Hesperornithes

Hesperornitheans were the only Mesozoic avialans known to colonize the oceans. They were wiped out in the Cretaceous–Paleogene extinction event, along with enantiornitheans and all other non-avian dinosaurs.

Anatomy and ecology

Most of what is known about this group rests on analyses of single species, as few provide sufficiently complete fossils for analysis. Although some of the smaller and more basal species, like those belonging to the subgroups Enaliornithidae and Brodavidae, might have been able to fly, the larger hesperornithids like Hesperornis and Baptornis had only vestigial wings. As in the case of modern foot-propelled diving birds, the femur and metatarsus of these animals were short, whereas the tibia was long. The legs were also set far back on the body, as in loons, grebes or penguins. Hesperornithids must have been powerful swimmers and divers but extremely ungainly on the land, and probably spent little time ashore except to nest. They were rather long-bodied, and measured about 6 feet (180 cm) long.

Some researchers think that on land they had to slide on their bellies and push with their legs; the hip and knee joints were shaped such that these species could not move them dorsoventrally, and in a resting position the feet projected sideways from the body, which would have prevented them from walking upright. The anatomy of their toes suggests that hesperornitheans had lobes of skin for propulsion underwater similar to grebes, rather than being webbed. The dense bones of these animals decreased their buoyancy, making diving easier. However, morphometric comparison with modern diving birds suggests that hesperornitheans share more similarities with diving ducks and cormorants rather than with loons or grebes.

The snout was long, and tipped with a slightly hooked beak. Behind the beak, the jaws were filled with a series of simple, sharp teeth which were set into a longitudinal groove. These probably helped to seize fish, like the serrated beak of mergansers. Unlike modern birds, they retained a joint between the lower jaw bones. It is believed that this allowed them to rotate the back portion of the mandible independently of the front, thus allowing the lower teeth to disengage.

Feather remains from specimens of this lineage have been preserved in coprolites. This lineage retained both modern and primitive feathers, which has been interpreted as one of the causes of its extinction.