Issue 6, March 2002
Catastrophic Events in the History of Life: Toward a New Understanding of Mass Extinctions in the Fossil Record - Part I
David B. Weinreb
Molecular Biophysics and Biochemistry & Geology and Geophysics, Yale University
article is Part One of a two-part series exploring the historical
changes in the theories of extinction and evolution.
Introduction: Who shall live?
history of life, as Charles Darwin taught us over a century ago,
is a struggle for survival. New organisms emerge and flourish because
they are somehow better equipped for their environments than the
creatures they replace. Extinction is evolution's way of weeding
out those who are simply not able to compete with more specialized
or more advanced organisms.
For the last 600 million years, this is how the saga of life unfolded:
new forms gradually evolved and replaced more archaic, less successful
forms, eventually driving them into extinction. Nearly every organism
that has ever existed is now extinct.
Is extinction simply evolution's way of destroying those life forms
that fail to adapt to constantly changing environments? Does a species'
extinction reflect its failure to compete with the more sophisticated
organisms with which it co-exists? Or, does extinction represent
a random event; indiscriminately destroying lineages that otherwise
could survive indefinitely? The best approach to answering such
questions is to investigate episodes in the fossil record when rates
of extinction have been phenomenally high: mass extinctions events.
There have been a handful of moments in the earth's history when
life on the planet came terrifyingly close to being totally eradicated.
At these moments, a great number of species, inhabiting diverse
ecosystems across the globe, have simultaneously, or nearly simultaneously,
been sentenced to extinction. Many of the condemned do not seem
to deserve their fate: in studying mass extinctions, it is difficult
to predict a priori which taxa shall live and which shall
perish. This has prompted many paleontologists to suspect that mass
extinctions do not represent a clearing of archaic, stagnant forms
to make way for more evolved, more successful species. In order
to fully understand why some species survive mass extinctions and
others don't, we must search for the causes of mass extinctions.
Unless we understand the geological events that trigger mass extinctions
and why they occurred at those particular moments in the earth's
history, we will not be able to comprehend the patterns of extinction
and survival that accompanied these devastating events.
Paleontologists, geologists, and physicists have collaborated over
the course of the last two decades to investigate the causes of
mass extinction. Collectively, they have achieved a more precise
understanding of why mass extinctions have occurred and offer insight
into why some of those outrageously fantastic creatures that once
wandered the earth are no longer with us.
Timescales and Mass Extinction Events
the 19th century, paleontologists divided the history of life on
Earth into four stages. The first era is the Paleozoic, beginning
570 million years ago (570 MYA) and lasting roughly 350 million
years. Next, the Mesozoic Era (literally, "time of middle
life") occurred from to 225 million to 65 million years ago.
The Mesozoic is subdivided into three periods: the Triassic
(225-195 MYA), which witnessed the first emergence of the dinosaurs,
the Jurassic (195 to 136 MYA), when the earliest birds are
documented in the fossil record, and the Cretaceous (136
to 65 MYA) when primates and flowering plants first appear. The
third chapter in the history of life is the Cenozoic Era,
beginning 65 million years ago, in which we are currently living.
The earth's biota has witnessed five major mass extinctions in the
last 600 million years. The "Big Five" include the Ordovician-Silurian
event (438 MYA), the Devonian extinction (367 MYA), the Permian-Triassic
event (250 MYA), the Triassic-Jurassic extinction (202 MYA), and
the Cretaceous-Tertiary (K-T) event (65 MYA). Most of these events
witnessed a 75% or greater drop in species diversity. The most recent
of these mass extinction events, the K-T extinction, brought about
the demise of the dinosaurs, signaling the end of the "Age
of Reptiles" and the beginning of the "Age of Mammals."
Perhaps, for this reason, paleontologists have devoted a tremendous
amount of attention to the underlying mechanisms of the K-T extinction.
The K-T extinction
was the final hurrah for some of the earth's most successful organisms.
The ammonites, squid-like creatures with coiled shells similar to
those of the chambered Nautilus, perished during the K-T extinction.
The ammonites represent one of the most evolutionarily successful
groups in Earth's history, existing for more than 300 million years
(by comparison, our species has existed for a brief 200,000 years,
if that long). These sophisticated cephalopods diversified in oceans
and seaways around the globe for hundreds of millions of years - only
to perish forever at the K-T boundary. As Peter Ward, a geo-biologist
from the University of Washington, comments, "The ammonites weathered
everything the world had ever thrown at them, and then, BANG! …the
ammonites have suffered many extinctions, but they've always come
back. This was the one when they didn't come back."
