Extinctions of Life · Extinctions of Life Precambrian Paleozoic V Vendian O OrdovicianS Silurian D...

Extinctions of Life

Precambrian Paleozoic

V Vendian O Ordovician S Silurian D Devonian

It is a delight to be here and to talkabout extinctions of life, althoughsome of you might find that titleincongruous. We usually use the

word life to refer to the collective prop-erties of living organisms. So extinc-tion of life suggests perhaps annihilationof all life. However, the study of ex-tinctions is in its infancy, and in newfields, where there is much more igno-rance than understanding, we often useorder-of-magnitude estimates, ballparkguesses, and first approximations. Giventhat, the title is okay, since, to a firstapproximation, life is extinct. Proba-bly more than 99 percent of all speciesthat have ever lived on this planet havedisappeared. The richness of the biotaaround us reflects only a slight excess ofspeciation over extinction.

Despite its magnitude and its appar-ent importance in the evolution of life,we know very, very little about whatextinction is, as either a phenomenonor a process. How does a particularspecies become extinct? What arrayof processes are operative during anextinction? How frequently are extinc-tions catastrophic? How can we predictwhat species or what kinds of specieswill become extinct in a given situation?And, how can we manage the biota tocontrol extinction in the present and fu-ture world? These are some questionsthat we are not sure how to answer.But they are certainly of vital contem-porary importance. As more and moreof the earth’s surface is altered and re-engineered, we are facing unprecedentedlevels of extinction, unprecedented atleast in historical time. And as we facethe possibility of nuclear winter, weneed to know what that might do to thebiota. Finally, from the standpoint ofpure science, we want to understandhow extinction has influenced the his-tory of life on this planet and perhaps

know something about extinction—howit operates and what results it produces.

Courtesy Department Library Services, AmericanMuseum of Natural History. Neg./Trans. No. V/C2827

Tyrannosaurus rex

when we think about extinction, theimage that immediately comes to

mind is the dinosaur. Dinosaurs havebeen known for well over a centurynow. the first fossils having been rec-ognized in the 1820s. The early con-ceptions about dinosaurs were that theywere a strange group of animals. Theywere very large animals, thought per-haps to be too big for terrestrial ecosys-tems, They were thought to be cold-blooded, like most modern reptiles, andtherefore too slow. They were thoughtto have too small brains and thereforeto be too dumb. In a nutshell, dinosaurs

were thought to have all of the charac-teristics that an extinct group of animalsought to have, and their disappearanceseemed perfectly understandable. Thatof course led to the use of the epithetdinosaur for anything that is beyond itstime and ought to be gone. I hope mystudents never refer to me as a dinosaur.

Many of the old ideas about dinosaurshave changed radically through researchof the last few decades. We now knowthat not all dinosaurs were large, al-though the average size was fairly great.Some dinosaurs were the size of birds,and, in fact. some dinosaurs were theancestors of birds. (Some people makethe statement that dinosaurs are notextinct; they have simply taken to thetrees. ) We know from their morphologythat some dinosaurs were very activeand were probably not cold-blooded.They may have been as homeothermicas you and 1 are. From studies of track-ways of dinosaurs as well as some oftheir morphological features, peoplehave argued that dinosaurs weren’t in-credibly dumb animals. Some of themtraveled in organized herds and probablyshowed some fairly complex behaviors.

Finally, we know that dinosaurs werethe dominant large animals on land forabout 150 million years, twice the spanduring which mammals have held thatposition. Dinosaurs arose in the late Tri-assic. at about the same time that mam-mals appeared. They then dominatedthe large-animal adaptive zone until theybecame extinct rather rapidly at the endof the Cretaceous.

The research of the last few decadesturned the disappearance of this verysymbol of extinction into very muchof an enigma. Many speculations were

Era Precambrian Paleozoic Mesozoic Cenozoic

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 CenozoicC Carboniferous P Permian J Jurassic K Cretaceus T Tertiary

published on what circumstances mighthave caused dinosaurs to become ex-tinct, but none seemed very satisfying,at least not until a discovery by Luisand Walter Alvarez in 1979.

Most of you are probably familiarwith that discovery. Walter Alvarezwas looking at some stratigraphic sec-tions, near Gubbio in central Italy, thatspan the Cretaceous-Tertiary bound-ary. He saw a peculiar clay layer, I to2 centimeters thick, sandwiched betweenolder Cretaceus rocks and younger Ter-tiary rocks (Fig. 1). Walter was curiousabout the clay and sent it back [o hisfather for analysis. Luis, Frank Asaro,and Helen Michel performed a numberof geochemical analyses of the clay andfound that it contained an excess of irid-ium (Fig. 2). The excess was far toolarge to explain on the basis of terres-trial surface sources, which are highlydepleted in iridium. They hypothesizedthat the excess iridium was due to theimpact of a large—perhaps 10 kilome-ters in diameter-xtraterrestrial objecton the last day of the Cretaceus.

