The changing spatial relationships generated by continental drift and sea level fluctuations must have had important in-fluences on climate.. James Valentine’s theory of climate change g
Trang 1are therefore not surprising from this point of view Area
re-duction itself might not be a potent agent of extinction
Sea-level 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
in-fluences on climate James Valentine’s theory of climate change
generated by continental assembly and fragmentation
at-tempted 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
contin-ental 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
max-imally 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
provinci-ality 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,
fol-lowing the disappearance of the Bolivar Trough marine
bar-rier 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 regu-lated 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 glaci-ation, and represent transgressive–regressive cycles (rises and falls of sea level) For example, sedimentary cycles in the la-custrine Early Mesozoic supergroup correspond to periodi-cities 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 Mid-Cretaceous 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, alter-nating 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 com-munities 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 Mid-Paleozoic 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
422 Extinction in the Fossil Record