Extinctions of Life Precambrian Paleozoic V Vendian O Ordovician S Silurian D Devonian
Total Page：16
File Type：pdf, Size：1020Kb
Extinctions of Life Precambrian Paleozoic V Vendian O Ordovician S Silurian D Devonian t is a delight to be here and to talk know something about extinction—how were thought to have all of the charac- about extinctions of life, although it operates and what results it produces. teristics that an extinct group of animals some of you might find that title ought to have, and their disappearance incongruous. We usually use the Courtesy Department Library Services, American seemed perfectly understandable. That I Museum of Natural History. Neg./Trans. No. V/C2827 word life to refer to the collective prop- of course led to the use of the epithet erties of living organisms. So extinc- dinosaur for anything that is beyond its tion of life suggests perhaps annihilation time and ought to be gone. I hope my of all life. However, the study of ex- students never refer to me as a dinosaur. tinctions is in its infancy, and in new Many of the old ideas about dinosaurs fields, where there is much more igno- have changed radically through research rance than understanding, we often use of the last few decades. We now know order-of-magnitude estimates, ballpark that not all dinosaurs were large, al- guesses, and first approximations. Given though the average size was fairly great. that, the title is okay, since, to a first Some dinosaurs were the size of birds, approximation, life is extinct. Proba- and, in fact. some dinosaurs were the bly more than 99 percent of all species ancestors of birds. (Some people make that have ever lived on this planet have the statement that dinosaurs are not disappeared. The richness of the biota extinct; they have simply taken to the around us reflects only a slight excess of trees. ) We know from their morphology speciation over extinction. that some dinosaurs were very active Despite its magnitude and its appar- and were probably not cold-blooded. ent importance in the evolution of life, They may have been as homeothermic we know very, very little about what as you and 1 are. From studies of track- extinction is, as either a phenomenon ways of dinosaurs as well as some of or a process. How does a particular their morphological features, people species become extinct? What array have argued that dinosaurs weren’t in- of processes are operative during an Tyrannosaurus rex credibly dumb animals. Some of them extinction? How frequently are extinc- traveled in organized herds and probably tions catastrophic? How can we predict hen we think about extinction, the showed some fairly complex behaviors. what species or what kinds of species wimage that immediately comes to Finally, we know that dinosaurs were will become extinct in a given situation? mind is the dinosaur. Dinosaurs have the dominant large animals on land for And, how can we manage the biota to been known for well over a century about 150 million years, twice the span control extinction in the present and fu- now. the first fossils having been rec- during which mammals have held that ture world? These are some questions ognized in the 1820s. The early con- position. Dinosaurs arose in the late Tri- that we are not sure how to answer. ceptions about dinosaurs were that they assic. at about the same time that mam- But they are certainly of vital contem- were a strange group of animals. They mals appeared. They then dominated porary importance. As more and more were very large animals, thought per- the large-animal adaptive zone until they of the earth’s surface is altered and re- haps to be too big for terrestrial ecosys- became extinct rather rapidly at the end engineered, we are facing unprecedented tems, They were thought to be cold- of the Cretaceous. levels of extinction, unprecedented at blooded, like most modern reptiles, and The research of the last few decades least in historical time. And as we face therefore too slow. They were thought turned the disappearance of this very the possibility of nuclear winter, we to have too small brains and therefore symbol of extinction into very much need to know what that might do to the to be too dumb. In a nutshell, dinosaurs of an enigma. Many speculations were biota. Finally, from the standpoint of pure science, we want to understand how extinction has influenced the his- Era Precambrian Paleozoic Mesozoic Cenozoic tory of life on this planet and perhaps the evolution of living systems else- 4600670 570 505 438 408 360 286 248 213 144 65 2 where in the universe. So we need to Millions of Years before the Present 38 Los Alamos Science Fellows Coloquium 1988 Extinctions of Life Mesozoic Cenozoic C Carboniferous P