Ward's World Activity Guides

Paleontology (continued) An evolutionist has two major interests—first, to know how the process of evolution works, which is accomplished largely by studying the genetics and population structure of modern organisms; second, to reconstruct the events produced by this process, that is, to trace the history of life. This is the paleontol- ogist's exclusive domain. Any modern animal group is merely a stage, frozen at one moment in time, of a dynamic, evolv- ing lineage. Fossils give the only direct evidence of previous stages in these lineages. Horses and rhinoceroses, for example, are very different animals today, but the fossil history of both groups is traced to a single ancestral species that lived early in the Cenozoic Era. From such evidence, a tree of life can be constructed whereby the relationships among organisms can be understood. Life properties Evolution is responsible for life's outstanding properties, including its diversity and its adaptation to the environment. Diversity A paleontologist studies diversity. Then, the number and geographic distribution of species in a more inclusive taxo- nomic group, such as vertebrates, can be tabulated for differ- ent periods of geologic history. When this is done for all major groups, very definite patterns emerge. Life history shows both periods of rapid diversification—evolutionary radiations—and brief intervals of rapid diversity decline—mass extinctions. These are major events in the history of life, and their causes have provided a major research agenda for paleontology. Evolutionary radiations may have resulted from new adaptive breakthroughs (for example, the evolution of legs and lungs in lobe-finned fishes), environmental triggers (for example, increased levels of atmospheric oxygen), or the availability of adaptive "niche space" (such as occurred in the Cambrian or fol- lowing mass extinctions). In contrast, at the close of the Perm- ian and Cretaceous periods, for example, extensive extinction of species occurred in many major groups at about the same time (Fig. 6). Long-term rates of evolution The fossil record provides information on the often linked rates of evolution and extinction of various large groups of organisms. Vertebrate paleontologist George Gaylord Simpson determined the average rates of taxonomic turnover of many different taxonomic groups of animals and found that various taxa have characteristic rates of change and that certain groups evolve much more rapidly than others. Rapidly evolving taxa include mammals and ammonites; conversely, other groups such as horseshoe crabs and lingulid brachiopods evolve slowly and provide excellent examples of living fossils, that is, extant organisms that have changed very little in millions of years. Many slowly evolving groups have very generalized modes of life or live in protected environments, but just why different groups show such varying rates of change is one of the great mysteries of paleontology Large-scale patterns and processes Fossils also provide very important insights into large-scale evolutionary patterns that cannot be studied with modern organisms. New species may arise by one of two processes: + ward ' s science Fig. 6: Patterns of diversity of families of marine organisms through the Phanerozoic Eon (the last 541 million years). Note the major drops in diversity at the end of the Ordovician, Late Devonian, Permian/Triassic, Late Triassic, and Cretaceous that correspond to the major mass extinctions of Phanerozoic life history. The individual diagrams show patterns for the Cambrian fauna dominated by trilobites; the Paleozoic fauna dominated by brachiopods, bryozoans, crinoids, and rugose and tabulate corals; and the Modern fauna dominated by mollusks and crustaceans. (Credit: Adapted from an original drawing by D. R. Prothero and R. H. Dott)

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