<P> The Western Interior Seaway was home to early birds also, including the flightless Hesperornis which had stout legs for swimming through water and tiny wings used for marine steering rather than flight; and the tern - like Ichthyornis, an early avian with a toothy beak . Icthyornis shared the sky with large pterosaurs such as Nyctosaurus and Pteranodon . Pteranodon fossils are very common and it was probably a major component of the surface ecosystem, though it was found in only the southern reaches of the Seaway . </P> <P> On the bottom, the giant clam Inoceramus left common fossilized shells in the Pierre Shale . This clam had a thick shell paved with "prisms" of calcite deposited perpendicular to the surface, giving it a pearly luster in life . Paleontologists suggest that its giant size was an adaptation for life in the murky bottom waters, where a correspondingly large gill area would have allowed the animal to cope with oxygen - depleted waters . </P>

What geological forces caused north america’s inland sea to disappear