Name means: Western Bird
Species: regalis, crassipes, gracilis, rossicus, altus, montana, lumgairi
Range: Late Cretaceous (Campanian, 84-72 MYA) from Kansas, Montana, South Dakota, Alaska, Canada (Alberta, Manitoba, Northwest Territories, Nunavut), Russia, Sweden
Size estimate: 3-6 ft length, 10-50 lbs
Discovery: Othniel Charles Marsh, 1872
Classification: dinosauria, maniraptora, avialae, euornithes, ornithurae
During the Cretaceous period, a shallow sea covered the modern Great Plains. Hesperornis fished these waters and the Arctic Ocean like a modern cormorant or loon. While most species seemed to prefer ocean habitats, a few may have moved into freshwater. Fossil evidence shows that they sometimes fell prey to mosasaurs and plesiosaurs. Though not ancestral to modern birds, they were nevertheless closely related.
Due to its aquatic lifestyle and its grebe-like swimming technique, its wings were reduced to thin stubs. Some reconstructions hold they did not appear outside the body at all. This would have made Hesperornis more streamlined in the water. Recent evidence suggests its feet were not fully webbed, but lobed like a grebe's. The joints of its legs also worked like a grebe's, making it agile in the water but awkward on land. Hesperornis probably only came on land to nest. Unlike modern birds, Hesperornis grew teeth along its lower jaw and the back of its upper jaw. Instead of resting in sockets like dinosaurs, they lay in a groove like the teeth of mosasaurs. Scientists describe such similar forms found in unrelated animals as "convergent evolution." The Champsosaurs on this river's bank and the Ophthalmosaurus further downstream also represent convergent forms.
Hesperornis bones tend to preserve better than other bird bones because they were solid. This feature acted like ballast, making it easier for the animal to dive. This also means that Hesperornis skulls sometimes survive better than those of other birds. Most skulls from other ancient birds are crushed during fossilization. Hesperornis skulls show a mix of unique features. The skulls of modern birds feature a mechanism known as "cranial kinesis." Special joints allow their upper jaws to move independent of the rest of the skull to a degree. Primitive birds generally lacked this feature. Recent studies show that Hesperornis had cranial kinesis, but accomplished it differently than modern birds. Thin areas of bone allowed the skull to bend, which accomplished the same movement in a different way.