Ward's World Activity Guides

Paleontology (continued) or phylogeny. Several different approaches to classification have been used by paleontologists. Traditional classifications and most current ones use a modified Linnean classification, a hier- archy of nested categories beginning with the species, defined qualitatively based on overall similarities of body (or skeletal) morphology. Species in turn are grouped into related groups, genera; genera into families; families into orders; orders into classes; classes into phyla; and phyla into kingdoms. In addition, cladistics or phylogenetic systematics attempts to group taxa into dichotomous trees based upon the shared possession of unique features. Such classifications take into account a number of characters to arrive at the most parsimonious or simplest trees of relationships between taxa and their sister taxa. Evolutionary paleontology Increasingly, systematic paleontologists are interfacing their studies of phylogeny based on the morphology of ancient or- ganisms with supplemental techniques, based on the biochem- istry of living organisms. For example, biologists have long examined the early development of organisms or embryology for clues as to deep relationships among groups of animals. Bi- ologists have developed techniques to determine relationships among various living taxa based on similarities of portions of their DNA or complex molecules (for example, proteins and hemoglobin). Measures of molecular distance between any two living species can give a sense of how closely related the two are, independently of their external form or morphology. Using these techniques, together with cladistics, paleobiologists and molecular biologists have come to some radical interpretations of the tree of life. Moreover, molecular biologists have found evidence that the base pairs in some strands of DNA undergo mutations in a stochastic manner, somewhat akin to radioac- tive decay. On this basis, they postulate that changes in DNA structure occur in a regular clocklike, time-governed manner. If so, the total amount of difference between the DNA of any two related species or other taxa can be used to calculate the amount of time that has elapsed since the two taxa diverged from a common ancestor. Thus, this molecular-clock technique provides a method, independent of paleontology, for deter- mining the timing of evolutionary branching. Biostratigraphy A major task of any historical science, such as geology, is to arrange events in a time sequence and to describe them as fully as possible. Arguably the most applied field of paleontol- ogy and its traditional linkage with geology is in the field of biostratigraphy. This field employs a principle established in the early 1800s by British engineer William Smith—the principle of "faunal and floral succession," that is, suites of particular, unique fossil species are typical of successive time intervals. Geology, the study of Earth history, did not become a modern science until the nineteenth century, when a worldwide timescale based on fossils was established; Earth history could not be deciphered until events that occurred in different places were related to one another by their position in a standard time sequence (Fig. 3). Because of evolution, there is no recurrence of species in later rocks; hence, fossils provide an arrow of time and are unambiguous markers of increments of relative time known as biozones. Certain fossils are more useful in this pursuit than others, and these form biostratigraphic guide or index fossils; Fig. 3: Biostratigraphy uses fossils to identify the age of particular stratigraphic levels. Note that certain types of fossils are characteristic of given levels that are assumed to be age-equivalent. (Credit: Adapted from an original drawing by S. M. Stanley) + ward ' s science

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