Extinction in the Fossil Record Jeffrey S Levinton, State University of New York, Stony Brook, NY, USA r 2013 Elsevier Inc All rights reserved Glossary Background extinction A distinctly lower rate of extinction, more typical of most of the fossil records Extinction rate Number or proportion of taxa becoming extinct per unit time or after an important geological temporal boundary Introduction Why the Fossil Record? Many species are threatened by impending extinction and attempts have been made to assess population declines and to enact policies of recognizing endangerment by rules, such as the International Conservation Union’s rule of three successive years of 80 percent decline It is difficult to develop a measure of extinction rates of entire flora or fauna, only because we usually have scant knowledge of the species pool before the impact This is particularly a problem in speciesrich tropical habitats, where cryptic species abound, yet have not been identified completely Surveys of specially rich fauna in tropical wet forests are only being organized now, and the decline of the areal extent of these forests in recent years makes it nearly impossible to measure extinction rates, except by means of indirect estimates of species–area relationships The same applies to species-rich marine communities such as coral reefs Extinction is very much the domain of a paleontologist It is believed that we are now possibly living through a mass extinction caused by human disturbance of high diversity tropical habitats But very little is known about the extinction of species, except by fairly obvious mechanisms such as hunting Can knowing that the Dodo or the Passenger Pigeon was hunted to extinction help very much with understanding climatically induced changes in key structural groups such as forest trees and reef corals, and their dependent species? Could such spotty knowledge be used to extrapolate to the broad sweep of geological time? Paleontological data has the advantage of large banks of ‘‘before and after’’ data on biodiversity on time scales that can be related to global climate change Its weakness, however, is in associating extinctions with unique causes, as shall be seen Invasions have caused extensive extinctions on oceanic islands, particularly when alien predators overwhelmed small populations of endemic species in a matter of decades Extinction on larger time scales, even over hundreds of years, is much more difficult to track Unfortunately, the time scale for larger-scale changes over 100,000 years or more is probably unapproachable by the neontologists, who can only observe ‘‘normal’’ extinction, and don’t have much understanding of what occurs normally It may be that fine-scale studies of the fossil record may eventually give us more insight into Encyclopedia of Biodiversity, Volume Mass extinction Extinction occurring over a short period of time that is of large magnitude, wide biogeographic impact, and involves the extinction of many taxonomically and ecologically distant groups species-level extinction than neontological studies ever will After all, the durations of animal species life spans range from the order of 105 (land vertebrates) to 106–107 years (marine species) Even with millions of living species, we are not likely to be able to document many cases of typical extinction of living animal species The fossil record is probably our only hope of a model for study of extinction rates, especially on the scale of ocean basins and continents Although its coverage of the total potential living biota is incomplete, the fossil record affords us a more complete glimpse of extinction rates of a number of readily fossilizable groups, both marine and terrestrial We have a reasonably complete database that has stabilized over the years and one can readily trace extinctions across geological time horizons A great advance is the Paleobiology Data Base (http://paleodb.org), which records as of the year 2011 nearly a million fossil occurrences with a wide variety of synthesis of data The fossil record moreover gives us a deeper insight into what extinction really means After all, we would like to produce a prospectus of the biological future of living communities following an extinction Does the loss of a species have a disproportional importance, resulting in the extinction of many associated species? Following an extinction event, is there enough redundancy that allows the surviving species to evolve a new diverse fauna? Does the extinction of certain species cause the snowballing of a larger extinction event? With some judicious reasoning we can infer the answers to some of these questions with the use of the fossil record Measures and Types of Extinction Rates Many extinctions in the fossil record appear to be precipitous, and occur over short time period of hundreds of thousands to a few million years Impacts of extraterrestrial objects may have caused changes in a year or less Such length of time is short when you consider the length of the record of the Phanerozoic Era (542 My) It is possible to quantify the extent of the extinction with the following data: The total pool of taxa before the extinction; the number of taxa that became extinct; the time span over which the extinction occurs http://dx.doi.org/10.1016/B978-0-12-384719-5.00051-4 413