A field guide to mesozoic birds and other winged dinosaurs

Hesperornitheans
Given the fact that most primitive euornitheans occupied semiaquatic
shorebird niches, it is unsurprising that some groups would diversify
into fully aquatic forms. The major lineage of aquatic Mesozoic birds was
the Hesperornithes, or “westbirds”. The clade Hesperornithes includes four
major lineages of foot-propelled divers: enaliornithids, brodavids, baptornithids,
and hesperornithids. The enaliornithids are a poorly known group
of primitive divers that may represent an unnatural grouping including the
ancestors of the hesperornithids and baptornithids. They, and the brodavids,
probably retained some flight ability, and hesperornithean feathers
found preserved in Albertan amber may belong to roosting enaliornithids
or brodavids.
The hesperornithoids, comprising the most specialized families
baptornithidae and hesperornithidae, include the most fully aquatic and
marine-adapted birds that ever lived, some almost completely forsaking
their ability to move about on land except to lay eggs. The leg and hip
anatomy of hesperornithids is very similar to that of modern loons, and
they probably employed similar foot-propelled diving techniques in pursuit
of aquatic prey. The anatomy of the toes, particularly the toe joints, is very
similar to modern lobe-toed birds like grebes, rather than web-footed birds.
Specifically, the toes probably carried one single large lobe each, rather than
multiple lobes as in coots. As in aerodynamic flight feathers, the lobes were
probably asymmetrical with respect to the skeletal toes for hydrodynamic
purposes. Like many specialized diving birds, many may have been colored
with counter-shaded patterns of dark on top and light on bottom to camouflage
them against surface and seafloor. The groove-set teeth of hesperornithoids
are also seen in mosasaurs and may be a specialization for catching
fast-moving prey in an open ocean environment. Additionally, their jaws
appear to have been prokinetic, i.e. the upper jaw was able to move up and
down relative to the base of the skull, as in many modern birds (Bhuler &
al. 1987).
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