Hesperornithes

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iHesperornithes Fossil range: Cretaceous
Hesperornis regalis skull
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Aves Subclass: Hesperornithes Fürbringer, 1888 Order: Hesperornithiformes Sharpe 1899
Families
Enaliornithidae Baptornithidae Hesperornithidae
Synonyms
Odontornithes Marsh, 1873 (partim) Odontolcae Marsh, 1875 Gaviomorphae Cracraft, 1982 (partim)

Hesperornithes are an extinct and highly specialized subclass of Cretaceous toothed birds. Hesperornithiform birds, apparently limited to former aquatic habitats in the Northern Hemisphere, include genera such as Hesperornis, Parahesperornis, Baptornis, Enaliornis, and probably Potamornis, all strong-swimming predatory waterbirds. Many, if not all species were completely flightless. The largest known hesperornithiform, described in 1999 and named Canadaga arctica, may have reached a maximum adult length of over 1.5 meters (five feet).

Hesperornithiform birds were, strictly speaking, the only dinosaurs to colonize the oceans; the aquatic reptiles of their time, such as the ichthyosaurs and plesiosaurs, were not dinosaurs.

[edit] Anatomy and ecology

Although some of the smaller species might have been able to fly, Hesperornis and Baptornis had only vestigial wings. Like living foot-propelled diving birds, the femur and metatarsus were short, whereas the tibia was long. The legs were also set far back on the body, as in loons, grebes or penguins. Hesperornithiformes must have been powerful swimmers and divers but extremely ungainly on the land, and probably spent little time ashore except to nest (indeed, it has been suggested that they did not incubate their eggs^{[citation needed]}). 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 birds could not move them dorsoventrally, and in a resting position the feet projected sideways from the body, which would have prevented them from walking. The joints of the toes resemble those of grebes, suggesting that the foot bore a series of lobes rather than a single web. The dense bones of these birds decreased their buoyancy, making diving easier (Chinsamy et al. 1998).

The beak was long, probably hooked at the tip, and bore a series of simple, sharp teeth which were set into a longitudinal groove. These and probably helped to seize fish, as does the serrated beak of mergansers today; unlike the reptilian teeth of other toothed birds, those of the Hesperornithes were unique (Gregory, 1952); given that teeth are induced by a rather simple genetic mechanism^{[citation needed]}, it is even possible that these birds re-evolved them and that the ancestors of the hesperornithids ancestors were toothless.

[edit] Systematics and evolution

These birds were originally combined with Ichthyornis in the paraphyletic Odontornithes by Othniel Charles Marsh, in 1873. In 1875, they were separated as Odontolcae. The group was considered to be allied to loons and grebes (Cracraft, 1982^[1]), or to the Paleognathae (based on a perceived similarity of the bony palate: Gingerich, 1973). These similarities, however, as the more recently determined fact that the osteons of their bones - at least in Hesperornis - were arranged in a pattern similar to that in Neognathae (Houde, 1987), are today considered to be due to convergent evolution.

Currently, the hesperornithiform birds are recognized as a very specialized lineage that is not ancestral or otherwise closely related to the birds of today. Still, their relationship is close enough to make Hesperornis regalis is the first species of essentially moden bird discovered, and they possibly diverged from the ancestors of today's birds as late as the Earliest Cretaceous. On the other hand, given the great number of convergent traits already discovered in hesperornithforms, it is not inconceivable that they represent a wholly independent avian lineage.

The discovery that Gansus yumenensis, which lived some time before the main radiation of the Hesperornithiformes, was a primitive member of the Ornithurae - that is, birds which had developed the type of pygostyle that is also found in modern birds - suggests that the ancestors of the Hesperornithes - also Ornithurae - may have been birds generally similar to Gansus and living some 130-150 mya. Given the tendency of birds that have become flightless to increase in size markedly in just a few million years, the probability of the Hesperornithes deriving from an animal at least superficially similar and not too far related to Gansus is quite high. The new fossil material of that species should assist in determining the degree of synapomorphy of the Hesperornithes and other Ornithurae, which have until now been muddied by the strong autapomorphy of the former.

Loss and/or fusion of caudal vertebrae in pygostyle-like structures was a general trend in Cretaceous avian evolution, and a full pygostyle and associated structures may have evolved more than once to similar shapes. The fact that Gansus had non-pneumatized dense bones as the Hesperornithes, although it was not a specialized diver, is interesting to note.

Similarly, the bone structure of Hesperornis indicates that as opposed to Enantiornithes and in line with other Ornithurae it showed rapid, uninterrupted growth to adult size (Chinsamy et al. 1998).

The earliest known hesperornithiform is the Early Cretaceous Enaliornis although these birds are somewhat tentatively assigned to this group because of the bad preservation of their remains; the majority of hesperornithiform taxa are known from the Late Cretaceous. Hesperornis has been found in Late Cretaceous marine chalks from Kansas and in marine shales from Canada. Small hesperornithiform bones are known from the freshwater deposits of the Late Cretaceous of the Judith River Group as well as the Hell Creek and Lance Formations. These birds were about the size of a cormorant or a loon.

