Hesperornis

Hesperornis is a genus of bird from the Late Cretaceous, living during the Campanian age and potentially into the early Maastrichtian. Its name comes from the Ancient Greek words for "western" and "bird." This cormorant-like ornithuran was one of the early finds in avian paleontology, discovered by O. C. Marsh during the late 19th-century Bone Wars, though it is not among his most famous finds. Fossils have been found in Late Cretaceous marine limestones in Kansas and marine shales in Canada. Of the nine recognized species, eight come from North America and one from Russia.

The bird was large, reaching about 1.5 to 2 meters in length and weighing around 10.6 kilograms. It had almost no wings and used its strong hind legs for swimming. Early studies suggested its feet had lobed toes like modern grebes, but more recent work on foot shape in hesperornithiforms and modern seabirds has made webbed toes equally plausible. Like many Mesozoic birds, Hesperornis had both teeth and a beak. Its teeth were arranged in a longitudinal groove along the lower jaw and the back of the upper jaw, a feature convergent with mosasaurs and unlike any other known bird. The front of the upper jaw and tip of the lower jaw lacked teeth and were likely covered in a hard, keratinous beak. The palate had small pits where the lower teeth locked when the jaws closed. A primitive joint in the lower jaw allowed the back portion to rotate independently, helping the teeth disengage.

The first Hesperornis specimen was found in 1871 by Marsh during his second western expedition with ten students. In Kansas, they uncovered a large, wingless bird skeleton with strong legs, about five feet tall, but missing its head. Marsh named it Hesperornis regalis, meaning "regal western bird." The following year, one of Marsh's students, Thomas H. Russell, discovered a nearly complete skeleton in western Kansas. This specimen had enough of its head intact to reveal teeth in the jaws. In an 1873 paper, Marsh declared the find helped break down the old distinction between birds and reptiles. The discovery also worsened Marsh's rivalry with Edward Drinker Cope, after Cope accidentally received boxes of fossils meant for Marsh. Cope called the birds "simply delightful," but Marsh accused him of stealing the bones. By 1873, their friendship turned to open hostility, fueling the Bone Wars. Marsh rarely went into the field after that, but paid collectors sent him parts of 50 Hesperornis specimens, allowing him to demonstrate a stronger evolutionary link between reptiles and birds than before.

Many species have been described, though some are known from very few bones and cannot be properly compared with more complete remains. Species are often separated by provenance, age, or size. The type species, Hesperornis regalis, is the best known, with dozens of specimens from the Smoky Hill Chalk Member of the Niobrara Formation (early Campanian, 90 to 70 million years ago). It is the only species with a nearly complete skull. Hesperornis crassipes, named by Marsh in 1876 and originally classified as Lestornis crassipes, was larger than H. regalis, had five ribs instead of four, and differed in bone sculpturing on the breastbone and lower leg. It is known from the same time and place as H. regalis, with one incomplete skeleton including teeth and skull parts. Marsh named Hesperornis gracilis in 1876 for an incomplete metatarsus from the same layers. This smaller species later became tangled in the taxonomy of a specimen that eventually formed the basis of a new genus and species, Parahesperornis alexi, whose type specimen was originally thought to belong to the same species.