Now an impact by a 10-kilometer-diameter object would wreak havoc on

Los Alamos Science Fellows Colloquium 1988

the earth. Various scenarios, which dif-fer quantitatively but agree qualitatively,suggest that huge amounts of dust werethrown into the atmosphere, blockingout sunlight for perhaps three months.The impact may have first heated theatmosphere and then cooled it. It mayhave produced large amounts of nitro-gen oxides, which would rain down asnitric acid. The list of damages can goon and on.

IRIDIUM-RICH DEPOSIT ATCRETACEOUS-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-

ium abundance in various stratigraphic sec-

tions from the vicinity of Gubbio, Italy. The

abundance rises abruptly at the end of the

Cretaceus to a value some twenty-five times

greater than the background level and then

falls back to that level within approximately

15,000 years. (Figure adapted from “Current

status of the impact theory for the terminal

Cretaceus extinction” by Walter Alvarez, Luis

W. Alvarez, Frank Asaro, and Helen V. Michel.

6

5

4

3

2

1Tertiary

I I I I I I I I I I10 5 10 4 10 3 10 2 10 0 10 20 102 10’ 104

Height below or above Cretaceous-Tertiary Boundary (centimeters)

39

Extinctions of Life



Various climatic and chemical mod-els suggest that the earth wouldn’t havebeen a very pleasant place on the lastnight of the Cretaceus, that is, thethree-month night that followed the im-pact. Photosynthesis by green plantswould have been shut down, and largeherbivores that fed upon them wouldhave starved, as would the carnivoresthat stalked the plant eaters. The ex-pected result would be extinctions. Wedon’t have an equation relating thespecies that would become extinct tothe size of the impacting body. The onething we do know is that if things got asbad as the models predict, more kindsof animals than just dinosaurs shouldhave become extinct. And indeed thatis what the fossil record shows. Theflying reptiles, which had a long his-tory in the Mesozoic, vanished at theend of the Cretaceus. In the oceans thelarge marine reptiles, such as the ple-siosaurs, disappeared. So did a largenumber of marine invertebrates, includ-ing the ammonites (well-known ma-rine fossils of the Mesozoic), almost allof the belemnites, and a large varietyof clams, snails, crabs, bryozoans, andbrachiopods.

Thus a whole suite of organisms be-came extinct at the same time that thedinosaurs did. From the fossil recordwe can estimate that about 45 percent ofmarine animal genera became extinct atthe end of the Cretaceus. Extrapolatingdown to the species level leads to esti-mates that 60 to 75 percent of marinespecies became extinct in the last 2 mil-lion years or less of the Cretaceus pe-

Belemnite

riod. So whatever happened was indeedquite devastating to the marine biota.

How do we know what became ex-tinct? How do we make quantita-

tive estimates of the magnitudes of massextinctions? Paleontologists use two ba-sic methods to study mass extinctionsand other events in geologic history.The traditional method is to collect in-formation about the types and numbersof fossils in the various strata of out-crops or core samples and then to deter-mine the times when the various fossiltaxa first appeared, flourished, and thendisappeared. Such data are then usedto assess patterns of origination and ex-tinction and perhaps to test hypothesesconcerning those phenomena.

This “normal” methodology givesmany details about extinction, such asthe abundance of an organism before itsdisappearance and the time scale of itsdisappearance. But usually such dataare available only for a single group—asingle order or class or even phylum—in a rather local region of the earth.And amassing the data is very labor-intensive. Despite a century and a halfof work by paleontologists worldwide,we still have detailed data on patternsof extinction for only a small number oflocalities, a small number of’ time inter-vals, and a small number of taxa.

To sidestep the gaps in the detailedpaleontological data—and to supple-ment them—a second way of studyingmass extinctions has been developed.This second way has been the subject ofmy work. Rather than studying detailedinformation on local patterns of extinc-tion over relatively short time intervals,I am trying to discern global patternsover longer time intervals. My approachis analogous to deducing the popula-tion demographics of ancient peoplesfrom the spotty records available. Whatrecords have been unearthed are assem-bled and correlated, as well as possibleconsidering the many records that are

missing. The focus is not on individu-als but on some higher group—families,perhaps, or tribes.

Like historical census data, the fossilrecord is incomplete, covering only asmall sample of the earth’s biota. Still,it contains a huge number of speciesfrom all parts of the world—too muchdata to assess well. We therefore usu-ally work at higher taxonomic levels,such as the genus or the family. Welose resolution doing that but sometimesget a better overall picture, because agenus, say, is included in our data seteven if all but one of its species aremissing from the fossil record.

I have attempted to obtain data on allanimals but have concentrated most ofmy attention on marine organisms. Thereason for doing so is that, although ter-restrial organisms, such as dinosaurs,flying reptiles, and giant mammals, arecertainly more spectacular, our fos-sil record for them is far poorer thanthat for marine organisms. After all,land is an area of net erosion, as youcan certainly see in the environs ofLos Alamos. The oceans are areas ofnet sedimentation. They end up with alarger and more complete fossil recordthat, for various historic and economicreasons, has been far better explored andfar better studied.

