| Hesperornitheans | |
|---|---|
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| Restored skeleton ofHesperornis regalis | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Avialae |
| Clade: | Ornithurae |
| Clade: | †Hesperornithes Fürbringer, 1888 |
| Subgroups[4] | |
| Synonyms | |
Hesperornithes is an extinct and highly specialized group of aquaticavialans closely related to the ancestors of modernbirds. They inhabited both marine and freshwater habitats in the Northern Hemisphere, and includegenera such asHesperornis,Baptornis,Parahesperornis,Enaliornis, andPotamornis, all strong-swimming, predatory divers. Many of the species most specialized for swimming were completely flightless. The largest known hesperornithean,Canadaga arctica, may have reached a maximum adult length of 2.2 metres (7.2 ft).[5]
Hesperornitheans were the onlyMesozoic avialans known to colonize the oceans. They were wiped out in theCretaceous–Paleogene extinction event, along withenantiornitheans and all other non-avian dinosaurs.
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Most of what is known about this group rests on analyses of single species, as few provide sufficiently completefossils for analysis. Although some of the smaller and more basal species, like those belonging to the subgroupsEnaliornithidae andBrodavidae, might have been able to fly, the largerhesperornithids likeHesperornis andBaptornis 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 inloons,grebes orpenguins. 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.[6]
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.[1] The anatomy of their toes suggests that hesperornitheans had lobes of skin for propulsion underwater similar togrebes, rather than being webbed. The dense bones of these animals decreased their buoyancy, making diving easier.[7] However, morphometric comparison with modern diving birds suggests that hesperornitheans share more similarities withdiving ducks andcormorants rather than withloons orgrebes.[8]
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 seizefish, like the serrated beak ofmergansers.[9][10] 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 themandible independently of the front, thus allowing the lower teeth to disengage.[6]
Currently, the hesperornitheans are recognized as a very specialized lineage that is not ancestral to modern birds. Still, their relationship is close enough that they probably diverged from the ancestors of modern birds as late as the earliestCretaceous.
The earliest known hesperornithean is theEarly CretaceousEnaliornis. The majority of hesperornithean species are known from theLate Cretaceous of North America. Small hesperornithean bones are known from the freshwater deposits of the Late Cretaceous of theJudith River Group as well as theHell Creek andLance Formations, and in severalEurasian sites. These species were about the size of a cormorant or a loon.
The clade Hesperornithes was originally named as a subclass ofAves by Furbringer in 1888.[11] However, it was generally ignored in the scientific literature in favor of the order-level nameHesperornithiformes, coined one year later. In 2004, Clarke became the first to define the hesperornithean group in terms ofphylogenetics. Clarke defined Hesperornithes as all species closer toHesperornis regalis than to modern birds, and regarded Hesperornithiformes as a junior synonym, though she did not define the latter name. Clarke also defined the more inclusive groupHesperornithidae as all hesperornitheans closer toHesperornis than toBaptornis.[3]
Hesperornitheans were originally combined withIchthyornis in theparaphyletic group "Odontornithes" byOthniel Charles Marsh, in 1873. In 1875, they were separated asOdontolcae. The group was often considered to be related to loons and grebes,[12] or to thePaleognathae (based on perceived similarities in thebony palate).[13] These similarities, however, as the more recently determined fact that theosteons of their bones – at least inHesperornis – were arranged in a pattern similar to that inNeognathae,[14] are today considered to be due toconvergent evolution.[15][16]
In 2015, a species-level phylogenetic analysis found the following relationships among hesperornitheans.[17]
| Hesperornithes | |
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