Permian J Jurassic K Cretaceus T Tertiary IRIDIUM-RICH DEPOSIT AT CRETACEOUS-TERTIARY BOUNDARY Fig. 1. Close-up of the iridium-rich clay layer at the boundary between Cretaceus and Tertiary rocks in a stratigraphic section near Gubbio, Italy. The high iridium content of the clay (see Fig. 2) is attributed to the impact with the earth of an extraterrestrial body. Since discovery of the Gubbio anomaly in 1978, deposits similarly rich in iridium have been found at Cretaceous- Tertiary boundaries worldwide. (Photo cour- tesy of Alessandro Montanari, Department of Geology and Geophysics, University of Cali- THE ALVAREZ IRIDIUM ANOMALY Fig. 2. A plot, versus height above or be- low the Cretaceous-Tertiary boundary, of irid- published on what circumstances might the earth. Various scenarios, which dif- ium abundance in various stratigraphic sec- have caused dinosaurs to become ex- fer quantitatively but agree qualitatively, tions from the vicinity of Gubbio, Italy. The tinct, but none seemed very satisfying, suggest that huge amounts of dust were abundance rises abruptly at the end of the at least not until a discovery by Luis thrown into the atmosphere, blocking Cretaceus to a value some twenty-five times and Walter Alvarez in 1979. out sunlight for perhaps three months. greater than the background level and then Most of you are probably familiar The impact may have first heated the falls back to that level within approximately with that discovery. Walter Alvarez atmosphere and then cooled it. It may 15,000 years. (Figure adapted from “Current was looking at some stratigraphic sec- have produced large amounts of nitro- status of the impact theory for the terminal tions, near Gubbio in central Italy, that gen oxides, which would rain down as Cretaceus extinction” by Walter Alvarez, Luis span the Cretaceous-Tertiary bound- nitric acid. The list of damages can go W. Alvarez, Frank Asaro, and Helen V. Michel. ary. He saw a peculiar clay layer, I to on and on. 2 centimeters thick, sandwiched between older Cretaceus rocks and younger Ter- tiary rocks (Fig. 1). Walter was curious about the clay and sent it back [o his father for analysis. Luis, Frank Asaro, 6 and Helen Michel performed a number of geochemical analyses of the clay and 5 found that it contained an excess of irid- ium (Fig. 2). The excess was far too 4 large to explain on the basis of terres- trial surface sources, which are highly 3 depleted in iridium. They hypothesized 2 that the excess iridium was due to the impact of a large—perhaps 10 kilome- 1 ters in diameter-xtraterrestrial object Tertiary on the last day of the Cretaceus. I I I I I I I I I I 5 4 3 2 Now an impact by a 10-kilometer- 10 10 10 10 10 0 10 20 102 10’ 104 diameter object would wreak havoc on Height below or above Cretaceous-Tertiary Boundary (centimeters) Los Alamos Science Fellows Colloquium 1988 39 Extinctions of Life Precambrian Paleozoic V Vendian O Ordovician S Silurian D Devonian Various climatic and chemical mod- riod. So whatever happened was indeed missing. The focus is not on individu- els suggest that the earth wouldn’t have quite devastating to the marine biota. als but on some higher group—families, been a very pleasant place on the last perhaps, or tribes. night of the Cretaceus, that is, the ow do we know what became ex- Like historical census data, the fossil three-month night that followed the im- Htinct? How do we make quantita- record is incomplete, covering only a pact. Photosynthesis by green plants tive estimates of the magnitudes of mass small sample of the earth’s biota. Still, would have been shut down, and large extinctions? Paleontologists use two ba- it contains a huge number of species herbivores that fed upon them would sic methods to study mass extinctions from all parts of the world—too much have starved, as would the carnivores and other events in geologic history. data to assess well. We therefore usu- that stalked the plant eaters. The ex- The traditional method is to collect in- ally work at higher taxonomic levels, pected result would be extinctions. We formation about the types and numbers such as the genus or the family. We don’t have an equation relating the of fossils in the various strata of out- lose resolution doing that but sometimes species that would become extinct to crops or core samples and then to deter- get a better overall picture, because a the size of the impacting body. The one mine the times when the various fossil genus, say, is included in our data set thing we do know is that if things got as taxa first appeared, flourished, and then even if all but one of its species are bad as the models predict, more kinds disappeared. Such data are then used missing from the fossil record.