The following classification is a synthesis of Marsh (1880), Feduccia (1996), Tokaryk, Cumbaa & Storer (1997), and Galton & Martin (2002) (via Haaramo, 2005), as well as Clarke (2004) and Sereno (2005).

Hesperornis regalis reconstruction by Marsh. Though a popular depiction, the bird is now known to have been completely unable to assume such a posture. See above for correct position of the legs

• Class AVES
  • Subclass HESPERORNITHES
    • Order HESPERORNITHIFORMES
      • Basal and unresolved taxa
        • Genus Asiahesperornis (Late Cretaceous of Kazakhstan) - hesperornithid?
        • Genus Judinornis (Nemegt Late Cretaceous of S Mongolia)
        • Genus Potamornis (Lance Creek Late Cretaceous of Buck Creek, USA) - hesperornithid?
        • Genus Pasquiaornis
        • Hesperornithiformes gen. et sp. indet. (Late Cretaceous of Kushmurun, Kazakhstan)
        • Hesperornithiformes gen. et sp. indet. TMP 89.81.12 (Dinosaur Park Late Cretaceous of Iddesleigh, Canada)
      • Family Enaliornithidae
        • Genus Enaliornis (Early Cretaceous of Cambridge, England)
      • Family Baptornithidae
        • Genus Baptornis (Late Cretaceous of C North America)
      • Family Hesperornithidae
        • Genus Hesperornis (Late Cretaceous of C North America)
        • Genus Parahesperornis (Late Cretaceous of C North America)
        • Genus Coniornis
        • Genus Canadaga

Sometimes assigned to the Hesperornithes, but actual relationship unknown

• Genus Neogaeornis
• Genus Parascaniornis

[edit] References

• Chinsamy, A.; Martin, Larry D. & Dobson, P. (1998): Bone microstructure of the diving Hesperornis and the volant Ichthyornis from the Niobrara Chalk of western Kansas. Cretaceous Research 19(2): 225-235. DOI:10.1006/cres.1997.0102 (HTML abstract)

• Clarke, J. A. (2004): Morphology, Phylogenetic Taxonomy, and Systematics of Ichthyornis and Apatornis (Avialae: Ornithurae). Bulletin of the American Museum of Natural History 286: 1-179 PDF fulltext

• Cracraft, Joel (1982): Phylogenetic relationships and monophyly of loons, grebes, and hesperornithiform birds, with comments on the early history of birds. Systematic Zoology 31: 35-56. DOI:10.2307/2413412 (HTML abstract)

• Feduccia, A. (1996): The origin and evolution of birds. Yale University Press, New Haven and London.

• Fürbringer, M. (1888): Untersuchungen zur Morphologie und Systematik der Vögel (2 vols). Von Holkema, Amsterdam.

• Galton, P. M. & Martin, L. D. (2002): Enaliornis, an Early Cretaceous Hesperornithiform Bird from England, with Comments on Other Hesperornithiformes. 317-338. In: Chiappe, L. M. & Witmer, L. M. (eds.): Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press, Berkeley, Los Angeles, London.

• Gingerich, P. D. (1973): Skull of Hesperornis and the early evolution of birds. Nature 243: 70-73. DOI:10.1038/243070a0 (HTML abstract)

• Gregory, Joseph T. (1952): The Jaws of the Cretaceous Toothed Birds, Ichthyornis and Hesperornis. Condor 54(2): 73-88. PDF fulltext

• Haaramo, Mikko (2005): Mikko's Phylogeny Archive: †Hesperornithiformes. Version of 2005-11-15. Retrieved 2006-09-21.

• Houde, Peter (1987): Histological Evidence for the Systematic Position of Hesperornis (Odontornithes: Hesperornithiformes). Auk 104(1): 125-129 PDF fulltext

• Marsh, Othniel Charles (1880): Odontornithes, a Monograph on the Extinct Toothed Birds of North America. Government Printing Office, Washington DC.

• Sereno, P. C. (2005): Hesperornithes. In: TaxonSearch: Stem Archosauria. Version 1.0, 2005 November 7. Retrieved 2006-09-21.

• Tokaryk, T. T.; Cumbaa, S. L. & Storer, J. E. (1997): Early Late Cretaceous birds from the Saskatchewan, Canada: The oldest diverse avifauna known from North America. Journal of Vertebrate Paleontology: 17(1): 172-176. HTML abstract

[edit] Footnotes

1. ^ An interesting example of how to arrive at an essentially phenetic - and utterly incorrect - analysis in spite of using cladistic methods.

[edit] External links

• Introduction to the Hesperornithiformes

• Hesperornis regalis Marsh 1872 Toothed marine birds of the Late Cretaceous seas by Mike Everhart, Oceans of Kansas website. Retrieved 2006-09-16.