 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.