Encyclopedia of biodiversity encyclopedia of biodiversity, (7 volume set) ( PDFDrive ) 1997

414 Extinction in the Fossil Record We could calculate the number of taxa that become extinct or the percent of the former pool of taxa that became extinct Time could be measured in years, but often we only have segments of relative geological time units such as geological stages (A set of stages comprises a geological series, and a set of series comprises a geological period (e.g., Cambrian, Cretaceous).) In many parts of the fossil record, the absolute time represented by a stage is not accurately known, and different geological stages are often of great difference in temporal extent Charles Lyell developed an ingenious technique to estimate extinction rate by charting the gradual diminution of living species as one went back in geological time This type of analysis can give longevities and extinction rates Such Lyellian curves demonstrate, for example, that the diminution of bivalve species on the Pacific coast of the USA is at a steady pace whereas a more precipitous extinction occurred in the Atlantic Problems in Measuring Extinction Rates Taxon-Level Bias We typically think of extinction rate as a measure of the loss of species To create a database for paleontology, the species level is very difficult to trust with any degree of confidence; most paleontologists tend to trust the genus and higher taxonomic levels in identifications In recent years, more and more effort has been directed toward accounting for the ranges of all named species in the fossil record, but most analyses have been done at the family or genus level The large-scale database we now employ owes its existence to the dedicated work of David Raup and especially the late Jack Sepkoski, who continuously sought to produce a more complete database of the geological ranges of all fossil groups Initially, the compilation was at the level of taxonomic order but subsequent analyses have moved down the taxonomic hierarchy to the family and generic levels Can extinctions of higher-level taxa be used to estimate species-level extinctions? To estimate species richness using numbers of higher-level taxa (e.g., orders), we assume that taxonomic diversity at higher taxonomic levels is correlated with species richness, but they are not necessarily correlated in this manner This can be seen clearly where changes in ratios of one taxonomic level to another occur over broad spans of time If the ratios change, then higher taxonomic units might be flawed estimators of changes in species diversity For example, the ratio of taxonomic orders to families decreased significantly from the Paleozoic to the Mesozoic Era Over short periods of time, the number of taxa at a higher taxonomic level (e.g., level of family) might have a regular relationship with a lower taxonomic level, such as species To estimate species-level extinction from family-level extinction, David M Raup used a rarefaction technique based on the sampling curve that relates the number of species collected at random to the number of families recovered This method has been modified and refined to be used in many estimates of standing fossil diversity The rarefaction approach is the best we have so far Nevertheless, we must be careful in applying it The biggest problem is the potential change in the relationship over geological time For example, the ratio of families to species decreases by a factor of two from the Mesozoic to the Cenozoic, and other cases are known of changing ratios of species to genera Selective extinction can also bias our conclusions For example, certain families may be much more prone to extinction, due to their presence in a particularly vulnerable habitat (e.g., coral reefs during a cooling event) This might overestimate total extinction, if these are added to a larger species list It is also difficult to get sufficient data to calculate good rarefaction curves for all but the most abundant fossil groups Raw Data for Analyses Any set of diversity data involves a count of the number of taxa But this number is strongly modulated by the intensity of sampling Thus diversity could be inflated simply because more studies have been done in a given locality at a given time interval Thus raw data must be corrected for sampling intensity In most instances, the genus is the level used for diversity estimates, so the number of genera must be estimated by some sort of sampling process that corrects for the total number of studies, the number of localities, and the number of fossils that are collected at any one locality and time horizon Two examples of these potential biases are the possible monograph effect of studying intensively one part of geological time and a bias that inflates the diversity of parts of the geological record just before the present, because species are still alive and can be sampled and studied more completely than their antecedents Biased Preservation and Convergence Estimates of extinction rates may be biased by preservation and abundance at the time of extinction Preservation of appropriate habitats during an extinction may be greatly reduced Thus a species might have survived, but there are no opportunities to see it because its usual facies of occurrence has not been preserved A common change in probability of preservation occurs when a systematic change in rock preservation occurs, as in the reduction of deposition during a regression phase of the sea, as at the end of the Permian and just before the end of the Cretaceous (regression refers to a lowering of sea level in a given area; transgression refers to a rise in sea level) Suppose the ranges of a group of species all ended at the very terminus of the Cretaceous A gradual reduction of deposition would, by sampling error alone, give the impression of a gradual disappearance of the fossil species Even if deposition does not decline, previously rarer species would be difficult to sample for presence during a general decline in abundance during extinction, just because we would be unlikely to find them These biases have come to be known as the Signor–Lipps effect, or ‘‘backward smearing,’’ because a sharp extinction might appear to be gradual from fossil sampling Only abundant forms would be sufficiently ‘‘findable’’ that we could assess their total geological range with confidence, especially up to the time of their extinction Even at smaller levels of geological resolution, the problem of a spurious component of estimated diversity due to the preservation of different amounts of rock of a given age is a major source of bias The problem is compounded by cases where some environmental factors