of the Niobrara, and that its habitat did not extend farther north than South Dakota. Some very fragmentary fossils belonging to pteranodontian pterosaurs, and possibly Pteranodon itself, have also been found on the Gulf Coast and East Coast of the United States. For example, some bone fragments from the Mooreville Formation of Alabama and the Merchantville Formation of Delaware may have come from Pteranodon, though they are too incomplete to make a definite identification. Some remains from Japan have also been tentatively attributed to Pteranodon, but their distance from its known Western Interior Seaway habitat makes this identification unlikely. Pteranodon longiceps would have shared the sky with the giant-crested pterosaur Nyctosaurus. Compared to P. longiceps, which was a very common species, Nyctosaurus was rare, making up only 3% of pterosaur fossils from the formation. Also less common was the early toothed bird, Ichthyornis. It is likely that, as in other polygynous animals (in which males compete for association with harems of females), Pteranodon lived primarily on offshore rookeries, where they could nest away from land-based predators and feed far from shore; most Pteranodon fossils are found in locations which at the time, were hundreds of kilometres from the coastline. Below the surface, the sea was populated primarily by invertebrates such as ammonites and squid. Vertebrate life, apart from basal fish, included sea turtles, such as Toxochelys, the plesiosaur Styxosaurus, and the flightless diving bird Parahesperornis. Mosasaurs were the most common marine reptiles, with genera including Clidastes and Tylosaurus. At least some of these marine reptiles are known to have fed on Pteranodon. Barnum Brown, in 1904, reported plesiosaur stomach contents containing "pterodactyl" bones, most likely from Pteranodon. Fossils from terrestrial dinosaurs also have been found in the Niobrara Chalk, suggesting that animals who died on shore must have been washed out to sea (one specimen of a hadrosaur appears to have been scavenged by a shark). Classification Timespan and evolution Pteranodon fossils are known primarily from the Niobrara Formation of the central United States. Broadly defined, Pteranodon existed for more than four million years, during the late Coniacian to late Maastrichtian stages of the Cretaceous period. The genus is present in most layers of the Niobrara Formation except for the upper two; in 2003, Kenneth Carpenter surveyed the distribution and dating of fossils in this formation, demonstrating that Pteranodon sternbergi existed there from 88 to 85 million years ago, while