The detailed data collected by pa-leontologists are usually presented as“biostratigraphic range charts.” Figure 3is an example showing data for the oc-currence of trilobite genera in MiddleCambrian strata in western North Amer-ica. Note that even this study dealt notwith species but with genera. Note alsothat the geologic zones are not plottedaccording to scale. That is, the time in-terval spanned by each zone is not thesame, although each is allotted an equalspace on the chart. We don’t have goodestimates of the duration of those geo-logic time intervals since our methodsfor determining time in the Cambrianare not accurate enough. Furthermore,

40 Los Alamos Science Fellows Colloquium 1988

Extinctions of Life

Mesozoic Cenozoic

C Carboniferous P Permian J Jurassic K Cretaceus T Tertiary

geologic time intervals usually can beaccurately characterized only over localareas.

Putting together data for all fossil ma-rine taxa from all over the world, wecome up with something like a smalltelephone book. Figure 4 is a pagefrom such a compilation giving firstand last appearances in the fossil recordfor Cambrian and Ordovician trilobitefamilies. The data set I have assembledcovers about 3500 marine families andabout 30,000 marine genera.

To develop some picture of extinctionpatterns from such a data set, the sim-plest thing to do is to count the numberof families or genera that are presentin each time interval. In the case offamilies, 77 standard geologic time in-tervals compose the last 600 million

LOS Alamos Science Fellows Colloquium 1988

BIOSTRATIGRAPHIC RANGECHART

Fig. 3. This chart presents paleontologic data

for the time ranges of trilobite genera through

the stratigraphic zones of the Middle Cambrian

period in western North America. Dashed lines

indicate lack of field data. (Figure adaptedfrom The Cambrian System in the Southern

Canadian Rocky Mountains, Alberta and Brit-

ish Columbia (Second international Sympo-

sium on the Cambrian System, Guidebook for

Field Trip 2), compiled by James D. Aiken,

edited by Michael E. Taylor, 31. Denver, Col-

orado: U.S. Geological Survey, International

Union of Geological Sciences, Geological Sur-

vey of Canada, 1981.) ➤

EXTINCTION DATA FORTRILOBITE FAMILIES

Fig. 4. A page from a summary of data on the

appearance and disappearance worldwide ofmarine families. The data shown are those

for trilobites. The abbreviations in parenthe-

ses denote subdivisions of the Cambrian and

Ordovician geologic periods. (Figure adapted

from A Compendium of Fossil Marine Fami-

lies by J. John Sepkoski, Jr. Milwaukee Public

Museum Contributions in Biology and Geology

Number 51. Milwaukee, Wisconsin: Milwaukee

Public Museum Press, 1982.)<

Class Trilobita

Order Agnostida (= Miomera)

Agnostidae

Clavagnostidae

Condylopygidae

Diplagnostidae

Discagnostidae

Eodiscidae

Pagetiidae

Phalacromidae

Sphaeragnostidae

Trinodidae

Order Redlichiida

Abadiellidae

Bathynotidae

Chengkouiidae

Daguinaspididae

Despujolsiidae

Dolerolenidae

Ellipsocephalidae

Emuellidae

Gigantopygidae

Hicksiidae

Kueichowiidae

Longduiidae

Mayiellidae

Neoredlichiidae

Olenellidae

Paradoxididae

Protolenidae

Redlichiidae

Saukiandidae

Yinitidae

Yunnanocephalidae

41

Extinctions of Life



900

600 400 200 0Geologic Time (millions of years)

years, which is often referred to as thePhanerozoic, the eras of geologic timefor which evidence of animal life on theearth is abundant. For genera the database I have is a little better, composedof about 100 time intervals (attained bycarefully subdividing some of the longerstandard intervals).

Figure 5 is a plot of the number ofmarine animal families versus time in-terval. The big mass extinctions showup as large and rapid drops in the num-ber of families. As you can see, the ter-minal Cretaceus, or Maestrichtian, ex-tinction, the one that led to the demiseof the dinosaurs on land, was fairlyrapid but not excessively large. About17 percent of marine animal familiesdisappeared in that time interval. Be-cause the disappearance of a family re-quires the disappearance of every generaand species within the family, a familykill of about 17 percent corresponds toa genus kill of about 45 percent and aspecies kill of around 60 to 75 percent.

80

60

40

20

0

Fig. 5. This history of marine animal diver-

sity reveals five principal mass extinctions, ofwhich the upper Permian, or Guadalupian, was

by far the most devastating. Lesser extinction

events are also visible. (Figure adapted from

“Mass extinctions in the Phanerozoic oceans:

A review” by J. John Sepkoski, Jr. In Silver

EXTINCTION RATE HISTORY FORMARINE GENERA

Fig. 6. This history of extinction rates shows

more clearly than the diversity curve (Fig. 5)

the many extinction events experienced by

marine fauna. (Figure adapted from “Phanero-

zoic overview of mass extinction” by J. J. Sep-

koski, Jr. In Patterns and Processes in the

History of Life (Report of the Dahlem Work-

shop on Patterns and Processes in the His-

tory of Life, Berlin 1985, June18-21), edited by

D. M. Raup and D. Jablonski, 277-295. Berlin:

The terminal Cretaceus event cer- when about 55 percent of marine fam-tainly isn’t the only large mass extinc- ilies became extinct. Virtually everytion we see in Fig. 5. And it certainly order and class of marine organisms lostisn’t the largest. The largest was the an extensive number of families. GoingGuadalupian at the end of the Permian, through the same sort of extrapolation,


Extinctions of Life

Mesozoic Cenozoic


we find that about 80 percent of marinegenera and perhaps more than 95 per-cent of marine species disappeared atthe end of the Permian period. Othermajor events visible in Fig. 5 includeone at the end of the Ordovician, whichis probably the second largest extinc-tion of marine animal fauna. But it isnot that much larger than the one at theend of the Cretaceus. Another extinc-tion occurred in the late Devonian, andanother in the late Triassic, right on thetail of the Guadalupian extinction.

