
bone structure shows that they likely had “compound rhamphotheca”, and
this may have been the ancestral condition for modern birds (Heironymous
& Witmer, 2010). While the quintessential bird beak is made up of a single
keratinous sheet covering the jaw, in species with compound beaks, the keratin
is arranged in discrete plates on the jaws. This can best be seen in some
modern seabirds like the Albatross.
Unfortunately, in interesting groups like Hollanda, Gansus, and Patagopteryx,
the condition of the jaws is unknown. However, we can use parsimony
and phylogenetic bracketing to try and develop an educated guess.
Most studies find these three groups to be euornitheans (“true birds”) more
primitive than the hesperornitheans and Ichthyornis, which have toothed
upper and lower jaws with beaked tips. While the three intermediate birds
may well have lost some or all of their teeth independently again, all things
being equal it is more parsimonious to suppose that like ichthyornithids,
hesperornithids, hongshanornithids and chaoyangornithids, they had
beaks restricted to the tips of the jaws, with teeth behind.
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