Geologic Time, History of Biodiversity in close of Early Cenozoic time, all of the present continents that are now separated had broken from each other Species that could once range widely across Pangaea became isolated on separate continents As the continents dispersed into different climatic zones and developed different topographies, their terrestrial biotas each evolved distinctively in response to local conditions and events For example, North America, Eurasia, and Australia each developed distinctive temperate biotas, and the tropics of the Americas became distinct from the Old World tropics and those of the western Pacific In the sea, the growing geographic differentiation was even more profound, as new shelves appeared on each side of the new oceans, significantly raising the number of isolated shelf segments As on land, each continent evolved a distinctive biota in response to the unique set of environmental conditions and events that developed in each region The heterogeneity of world environments thus increased greatly during this period, on a global scale These plate tectonic effects are responsible for a significant part of the important rise in standing global marine diversities during the Mesozoic and Cenozoic as represented at the family level in Figure 4; an accompanying rise occurred in the continental biota Climate Change Climate is another major factor governing global biodiversity For example, earth underwent a general cooling trend during the Cenozoic, although there were shorter warming and cooling events superimposed on the generally falling temperature curve The temperature change was greatest in high latitudes, thus increasing the temperature differential between the poles and the equator The margins of the tropics were shifted toward the equator, the tropics became compressed, and cooler climates came into existence in high latitudes, shifting equatorward themselves as cooling continued, to be replaced poleward by still cooler climatic regimes In the sea, the increasing temperature gradient made for a markedly increased provincialism Tropical species whose biology was related to climate became progressively more restricted to lower latitudes, with some occasional, relatively short-lived reversals in which species tolerant of lower temperatures arose and diversified in the more poleward regions Because the major rifting that produced the Cenozoic continental pattern was primarily in a north–south direction, most major continental shelves trend north–south in the Cenozoic, as they today, and so there was a rise in species richness on each shelf as the latitudinal thermal gradient increased, multiplying the effect on global marine biodiversity by the number of isolated shelves The Cenozoic global marine rise in family richness shown in Figure is compounded partly of the increase in the number of isolated shelves following continental rifting and partly of the increased provincialism on each shelf created by an increasing thermal gradient; together they have formed a powerful engine of global species enrichment In terrestrial environments, family richness also increased during those Cenozoic events, and for the same reasons The continental expanses provided for a three-dimensional array of habitats (including high mountains and basins) unlike the 689 rather two-dimensional array along the narrow, shallow continental shelves, and therefore the biotic response has been correspondingly more complicated in terrestrial than in marine environments Not only did high-latitude climate zones open up, but high-altitude regions became cooler, increasing the contrasts between mountain and lowland environments A lowering of rainfall in some continental interiors produced semiarid plains and steppes The biota responded to these increases in environmental heterogeneity by producing, for example, alpine and grassland plants and animals, thus enriching both the flora and fauna The species richness of angiosperms among plants and of insects among animals profited the most from these events During the Late Cenozoic, as cooling has continued in high latitudes, massive ice accumulations on a subcontinental scale have produced a series of glacial ages interrupted by warmer interglacial periods, most pronounced during the Pleistocene, which began about 1.6 million years ago Some regional extinctions occurred during the early onset of cold periods, but in general there has been little extinction, and thus no significant lowering of global biodiversity, associated with these climatic swings Instead, the biotas of both land and sea have tended to migrate with their climatic zones, southward during glaciations and northward during interglacial times Thus regional diversities have changed as species have migrated, but overall diversity has not been significantly affected Indeed, the Recent biosphere may be the richest in species during the entire history of animals and plants, at least prior to the deleterious influences of human activities on biodiversity Extinction Events Changing environments have often provided opportunities for the origin of species, some of which have clearly led to major morphological novelties and enhanced morphological disparity At the same time, changing conditions have led to the extinction of species, which has occurred more or less continually over Phanerozoic time As noted earlier, extinction rates vary among taxa, whose histories tend to reflect their rates of turnover The species richness of a taxon results from an interaction between its rates of speciation and extinction Thus when there has been some unusually profound disturbance to the global environment, and extinction rates have become unusually high, species richness has fallen dramatically in events called mass extinctions The most drastic such event, recorded in marine fossils, was the Permian–Triassic (P/T) extinction about 250 million years ago, at the close of the Paleozoic era As many as 90% of marine species may have become extinct across the P/T boundary (see Figures and 6) This extinction was highly differential; the groups that did best in surviving the extinction and in rediversifying were those that dominate the marine fauna today Disparity as well as richness was affected; some entire classes and orders of marine invertebrates disappeared Brachiopods, crinoids, and other groups that were dominant before the extinction were reduced to minor roles The causes of this extinction, and the extent of its effects on the terrestrial biota, are not yet clear