mark the geological record beginning 34-million-years ago exemplify these points. This crisis is especially amenable to study because it happened relatively recently. The stratigraphic record is of high-quality, and the well-established pattern of magnetic reversals provides an excellent temporal framework. Fossil records of planktonic foraminifera, calcareous nannoplankton, and terrestrial mammals show that the crisis was protracted, suggesting multiple pulses of extinction. As we have seen, changes in terrestrial paleofloras indicate that climates cooled in many areas; shifts in oxygen isotopes of foraminiferan shells reveal that water masses underwent major changes; and plate reconstructions suggest that the Antarctic cooling system for the deep-sea originated at this time. Nonetheless, the ultimate cause of this crisis remains controversial.

Pulses of extinction, when they have removed large numbers of species by imposing unusual and lethal conditions, can reset biological systems in ways that may have had nothing to do with the victim's ability to adapt successfully to ordinary conditions. This circumstance is epitomized by the extinction of the dinosaurs, which emptied ecospace to the benefit of the mammals. In recent years, dinosaurs have been recast as active, ecologically adept creatures—not backward lumbering forms that were inherently inferior to mammals.

A continuing question about global biotic crises in general is whether, through geological time, they form the tip of one tail in a unimodal distribution of extinction rates or whether they represent a statistical outlier; a sparse data base currently frustrates valid statistical testing. The second condition would imply unique causation rather than simply an accentuation of normal agents of extinction. On the other hand, periodic spacing of mass extinctions may suggest a highly abnormal cause. The possible regularity of these crises stimulates current debates.

At the other end of the distribution of extinction rates lies another question: When the record shows extinctions occurring at minimal rates, do species still disappear in groups or do they die off independently in piecemeal fashion? The former condition would require an expansion of catastrophic ideology to embrace even relatively calm intervals of biotic history. Finer resolution of the stratigraphic record may solve this basic problem of normal, or background, extinction levels.

Macroevolutionary Trends

Macroevolution transcends species boundaries, involving changes in the more generalized taxonomic levels, such as genus or family. Macroevolutionary changes include those trends in the marine realm that have been driven by the expansion of sophisticated predators on the seafloor over the past 100-million-years. Phyletic evolution can produce macroevolutionary trends, as can differences in rates of extinction and speciation among groups of species. An important question here relates to the relative importance of phyletic evolution in producing trends in the history of life. If it has been of secondary importance, as asserted in the punctuational model, differential rates of extinction and speciation gain importance. A further question concerning phyletic evolution addresses the degree to which it has been gradual and the degree to which it has followed a stepwise course, in which established species have undergone most of their changes during very brief intervals of geological time.

Trends can be documented only on the basis of careful taxonomic and biostratigraphic studies, and they can be interpreted only by considering the functional morphology and ecology of component species.

Origins of Major Biological Categories

Fossil data provide estimates of the times when higher biological groups evolved. For groups with excellent fossil records, early estimates remain unchanged even though large volumes of new data have accumulated. For those with poor fossil records, the estimates are vulnerable to new discoveries. Thus, although Archaeopteryx is remarkable for its intermediate morphological position between dinosaurs and birds, the early fossil record of birds is so poor that a recent claim for a much earlier bird cannot be rejected out of hand. The fossil record offers a unique opportunity to assess the origins of taxonomic groups at general levels—the origins of kingdoms in Precambrian time and then of phyla.

Times of origin for the more general groups, which are estimated from molecular data, do not always agree with those from fossil data. Molecular data clearly have great potential for the assessment of relative times of origin, but only well-dated fossils will estimate actual rates of divergence between forms. The environmental sites of major evolutionary breakthroughs are difficult to predict. These events may be concentrated in relatively unstable habitats, such as those of high latitudes or nearshore marine habitats, or in more stable habitats, such as those of the tropics or offshore marine habitats at middle or low latitudes. Hostile habitats generally offer more vacant ecospace, but more

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