Hesperornis were large flightless birds that swam in the oceans of the Late Cretaceous (~83 mya) and preyed upon small fish. It is likely that they swam and fed much like modern loons. The jaws were lined with sharp teeth, much different than birds of today. Interestingly, the jaws of Hesperornis were originally described as those of a small mosasaur (extinct marine reptiles), due to the unexpected teeth and the convergent evolution of a specialized jaw hinge (Gregory, 1952).
The type fossil, Hesperornis regalis, was discovered in the Niobrara Chalk formation of Trego County, Kansas by Professor O.C. Marsh on July 25, 1871. It lived during the Campanian of the Late Cretaceous, 83 mya to 72 mya.
The majority ofHesperornisremains are found within major seaways, such as the Western Interior Seaway (an inland sea that occured in the middle of North America during the Cretaceous), but some individuals come from freshwater deposits. It is theorized that some individuals followed river systems searching for fish.
H. regalis had a length of 1.2 - 1.8 m (4-6 ft) and resembled an enlarged version of a modern diving bird, such as a loon. The thighbone projects from the hip at a sharp background angle and were positioned to utilize its large webbed feet as paddles. This adaptation would make terrestrial locomotion difficult and impractical (the terrestrial locomotion would resemble the awkward terrestrial ambling of the modern loons). H. regalis was flightless and possessed highly reduced wings (Marsh, 1880; Reynaud, 2006).
A powerful swimmer, this large toothed-bird has a slender, gracile build. This slender morphology improved swimming efficiency. Extreme loss in muscle bulk occured when the loss of the forelimbs, or wings. Unlike the modern penguin, Hesperornis relied solely on its hind flippers for swimming, making its hind-region muscular and relatively bulky (Marsh, 1880; Chinsamy, 1998; Reynaud, 2006).
Another notable feature of Hesperornis regalis is that it is one of a few toothed-birds from that time period. These teeth must have helped H. regalis catch and grasp the small fish that it preyed upon in the Late Cretaceous seas of North America.The lower jaw of Hesperornis held a complete toothrow and the upper jaw had teeth on both sides of the back portion. The tooth structure found within these birds is a example of convergent evolution, as they are not lodged within sockets as those of their dinosaur ancestors, but sat in a singular groove that ran down their beak. Small holes were present in the tip of the upper beak, so that the lower teeth could slip into them when closed.These teeth must have helpedH. regalis catch and grasp the small fish that it preyed upon in the Late Cretaceous seas of North America.(Gregory, 1952; Elzanowski, 1991).
Further, Hesperornis had a specialized jaw hinge that is only seen in its relatives and the distantly related aquatic mosasaurs. However, it may be an example of convergent evolution of unrelated organisms to similar habitats.
H. regalis lived within the shallow coastal waters of contemporary seaways and had a piscivorous diet composed of fish and squid (Marsh, 1880; Gregory; 1952; Carpenter, 2003).
Due to the fossil remains of Hesperornis being found in the stomach cavity of a Tylosaurus, it is known that this large bird fell prey, at least occasionally, to mosasaurs (Cicimurri & Everhart, 2001).
Hesperornis regalis were thought to have legs that were largely enclosed within the body wall, which would have caused an odd appearance in life where the feet appear to be the only component of the leg. These paddle-like feet would protrude out of the sides of the animal near the tail, much like the loon (a modern diving bird). This morphological adaptation to an aquatic existence would have prevented them from bringing their legs underneath the body to stand or bear weight. This would have hindered terrestrial movements greatly and would have forced H. regalis to push themselves along on their bellies (Reynaud, 2006; Martin et al., 2012).
Hesperorniswas a marine-dwelling organism that occupied habitat in large subtropical to tropical seas, such as theprehistoric North Sea, the Western Interior Seaway and theTurgai Strait (Hills et al., 1999; Rees & Lindgren, 2005)
Interestingly, there have been several Hesperornis specimens thathave been found in inland freshwater deposits. Most of these specimens are some of the earliestHesperorniswithin the lineage. Because it is thought that H. regalis had a marine origin, these finds are indicative that Hesperornismay have eventually moved up river systems and away from their original marine ecosystem (Fox, 1974).