In addition to the large mass extinc-tions, many smaller extinction eventshave occurred-in fact, around twodozen. Simple diversity data don’t re-veal the smaller extinctions, but othermetrics of extinction intensity do.

Figure 6 shows one such metric, aplot of the extinction rate for marinegenera in each of the hundred or sosampling intervals spanning the Phaner-ozoic. Most of the spikes, or local max-ima, correspond to extinction events.The larger spikes—the Maestrichtian,the Norian, the Guadalupian, the Fras-nian, and the Ashgillian—are the samemajor mass extinctions that we seein the familial diversity data (Fig. 5).Many of the other spikes have been rec-ognized by paleontologists in detailedfield data on localized regions and re-stricted groups of organisms.

The data of Fig. 6, particularly whendisplayed as in Fig. 7, reveal a very

interesting feature of extinctions—a re-markable regularity in their timing dur-ing the past 300 million years. Thatobservation was first made by Al Fisherin the late seventies and was then redis-covered by my colleague David Raupand me about five years ago when wewere looking at the family data.

Figure 8 is another attempt to por-tray the regularity. Here I have sim-ply assigned a cycle number to extinc-tion events during the last 250 mil-lion years and plotted the cycle num-

TIMING OF MARINE GENERA EXTINCTIONS

500 400 300 200 100 0Geologic Time (millions of years)

Fig. 7. At Ieast during the most recent 300 mil-

lion years of geologic time, extinctions have

occurred with considerable regularity, as this

display of the data of Fig. 6 reveals. The

lengths of the arrows indicate the magnitudes

of the extinction rates. (Figure adapted from

“Phanerozoic overview of mass extinction” by

bers against the estimated times of theevents. Note the good fit of the datapoints to a straight line, which indicatesa constant, or stationary, periodicity.Dave Raup and I have performed a va-riety of analyses and have found thatthe probability of such a periodic ex-tinction pattern occurring at random isextremely low. A stationary periodicitydescribes the extinction events far betterthan any sort of random or semi-randommodel we can conceive of. I am quiteconvinced that, at least over the last 250million years of the earth’s history, ex-tinctions have occurred with a stationaryperiodicity of 26 million years.

That observation, however, does notagree with traditional views of massextinctions, which implicitly assumethat each extinction event was producedindependently by some random envi-ronmental perturbation or perhaps bya random coincidence of several envi-ronmental variables. And, since eachextinction event was independent of

J. J. Sepkoski, Jr. In Patterns and Processes

in the History of Life (Report of the Dah\em

Workshop on Patterns and Processes in the

History of Life, Berlin 1985, June 16-21), edited

by D. M. Raup and D. Jablonski, 277-295.

Berlin: Springer-Verlag, 1986.)

the others, it therefore could be stud-ied independently. But if the extinctionevents recur regularly, they cannot beindependent of one another, at least notin terms of their timing. Perhaps we aredealing with a series of events causedby a single, ultimate forcing agent thathas clock-like behavior.

When Dave Raup and 1 published thatspeculation, we didn’t know what theagent was. However, one event, the ter-minal Cretaceus mass extinction, wasknown to be associated with the impactof a large extraterrestrial object withthe earth. If an impact caused one massextinction in the periodic sequence, per-haps impacts caused all the others aswell.

The idea that most mass extinctions,at least over the last 250 million years,are the result of impacts of one or moreextraterrestrial bodies leads of courseto the next question: What could bethe cause of regularly periodic impacts?Several hypotheses have been offered:

43

Extinctions of Life

Precambrian PaleozoicV Vendian O Ordovician S Silurian D Devonian

the best known is the Nemesis, or so-called death-star, hypothesis (Fig. 9),which was put forward independentlyby several groups. The idea is that thesun is not alone, that it is accompaniedby a small companion star in a highlyelliptic orbit with an orbital periodicityof 26 million years or so. That com-panion, Nemesis, is usually far from thesun, but during the small portion of itsperiod when it is passing through theOort Cloud, it scatters up to a billioncomets into the inner solar system. JackHills has calculated that, out of that bil-lion or so comets, perhaps an average ofabout two dozen of various masses hitthe earth, wreaking havoc and causingextinction of many species on land andin the ocean.

