Ichthyornithes
Toothed avialans close to the ancestry of modern birds.
Ichthyornithes were a group of toothed, bird-like dinosaurs that went extinct at the end of the Cretaceous period, alongside all non-avian dinosaurs and many other creatures. They are considered very close relatives of the common ancestor of all modern birds. Their fossils have been found mainly in North America, spanning much of the Late Cretaceous, and several genera—including *Ichthyornis*, *Janavis*, *Iaceornis*, and *Guildavis*—are known from relatively complete remains.
The earliest known ichthyornitheans appear in the fossil record around 95 million years ago. Fragmentary fossils from the Ashville Formation in Saskatchewan represent two unnamed species that closely resembled *Ichthyornis dispar*. That species itself had an unusually long existence, with specimens attributed to it or very similar forms persisting for nearly 10 million years with little change, aside from some variation in average adult size. After that, *I. dispar* was replaced by other, still similar, ichthyornithean species. This long period of stability suggests their evolution was in relative stasis compared to other contemporary avialans, such as the hesperornitheans. Both groups were strongly tied to the Western Interior Seaway that split North America during the Cretaceous. It may be that the flightless, aquatic hesperornitheans were more sensitive to changes in sea level and shoreline, driving them into more specialized forms, while the flying, tern-like ichthyornitheans were less dependent on specific coastal conditions and could occupy the same ecological niche for a very long time. The last definitive ichthyornithean fossils date to the late Campanian or early Maastrichtian, with no well-established record within 300,000 years of the Cretaceous–Paleogene boundary.
A study on an *Ichthyornis* brain endocast found it had a relatively primitive brain, similar to that of *Archaeopteryx* and other non-avian theropods, but its palate was remarkably convergent with that of modern neognath birds. Another study noted striking similarities between the jaw structure of ichthyornitheans and pelagornithids, though it still classified pelagornithids as neognaths based on post-cranial features and placed them in a polytomy with Galloanserae and Neoaves, noting the pelagornithid palate is unknown.
Quick Facts
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Facts from the source article.
Lore & Background
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 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 form for millions of years suggests a stable ecological niche. However, contrary to earlier claims, the last definitive ichthyornithean fossils are from the late Campanian or early Maastrichtian, not from within 300,000 years of the K-Pg boundary. This means their extinction likely occurred earlier than previously thought, though the exact timing remains uncertain.
Reader's Guide
Ichthyornithes holds a key position in the evolutionary history of birds, representing a lineage of toothed avialans that diverged just before the origin of modern birds. Their fossils, primarily from the Western Interior Seaway of North America, show a remarkable evolutionary stasis over tens of millions of years, contrasting with the more rapid changes seen in contemporary hesperornitheans. This stability suggests that as flying, tern-like animals, ichthyornitheans occupied a broad ecological niche less affected by sea-level fluctuations. The group's extinction at the Cretaceous–Paleogene boundary, alongside all non-avian dinosaurs, underscores the severity of that event. Only two genera, Ichthyornis and Janavis, are well enough known to be named, but fragmentary remains from Canada, the United States, and Russia indicate a wider northern hemisphere distribution. Studies of an Ichthyornis endocast reveal a relatively primitive brain similar to that of Archaeopteryx, while its palate shows remarkable convergence with modern neognaths. The classification of ichthyornitheans has been refined through phylogenetic analyses, with the clade defined as all descendants of the most recent common ancestor of Ichthyornis dispar and modern birds. Their study continues to illuminate the transition from toothed, dinosaur-like ancestors to the toothless, modern bird radiation.
Did You Know?
- Only two genera of ichthyornitheans, Ichthyornis and Janavis, are represented by complete enough fossils to have been named.
- Ichthyornitheans appear to have existed virtually unchanged in size and anatomy for nearly the entire duration of the late Cretaceous period.
- The last ichthyornithean fossils are found in the Hell Creek Formation within 300,000 years of the K-Pg boundary.
More in Late Cretaceous dinosaurs 1-24
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