422 Extinction in the Fossil Record are therefore not surprising from this point of view Area reduction itself might not be a potent agent of extinction Sealevel drops would hardly affect the shallow-water habitat distribution of oceanic islands, where most modern families are widely distributed Sea-level drop may just be a correlate of another change The changing spatial relationships generated by continental drift and sea level fluctuations must have had important influences on climate James Valentine’s theory of climate change generated by continental assembly and fragmentation attempted to relate climate and sea level to sea-floor spreading Periods of continental assembly were envisioned as times when interior continental climates were severe, affecting the continental shelf faunas In contrast, times of fragmentation were times when the continents’ climate was more moderate due to ameliorating marine conditions; this permitted the buildup of shallow-water diversity Although the post-Permian expansion may fit this pattern, evidence from the Paleozoic does not seem to show an increase in continental fragmentation during the Early-mid-Paleozoic Indeed, the continents were maximally fragmented and arrayed along the equator during the Cambrian Continental drift and arrangement nevertheless have had profound effects on climate and probably extinction During the Ordovician and Silurian Periods, Gondwana drifted southward from its Cambrian position at the equator, and came to rest on the geographic south pole This coincides with the Late Ordovician glacial tillites that have been found in North Africa, and a large reduction in the degree of marine provinciality relative to the Early Ordovician In the Cenozoic, the spatial arrangements of the continents about the Pacific and Atlantic Ocean made for a quite different climatic history The North Atlantic was a more enclosed basin and was far more severely affected by the Late Cenozoic polar cooling The Pleistocene initiated severe enough climates to cause a major molluskan extinction in the southeastern US Shelf, whereas Pacific American faunas showed no increased extinction The effects of increasing access between biogeographic realms can be illustrated by the large-scale interchange of mammals between North and South America after the Pliocene establishment of the Isthmus of Panama, following the disappearance of the Bolivar Trough marine barrier Before the interchange, there was long-term stability in a number of mammalian families As a probable result of North America’s initial higher taxon richness, more taxa moved from north to south than in the reverse direction In South America, where taxon richness now exceeded previous ‘‘steady state’’ levels by more than 50 percent, there was about a 70 percent increase in extinction rates Descendants of the North American invaders participated in an evolutionary radiation, resulting ultimately in an overall richness higher than previous levels Mammalian diversity is now higher in South America, in contrast to the situation previous to the exchange This suggests that area does have an effect on regulating diversity, but evolutionary changes can impose a significant overprint on diversity Periodicity in Extinction, or Just Ups and Downs? Periodicity of extinction or climatic change predicted by astronomical or geophysical theories would be the most convincing way to establish a terrestrial or extraterrestrial cause of extinction If extinctions are measurably periodic, it may be that only one credible cyclic theory would fit in the available pattern The precedent for such an approach lies with the longstanding theories of the periodicity of Pleistocene glaciations The Yugoslav astronomer Milankovitch theorized that Pleistocene glacial advances and retreats might be regulated by changes in high-latitude insolation, caused by cyclic changes in the earth’s orbital eccentricity, tilt, and time of perihelion A power spectrum analysis of temporal changes of abundance of Pleistocene planktonic fossils in oceanic cores corresponded well to climate changes estimated by stable oxygen isotopes and to periodicity peaks predicted by the Milankovitch theory A number of studies in recent years have taken up this theme and related these cycles to sedimentary cycles, including some of the classic mid-continent alternations of carbonate and mudstone Many of these cycles occurred during times when there was no significant amount of continental glaciation, and represent transgressive–regressive cycles (rises and falls of sea level) For example, sedimentary cycles in the lacustrine Early Mesozoic supergroup correspond to periodicities of approximately 25,000; 44,000; 100,000; 130,000; and 400,000 years These periodicities, in turn, correspond to those expected from celestial processes, such as the precession of the equinoxes, the obliquity cycle, and the eccentricity cycle Cyclic processes such as the precession of the equinoxes may have driven continental heating cycles that rearranged wind and climate Milankovitch climatic rhythms also appear in MidCretaceous black shale sedimentary cycles These cycles consist of alternations of carbonate and shale, with intervals of highly oxidized (red) and highly reduced (black) strata They are particularly interesting, as they occur in marine sequences and must have reflected periods of ocean bottom anoxia, alternating with vigorous bottom mixing and high productivity in the water column On an even smaller scale, El Nin˜o-La Nin˜a cycles and the North Atlantic Oscillation whose forcing mechanisms of periodicity are not well understood, are known to cause cycles of benthic abundance in coastal communities and in small bays and fjords, such as the Swedish Gullmar Fjord The earth’s history has been dominated by large-scale changes in climate, arrangement of continents, volcanism, and sea level Alfred G Fischer developed a theory connecting physical conditions with the overall pattern of Phanerozoic life Global sea level was relatively high in both the MidPaleozoic and Mesozoic Periods of continental breakup, when dispersed and thinner continents resulted in smaller ocean basins, would be associated with higher sea levels Periods of continental aggregation, when continental crust was bunched up due to collisions and ocean basins were therefore more commodious, which resulted in lower stands of sea level The temporal variation in granite emplacement matches the sea-level curve This suggests a causal link between active continental fragmentation, volcanism, and sea level, an environmental condition of obvious importance to the world marine biota Fischer, a pioneer in global climate thinking with regard to the evolution and extinction of life, speculated on the presence