Several years ago we were all veryexcited about such ideas, but time has

REGULAR PERIODICITYOF MESOZOIC EXTINCTIONS

Fig. 8. The data points in this graph consist

of “cycle numbers” assigned to the Mesozoic

and Cenozoic extinction events and the times

of their occurrence. The good fit of the points

to a straight line indicates that the extinctions

are regularly periodic. (Figure adapted from

“Periodicity in marine extinction events” by

J. John Sepkoski, Jr., and David M. Raup. In

Dynamics of Extinction, edited by David K. El-

liott, 3-36. New York: John Wiley & Sons,

1986.)

THE NEMESIS HYPOTHESIS

Fig. 9. The Nemesis hypothesis has been pro-posed as an explanation for the apparent reg-ular periodicity of extinctions. According tothat hypothesis, Nemesis, a companion star

tempered our excitement somewhat.Some of the predictions of the modelsare now looking a little cloudy, if youwill permit me. Carl Orth, Frank Kyte,and others have failed to find iridiumor other geochemical anomalies appear-ing consistently with the various peri-odic extinctions. Although an iridiumanomaly and microtektites are associ-ated with the Eocene extinction event,the one that occurred about 26 millionyears after the end of the Cretaceus,there is no good evidence of impact sig-natures at many of the others. Also,

Nemesis has not yet been found, andthere are some unresolved theoreticalproblems with the death-star hypothe-sis, especially about the stability of thecompanion star’s orbit.

The only thing that I think has sur-vived is the regular periodicity of theextinctions. To me that still looks good,especially now that some of the gaps inthe periodic series have been filled in.

But some better data have also led tonew observations and new questions.

one of the more remarkable obser-vations that come from the latest

of the sun, scatters comets into the inner solarsystem when it passes through the Oort Cloudevery 26 million years. The impacts of a smallnumber of the scattered comets with the earthcause the observed extinctions.

generation of data is shown in Fig. 10,a plot of the per genus extinction rateper million years. That metric is essen-tially the probability of extinction pertime interval. Figure 10 seems to showa remarkable uniformity not only in thetiming but also in the magnitude of thesmaller extinction events. Within theresolution of the data, the smaller eventsare identical in amplitude. In addition tothe smaller, constant-amplitude events,we have a few outliers, particularly theMaestrichtian, Norian, and Guadalupianevents. Perhaps—and this is pure spec-ulation now—the impact or whateverit was that happened at the end of theCretaceus, say, was simply coincidentalwith a peak of extinction produced byan independent periodic forcing agent,and the combination of the two causedabsolute havoc. But if the impact hadoccurred in a trough between periodicevents, it would have caused a muchsmaller, aperiodic extinction event.

Figure 11 is a similar plot for the Pa-leozoic era. The extinction peaks in thePermian and Carboniferous periods stillgive an impression of some regularity intheir timing. There is a little more vari-


Extinctions of Life


T

COMPARISON OF 26,000,000-YEARPERIODICITY AND MESOZOICEXTINCTION PEAKS

Fig. 10. This superposition of a 26,000,000-

year periodicity on data for genus extinction

rates during the Mesozoic shows how closely

such a regular periodicity fits the extinction

peaks. Note also the similarity in magnitude

among most of the extinction rate peaks. (Fig-

ure adapted from Sepkoski 1986.)

200 1 0 0 0Geologic Time (millions of years)

PALEOZOIC EXTINCTION

500 400 300Geologic Time (millions of years)

ation in the timing, but then our abilityto estimate geologic time during that eraisn’t so good. However, our best esti-mates suggest that the spacing betweenthe Permian and Carboniferous events ison the order of 30 to 35 million years,somewhat longer than the 26-million-year spacing between the Mesozoicevents. Perhaps that indicates a vari-able periodicity. Back beyond the Car-boniferous the pattern seems to break


down into chaos. It is not clear whetherthe lack of pattern, or at least of peri-odic pattern, represents problems withthe fossil data or with our ability to tellgeologic time accurately. It is also pos-sible that the apparently chaotic patternreflects a combination of periodic andaperiodic events. And it is eminentlypossible that there is no periodicity atall in the Paleozoic.

PEAKS

Fig. 11. During the Permian and Carbonifer-ous periods of the Paleozoic era the peaks ofthe genus extinction rate history exhibit a fairlyregular periodicity but one closer to 30 to 35millon rather than 26 millon years. In con-trast, the earlier extinction peaks (during theDevonian, Silurian, Ordovician, and Cambrianperiods) seam to lack any periodicity. (Figureadapted from Sepkoski 1966.)

Despite the many unanswered ques-tions about extinction, one thing

is clear: Many extinction events haveoccurred, some of them rather large.And that fact raises a question that’snot easy to answer: What are the ef-fects of those frequent extinction eventson the course of evolution, on the his-tory of the earth’s biota? Our feelingis that the effects were more profoundthan the simple elimination of various

45

Extinctions of Life


taxa, such as the “outmoded” dinosaurs.Indeed, the extinction events may havehad some very constructive effects.

Looking back at Fig. 5, we see thatthe number of marine families risesrapidly to a sort of equilibrium duringthe Paleozoic era. That equilibrium ispunctuated by extinction events of var-ious amplitudes but the system seemsto rebound and to fill up again ratherquickly. Then the great Permian massextinction seems to destabilize the sys-tem, and the subsequent number of fam-ilies rises above the former equilibriumvalue. But, in fact, arguments can bemade that diversity was already increas-ing before that event, and what appearsto be a great increase in the number offamilies during the Mesozoic and earlyCenozoic eras is a combination of re-bound from the Permian event and anatural rise that would eventually havemoved asymptotically toward a greaterequilibrium value.

