Ichthyornis (meaning "fish bird", after its fish-like vertebrae) is an extinct genus of toothy seabird-like ornithuran from the Late Cretaceous of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansas (Greenhorn Limestone), New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway, spanning from the Cenomanian to the Campanian ages of the Late Cretaceous. Ichthyornis is a common component of the Niobrara Formation fauna, and numerous specimens have been found.

Ichthyornis has been historically important in shedding light on bird evolution. It was the first known prehistoric bird relative preserved with teeth, and Charles Darwin noted its significance during the early years of the theory of evolution. Ichthyornis remains important today as it is one of the few Mesozoic era ornithurans known from more than a few specimens.

Description

thumb|left|Relative size of two I. dispar specimens: YPM 1742, blue, and YPM 1450, green, compared with a human. Both specimens were adult, but YPM 1450 is approximately one million years older.

It is thought that Ichthyornis was the Cretaceous ecological equivalent of modern seabirds such as gulls, petrels, and skimmers. An average specimen was the size of a pigeon, long, with a skeletal wingspan (not taking feathers into account) of around ,

The wings and breastbone were very modern in appearance, suggesting strong flight ability and placing it with modern birds in the advanced group Carinatae. Unlike earlier avialans such as the enantiornithines, the species appears to have matured to adulthood in a rather short, continuous process.

Timespan and evolution

thumb|right|Restoration of I. dispar

Ichthyornis fossils have been found in almost all levels of the Niobrara Chalk, from beds dating to the late Coniacian age (about 89 million years ago) to the Campanian age (about 83.5 million years ago). There are fossils from the later Pierre Shale. Specimens of Ichthyornis from earlier eras were, on average, smaller than later ones. The holotype specimen of Ichthyornis dispar, YPM 1450, had a humerus about long. In many geologically younger specimens like YPM 1742, the same wing bone was long. Both the older, smaller specimens, and the more recent, larger specimens show signs that they had reached skeletal maturity and were adults, and came from the same geographic area. It is likely that Ichthyornis dispar as a species increased in size over the several million years it inhabited the Western Interior Seaway ecosystem.

Soon after these discoveries, Ichthyornis was recognized for its significance to the theory of evolution recently published by Charles Darwin. Darwin himself told Marsh in an 1880 letter that Ichthyornis and Hesperornis offered "the best support for the theory of evolution" since he had first published On the Origin of Species in 1859. Others at the time also recognized the implications of a nearly modern bird with reptilian teeth, and feared the controversy it caused. One Yale student described various men and women urging Marsh to conceal Ichthyornis from the public because it lent too much support to evolutionary theory. All other supposed species of Ichthyornis have not been supported as valid. The presumed "Ichthyornis" lentos, for example, actually belongs into the early galliform genus Austinornis.

References

Further reading

  • Asteriornis
  • Ichthyornis dispar: A toothed, flying bird from the Late Cretaceous of Kansas by Mike Everhart, Oceans of Kansas website. Retrieved 2006-09-16.