Summary: The Cretaceous Interior Seaway of western North America was a great abode for life, both in the waters and on the adjacent tropical or subtropical lands. You remember the chalk cliffs of Dover, England – the chalk that gave the name to the Cretaceous, which means “chalk-bearing”? The North American interior seaway also contained a lot of those microscopic coccolithophores whose shells accumulated to make chalk. Probably the most famous example is the Niobrara Chalk, a prominent part of the Cretaceous stratigraphic section especially in Nebraska and Kansas which were under the waters of the seaway. Most of the fossils from the Niobrara are, as you’d expect, marine fossils – fish, turtles, plesiosaurs. But it also contains occasional land-dwelling dinosaurs, whose bodies must have been washed offshore, and there are winged vertebrates as well. Pteranodons, winged reptiles that are not dinosaurs, flew in the late Cretaceous skies above and near the western interior seaway and what is now the Gulf Coastal Plain around 85 million years ago. Despite their relatively short existence, probably not much more than two or three million years, they are well known because more than 1,200 specimens have been found, the most of any pterosaur. “Pteranodon” means “winged, toothless,” and they had typical wingspans of 6 meters, about 20 feet. Pteranodons shared the skies with birds, which were becoming much more diverse during the Cretaceous. Ichthyornis was a small toothed bird that probably occupied ecological niches similar to gulls. Many specimens of Ichthyornis have been found in the Niobrara Chalk. Its time range is about 95 to 85 million years ago, and it is interpreted to be of a bird lineage that was near that of modern birds, but it was probably not a direct ancestor to them. Its name means “fish-bird,” and it’s among the best-represented birds in the fossil record – but “well represented” probably means a few hundred specimens, not thousands. Hesperornis fossil (Smithsonian) photo by Quadell, used under Creative Commons license. Hesperornis, whose name means “western bird,” was a flightless bird – based on its size, its wings were useless for flight. It was most likely a penguin-like, or more accurately loon-like, fully aquatic bird, demonstrating that birds had evolved and adapted to the marine realm as early as 84 million years ago. Hesperornis was pretty big, about 6 feet or almost 2 meters long. Its fossils have been found in Russia and Europe as well as many places in the western North American Interior Seaway, so as a group it clearly had a wide range. Both Ichthyornis and Hesperornis were extinct well before the end of the Cretaceous, at about 85 and 78 million years ago, respectively. —Richard I. Gibson Ichthyornis Hesperornis Hesperornis fossil (Smithsonian) photo by Quadell, used under Creative Commons license.