The reason the system fills up is thatthe whole-ocean ecosystem is finite interms of habitat space and other re-sources. Therefore it can hold only alimited number of kinds of animals.And the reason the system reboundsvery quickly after an extinction eventis that ecospace has been opened up,

which leads to a very rapid radiationinto specialized taxa. Even during thelong-term rise in diversity during theMesozoic and Cenozoic, we see rapidrebounds after the Norian and Maes-trichtian mass extinctions. So thoselarge mass extinctions are opening upecospace and promoting very rapid evo-lution in their wake.

Let’s look at evolutionary innova-tion, that is. at the appearance of newkinds of animals, in the marine sys-tem, We find that after the Ordovicianperiod nearly two-thirds of the newtaxonomic orders that appeared in theoceans originated during the reboundsthat followed extinction events. Thoserebounds, though, constitute only one-

third of the time span. So mass extinc-tions increase evolutionary innovationby a factor of about 2 and in that senseseem to be tilling a creative, construc-tive role. However, the best example ofthis by far is seen not in the oceans buton land.

Did you know that your ancestorswere vermin? During most of its his-tory, the mammal class consisted of tinyquivering vermin living in the intersticesof a dinosaurian world. Mammals havebeen pre-eminent only for the last 65million years, that is, only following therapid extinction of dinosaurs. Withinapproximately a dozen million yearsof the early Tertiary, virtually everymodern order of mammal—from miceto whales, from bats to elephants—appeared in terrestrial ecosystems. Itwas as if an inhibiting force on inno-vative mammalian evolution had beenlifted with elimination of the dinosaurs.

This constructive role of mass extinc-

tion might be absolutely necessary inthe earth’s evolutionary system and per-haps in evolutionary systems elsewherein the universe, as George Wald cer-tainly argued and I’m sure Frank Drakewill argue.

Another feature of extinction in gen-eral that increases the evolutionary im-portance of the large mass extinctionsis the following. In some of the graphsshown previously, you may have no-ticed a secular decline in the “back-ground” extinction rate through thePhanerozoic. (Background extinctionis total extinction minus that occurringduring the big mass extinctions.) Therates tend to be very high early in theCambrian and decline through the laterPhanerozoic. Figure 12 shows how asimple exponential fits that decline formarine families. The decline suggeststhat marine taxa are becoming more andmore resistant to whatever processescause extinction, at least at the family


Extinctions of Life


Geologic Time (millions of years)

DECLINE OF BACKGROUNDEXTINCTION

Fig. 12. Extinction is an ever-present featureof geologic history. The background extinc-tion (that is, total extinction minus the largepeaks of extinction) shows a decline through-out the Phanerozoic that is fitted quite well bya simple exponential. Such a decline has im-plications for evolutionary innovation. (Figure

level. We might speculate that back-ground extinction will asymptoticallygrind to a halt. If that should happenand if no more mass extinctions occur,there would be very little potential forevolutionary innovation or for furtherevolutionary development of the ecosys-tem. The evolutionary machine mightnot halt completely, but it would cer-tainly slow down without major massextinctions to reset it. Thus extinctionsmay be a necessary force in the devel-

adapted from “Some implications of mass ex-tinction for the evolution of complex life” byJ. John Sepkoski, Jr. In The Search for Ex-traterrestrial Life: Recent Developments (Pro-ceedings of the 112th Symposium of the In-ternational Astronomical Union held at BostonUniversity, Boston, Mass., U. S.A., June 18-21,1984), edited by Michael D. Papagiannis, 223-232. Dordrecht, Holland: D. Reidel PublishingCompany, 1985.)

opment of complex life and, from whatwe see of patterns at the end of the Cre-taceous, perhaps even for the appearanceof consciousness in an evolutionary sys-tem.

I hope I have shown that our under-standing of extinction is still very

limited and that this aspect of the sci-ence of life presents numerous unsolvedproblems. ■

Questions and Answers

Question: Has anybody tried to cor-relate the dates of large craters withthose of extinction events? Also, a lotof meteors are carbonaceous chondrites,which probably wouldn’t be expected tocontain much iridium. So wouldn’t it bea mistake to say that if you don’t findiridium there was no impact?Sepkoski: In 1982 Greeve publisheda compendium of the best estimates ofcrater ages at that time. An analysis byWalter Alvarez and Rich Muller sug-gested a periodicity in those crater agesthat wasn’t too different from the pe-riodicity we see in extinction events.Since then a lot of the crater dates werecleaned up, and on reanalysis the peri-odicity didn’t look as good. But severalmanuscripts now in press or review [andsubsequently published] indicate that aperiodicity in crater ages has been es-sentially refound. If it is assumed thatmaybe 50 to 65 percent of the cratersare due to random cratering events,perhaps impacts of Apollo asteroids orsomething of that nature, the timing ofthe rest of the craters looks quite peri-odic statistically. However, the periodic-ity is about 30 million years, which isn’tthe same as 26 million years. Also, overat least the most recent part of the ex-tinction time series, the crater dates areout of phase by about 9 million years.