The K-T extinction also brought about the demise of the dinosaurs,
which, in the words of journalist Ian Warden, are "creatures
of interest to all but the veriest dullard." Several decades
ago, paleontologists would have explained that dinosaurs were too
oversized, too cold-blooded, and too stupid to survive. But in reality,
the dinosaurs, which inhabited the earth for more than 150 million
years, represent one of the greatest success stories in the history
Princeton University geologist Glenn Jepsen observed in 1964 that,
"Authors of varying competence have suggested that dinosaurs
disappeared because the climate deteriorated… [or because of] changes
in the pressure of composition of the atmosphere, poison gases from
volcanic dust…continental drift, extraction of the moon from the Pacific
Basin…even lack of standing room in Noah's Ark."
More recently, Peter Ward argued that the extinction of the dinosaurs
truly becomes a profound enigma when we reject the view that dinosaurs
"simply seemed to have died out due to uncompetitiveness in a
very competitive world." Ward describes how the changing views
of dinosaur paleobiology in the last three decades have made their
disappearance seem even more perplexing:
"Old view: Dinosaurs were slow, clumsy, and so stupid
that they needed a second brain in their pelvic region just to be
able to walk; they were cold-blooded creatures with dull gray hides
whose time came and went. They sort of faded away in the face of
climate change, lots of volcanoes exploding everywhere, and superior
competition from the warm-blooded, egg-eating, all around nasties
of the Late Cretaceous period, the mammals."
"New view: Dinosaurs were fast, graceful, smart wildlife with warm
blood, brilliant coloring, and excellent parenting skills whose
time went by all too fast. They were so wonderful that they really
ought to still exist, and the cause of their extinction is a mystery."
At the close of the Cretaceous, nearly half of all species on Earth
vanish. But was the extinction sudden and catastrophic? Was it some
single disastrous event that abruptly snuffed out so much of Mesozoic
life? Or, perhaps, was it a less dramatic gradual event? Did the dinosaurs
slowly march toward an eventual demise, going extinct, in the words
of T.S. Elliot, "not with a bang but a whimper?"
Marine ecosystems were also decimated at the K-T boundary. Perhaps
as many as 90% of all marine species died out at the boundary: all
of the ammonites, 93% of marine reptiles, 83% of planktonic foraminifera
and 65% of all sponges. More than half of all terrestrial reptiles
died, including all of the dinosaurs. No terrestrial organism larger
than 25 kilograms survived the K-T extinction. Interestingly, most
of the higher forms of plants were only mildly affected, and mammals
actually experienced an increase in diversity at the start of the
Searching for the Trigger
In the late 1970s,
Walter Alvarez was studying a sequence of clay from Gubbio, Italy.
The sequence of clay bracketed the K-T boundary and contains deposits
from the final dusk of the Maastrichtian to the early dawn of the
Danian. Below the K-T boundary section in the Gubbio clay, there is
an abundance of fossil foraminifera of the genus Globotruncana.
The Cretaceous deposits in this section consist of a white limestone,
filled with Globotruncana. Above the Cretaceous limestone,
there is a veneer of reddish clay, only about 1 centimeter in thickness
and completely devoid of any fossils, above which lies the red limestone
of the Danian deposits. Globotruncana is entirely absent from
the red Danian limestone, although, in its place is the foraminifera
Alvarez's objective was to quantify the rate at which the faunal transition
from Globotruncana to P. eugubina had occurred. This
would require knowing only how long it took for the 1-centimeter thick
red clay at the boundary to be deposited. Alvarez postulated that
it probably took about 5,000 years for the clay to form, but he didn't
know how to precisely measure the rate of deposition.
Alvarez brought a polished specimen from the Gubbio boundary clay
to his father, Luis Alvarez, a Nobel Laureate and physicist at the
Lawrence-Berkeley National Laboratory who also had a reputation for
being a brash, arrogant man. While the range of research topics that
captured his attention extended into many fields of science, he often
looked down on researchers in scientific disciplines that he considered
to be less rigorous than physics. He was unhesitant to criticize the
credentials and character of those with whom he disagreed.
