 "I. dispar" itself had a very long temporal range, and specimens referred to it or very similar species existed relatively unchanged (other than some fluctuations in average adult size) 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 size and anatomy for nearly the entire duration of the late Cretaceous period suggests that their evolutionary pace was in relative stasis compared to other, contemporary species of closely related avialans, like the hesperornitheans.Both the ichthyornitheans and hesperornitheans were strongly linked to their ecosystem in the Western Interior Seaway that bisected North America for much of the Cretaceous.