Your second question would best beanswered by an expert on meteorites,which I am not. But it is my under-standing that virtually all meteorites,except eucrites, are enriched in iridiumrelative to earth crustal rocks, often byseveral orders of magnitude.

Question: Is the type of extinction dueto humans the same as that of the olderextinctions?Sepkoski: I think that the advent ofhumans has probably caused two massextinctions. There was certainly a ma-jor extinction on land—but not in the

Los Alamos Science Fellows Colloquium 1988 47

Extinctions of Life


oceans—about twelve thousand yearsago. The Holarctic continents, SouthAmerica, and Australia lost their largemammal fauna then. According to somepretty good arguments, now coupledwith some pretty good evidence, thatextinction event was related to the ap-pearance of fairly efficient hunting bandsat the end of’ the last ice age. Of’ course.the extinction was an aperiodic event,and so I would expect some extraordi-nary agent, such as human predation, tohave been responsible. Like many ear-lier mass extinctions, the event twelvethousand years ago affected large ter-restrial animals but not marine fauna,There are also good arguments that inhistorical times we have entered a sec-ond mass extinction that is much moreextensive in terms of the kinds of or-ganisms that are being affected. It isdifficult as yet to get good informationon what kinds of organisms are beingaffected at present, so comparisons witholder events are tenuous,

The closing statements I made aboutthe beneficial effects of extinction mayneed a little clarification. As a pale-ontologist, an evolutionary paleobiol-ogist, I am looking at how the wholeevolutionary system behaves over vastspans of time—tens of millions of years.That is very different from processesthat happen over human time scalesof days, weeks. and years. I fear thatsome of the animals and plants disap-pearing right now may be very usefulfor a variety of purposes, We shouldn’tbe too relaxed to see them disappearbefore we can characterize them betterand know what the short-term ramifi-cations of their extinction arc. The re-bounds from mass extinctions, whichtake place over 10 million years or so,may be good from the standpoint of alarge-scale evolutionary system such asthe entire biosphere of the earth. But,from the human standpoint. the first fewdecades or centuries after the initiationof an extinction event may in fact be

43

quite catastrophic. Thus, my commentsabout the constructive aspects of’ extinc-tion are meant to give solace.

Question: Are there correlations be-tween the changes in the earth’s mag-netic field and the extinction events?Sepkoski: Work by David Raup andseveral others suggests that the rever-sals of the earth magnetic field overthe last 200 or 250 million years show aperiodicity’. But there is some questionas to whether that periodicity is station-ary or nonstationary. Also the purportedperiodicity isn’t the same as that of theextinction events, It is about 30 millionyears. more in tune in both frequencyand phase with the cratering periodicitythan with the extinction periodicity.

Question: The effects of an impactthat creates a crater a couple hundredkilometers in diameter are obviouslyhorrendous-far worse than those of anuclear war, So how can it be that anextinction is not associated with everylarge crater?Sepkoski: An extinction of some mag-nitude could well be associated withevery large crater, but that doesn’t mean

we would see such a correlation in thedata. I could, for instance, sweep thatwhole question under the rug by simplysaying that our ability to date craters isstill rudimentary. not nearly approach-ing even our ability to date fossils. Theproblem may also lie in the loss of res-olution we incur by dealing with highertaxonomic levels. Remember that eventhe impact at the end of the Cretaceus,which spread a I -centimeter dust layerover the entire face of the earth, elim-inated only about 17 percent of theanimal families in the oceans, and onland it eliminated only about 10 per-cent of the vertebrate families. So atthe family level the biosphere seemsrather insensitive to perturbation. Thecombination of a small response andimperfections in the data for highertaxonomic levels could obliterate anyobservable response. Alternatively, ab-sence of a marked response in associ-ation with an impact or the like couldmean that the impact was completelyout of phase with the periodic extinctionforce, We are trying to use statisticalmodels to sort out these problems andto learn how to start attacking when wesee associations, but we arc really justbeginning.

Question: Are there any explanationsfor the rebound phenomenon, and doesthe nature of the animals that survive anextinction provide information about thenature of the extinction force’?Sepkoski: That is a very good question.One thing that wc know from lookingat radiations, including rebounds, isthat evolution can go on extraordinar-ily rapidly, at least on geologic timescales. If the rate of evolution acrossthe Precambrian-Cambrian boundaryhad continued to the present, the oceanswould now contain on the order of1027 families, in contrast to the about3 x 103 that in fact they do contain, (Wewould essentially have bouillabaissefrom New York to London.’) What we


Extinctions of Life

Mesozoic Cenozoic


see as normal rates of evolution throughmost of the fossil record seem to bevery, very damped, which I suspect isjust a crowding effect. The clearing ofecospace by the extinction of a lot ofspecies may take the brakes off evolu-tion, so that the initial, unconstrainedevolutionary rates are again in effect,rapidly refilling the open ecospace. Theevolutionary rates during the reboundscan be of about the same magnitude asthat across the Precambrian-Cambrianboundary, when animals were first ap-pearing in large numbers in the marinesystem.

