Geographical distributions of terrestrial or aquatic taxa that are broken up by oceans can be explained by either oceanic dispersal or vicariance in the form of fragmentation of a previously contiguous landmass. Oceanic dispersal may be the most important factor in the distributions of animals. Present distribution patterns of animals are largely a result of shifting landmasses. Distribution of animals is limited by various environmental conditions which the most restrictive barriers are the separation of the land by water, high mountain ranges and deserts. Extinction is a gradual process driven by the interactions among species and between species and their physical environment. Mass extinctions affected most different taxonomic groups present at the time such as invertebrates and vertebrates including birds, mammals, reptiles, amphibians and fish. Over 99% of species that ever lived are now extinct, but extinction occurs at an uneven rate. Major extinction events include Ordovician-Silurian extinction events, Late Devonian extinction events, Permian-Triassic extinction events, Triassic-Jurassic extinction events and Cretaceous-Tertiary extinction events.