Ichthyornithes is an extinct group of toothed avialan dinosaurs very closely related to the common ancestor of all modern birds. They are known from fossil remains found throughout the late Cretaceous period of North America, though only two genera, Ichthyornis and Janavis, are represented by complete enough fossils to have been named. Ichthyornitheans became extinct at the Cretaceous–Paleogene boundary, along with enantiornitheans, all other non-avian dinosaurs, and many other animal and plant groups.
Ichthyornithes is an extinct group of toothed avialan dinosaurs very closely related to the common ancestor of all modern birds. They are known from fossil remains found throughout the late Cretaceous period of North America, though only two genera, Ichthyornis and Janavis, are represented by complete enough fossils to have been named. Ichthyornitheans became extinct at the Cretaceous–Paleogene boundary, along with enantiornitheans, all other non-avian dinosaurs, and many other animal and plant groups.
==Origin and evolution== The earliest known ichthyornitheans appear in the fossil record about 95 million years ago, during the Cenomanian age of the Late Cretaceous. Based on fragmentary fossil remains, the two known species present in the Ashville Formation have not been given names, but overall were very similar to Ichthyornis dispar. I. dispar itself had a very long temporal range, and specimens referred to it or very similar species existed relatively unchanged (other than some fluctuations in average adult size) for nearly 10 million years, when species referable to I. dispar disappeared and were replaced by other, though still somewhat similar, species of ichthyornitheans. The fact that ichthyornitheans appear to have existed virtually unchanged in their general size and anatomy for nearly the entire duration of the late Cretaceous period suggests that their evolutionary pace was in relative stasis compared to other, contemporary species of closely related avialans, like the hesperornitheans. Both the ichthyornitheans and hesperornitheans were strongly linked to their ecosystem in the Western Interior Seaway that bisected North America for much of the Cretaceous. It may be that while the flightless, aquatic hesperornitheans were much more sensitive to changes in shoreline extent and sea level during that time, driving their evolution and adaptation into more specialized forms, the flying, tern-like ichthyornitheans were not as dependent on specific coastal or sea-level conditions, and they were able to inhabit basically the same ecological niche for a very long period. The icthyornithean lineage finally became extinct in the Cretaceous–Paleogene extinction event that caused the extinction of many major land and marine animal groups at the end of the Mesozoic era. The last ichthyornithean fossils are found in the Hell Creek Formation very close (within 300,000 years at least) of the K-Pg boundary, dated to 66 million years old.
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