At this time only a few systematicstudies of victims and survivors of massextinctions exist, and so little can bededuced from them about the natureof extinctive forces, At the end of theCretaceus, small animals and animalsin detritus-based food chains preferen-tially survived, which seems consistentwith impact scenarios. On the otherhand, warm-blooded, high-energy birdsalso survived, which seems problematic.Whatever the forces, David Jablonskirecently completed a study for the Cre-taceus that suggests the rules of thegame change during mass extinctions:Victims of those events do not have thesame sort of properties as species thatare vulnerable to extinction during nor-mal “background” times. Thus, massextinctions represent more than sim-ply intensification of extinction; theyrepresent real changes in the nature ofextinctive forces.

J. John Sepkoski, Jr., received a B.S. in geol-ogy from the University of Notre Dame in 1970and a Ph.D. in geological sciences from HarvardUniversity in 1977. After serving from 1974 to1978 as an Instructor and then Assistant Profes-sor in the Department of’ Geological Sciences atthe University of Rochester, he moved to the De-partment of the Geophysical Sciences at the Uni-versity of Chicago, where he is now a Professorof Paleontology. He is also a Research Associateat the Field Museum of Natural History. In 1983he received the Charles Schuchert Award fromthe Paleontological Society. He has served as co-editor of Paleobiology and is a consulting editorfor McGraw-Hill’s Encyclopedia of Science andTechnology. He is a member of the American As-sociation for the Advancement of Science, the Pa-leontological Society, the Society of Sigma Xi,and the Society for the Study of Evolution.

Further ReadingLuis W. Alvarez, Walter Alvarez, Frank Asaro,and Helen V. Michel. 1980. Extraterrestrial causefor the Cretaceous-Tertiary extinction. Science208: 1095-1108.

Walter Alvarez. 1986. Toward a theory of impactcrises. EOS 67: 649, 653–655, 658.

Walter Alvarez, Erie G. Kauffman, Finn Surlyk,Luis W. Alvarez, Frank Asaro, and Helen V. Mi-chel. 1984. Impact theory of mass extinctions andthe invertebrate fossil record. Science 223:1135–1141.

Marc Davis, Piet Hut, and Richard A. Muller.1984. Extinction of species by periodic cometshowers. Nature 308: 7 15–7 17.


Donald Goldsmith. 1985. Nemesis: The Death-Star and Other Theories of Muss Extinction. NewYork: Walker and Company.

Stephen Jay Gould. 1984. The cosmic dance ofSiva. Natural History August 1984, 14–19.

David M. Raup. 1986. The Nemesis Affair: AStory of the Death of Dinosaurs and the Ways of’Science. New York: W. W. Norton & Company.

David M. Raup. 1986. Biological extinction inEarth history. Science 231:1528–1533.

David M. Raup. 1987. Mass extinction: A com-mentary. Paleontology 30: 1–13.

David M. Raup and J. John Sepkoski, Jr. 1982.Mass extinctions in the marine fossil record.Science 215:1501-1503.

David M. Raup and J. John Sepkoski, Jr. 1984.Periodicity of’ extinctions in the geologic past.Proceedings of the National Academy of Sciencesof the United States of Arnerica 81:801–805.

David M. Raup and J. John Sepkoski, Jr. 1986.Periodic extinction of families and genera. Science231:833-836.

J. John Sepkoski, Jr. 1982. Mass extinctions inthe Phanerozoic oceans: A review. In GeologicalImplications of’ Impacts of” Large Asteroids andCornets on the Earth, edited by Leon T. Silver andPeter H. Schultz, 283–289. Geological Society ofAmerica Special Paper 190. Boulder, Colorado:The Geological Society of America, Inc.

J. John Sepkoski, Jr. 1985. Some implicationsof mass extinction for the evolution of complexlife. In The Search for Extraterrestrial Life:Recent Developrnents (Proceedings of the l12thSymposium of the International AstronomicalUnion held at Boston University, Boston, Mass.,U. S.A., June 18–21, 1984), edited by MichaelD. Papagiannis, 223–232. Dordrecht, Holland:D. Reidel Publishing Company.

J. John Sepkoski, Jr. 1986. Global bioeventsand the question of periodicity. In Global Bio-Events: A Critical Approach (Proceedings of theFirst International Meeting of the IGCP Project216: “Global Biological Events in Earth His-tory”), edited by Otto H. Walliser, 47–61. Berlin:Springer-Verlag.

J. John Sepkoski, Jr., and David M. Raup. 1986.Periodicity in marine extinction events. In Dynam-ics of Extinction, edited by David K. Elliott, 3–36.New York: John Wiley & Sons.

Leon T. Silver and Peter H. Schultz, editors.1982. Geological Implications of Impacts of LargeAsteroids and Comets on the Earth. GeologicalSociety of America Special Paper 190. Boulder,Colorado: The Geological Society of America.

Steven M. Stanley. 1987. Extinction. New York:Scientific American Books, Inc., Scientific Ameri-can Library.

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