The Iridium Clock
Luis Alvarez suggested
to his son that they could measure the abundance of cosmic dust
in the clay. The platinum group elements (iridium, osmium, palladium,
rhodium, and ruthenium) are incredibly rare in the earth's crust,
although their abundance in meteorites is much higher. Broken shards
of meteorites produce a cosmic rainfall, reaching the earth's surface
at a steady rate. Currently, between 107 and 109
kilograms of meteoritic dust fall to Earth each year. Presuming
that the sparse rain of cosmic dust and fragments has been constant
and uniform over the thousands of year during which the clay was
deposited, the amount of platinum group elements in the clay could
provide a clock for assessing sedimentation rates.
Iridium is the easiest to measure of all the platinum group elements.
The Alvarezes decided to test an "iridium clock." By comparing
the levels of iridium in the clay with the levels in the two layers
of limestone that sandwiched it, they could assess the relative
rates of sedimentation. Iridium can be measured by neutron activation
analysis, in which the clay is bombarded with neutrons, causing
the iridium to become radioactive and then quickly decay by emitting
gamma rays at a particular, detectable energy level.
What the Alvarezes found was nothing short of shocking. The boundary
clay contained 30 times the amount of iridium as the bracketing
limestone. If the rain of cosmic dust to Earth had been constant
during the time of the interval of clay deposition, then the clay
would have required an absurdly long time to form.
The "iridium spike" in the Gubbio boundary clay was not
a localized phenomenon. The Alvarez team found an even more impressive
enrichment of iridium in a contemporaneous K-T section near Copenhagen,
Denmark. By 1981, an iridium enrichment had been detected in more
than 100 K-T sections around the world.
In June, 1980, the Alvarez team, which now included nuclear chemist
Frank Asaro and Berkeley paleontologist Helen Michel, published
a groundbreaking study in the journal Science. In the introduction
of their paper, they observed that nearly everything known about
the mass extinctions is related to paleobiological interpretations
of the fossil record. They claimed that a lack of evidence about
how the earth had somehow changed at the K-T boundary had made it
challenging to hypothesize about the underlying cause of the mass
The abundance of iridium in the crust is less than 0.1 parts per
billion, although extraterrestrial sources have iridium concentrations
greater than hundreds of parts per billion. The Alvarez team concluded
from these results that the source of global iridium enrichment
was an asteroid that struck the earth at the precise time of the
K-T boundary and ejected debris from the impact crater. It may have
taken as long as three years before the dust settled to the earth,
during which time sunlight would have been blocked and photosynthesis
stopped. The impact would have created a type of nuclear winter.
Impact at the Boundary
estimated that, given that there are presently 700 Earth-orbit-crossing
asteroids with diameters greater than 1 kilometer, one object with
a diameter greater than 10 kilometers will strike the earth on average
every 100 million years. Thus, it is certainly not impossible that
one such object, with a diameter of about 6 kilometers, could have
struck the planet 65 million years ago.
The 1980 Alvarez team paper was truly a bold step. They not only
provided evidence for a major impact at the end of the Cretaceous,
they took the theory a giant leap further when they proposed that
this impact single-handedly created the environmental havoc leading
to a mass extinction.
In the year after the Alvarez impact theory was proposed, paleontologists
were reluctant to accept the notion of a catastrophic impact. They
believed the extinction at the boundary was not instantaneous and
sudden, as the Alvarez team proposed, but rather a gradual turnover.
Over a period of at least several million years, the majority of
paleontologists believed, most species slowly died off. They had
been arguing for decades that the K-T event was a gradual transition.
The dinosaurs, the ammonites, and their contemporaries had been
dwindling in diversity for millions of years; for these paleobiologists,
the K-T boundary didn't represent a sudden sweep of devastation,
but rather a last whimper for taxa that were long destined for extinction.
Could a meteorite impact have been devastating enough
to initiate a global catastrophe, one in which 90% of all marine
Imagine a meteorite slightly larger than Mount Everest, racing toward
Earth at a speed 100 times faster than a commercial airliner. The
air between the racing meteorite and the surface is compressed,
resulting in perhaps the loudest sonic boom ever heard. The compressed
air reaches a temperature five times hotter than the surface of
the sun, creating a blinding flash of light. The impact releases
a burst of energy equivalent to 100 million megatons of TNT - 1,000
times more powerful than a simultaneous detonation of the entire
world's nuclear arsenal. When it crashes into the crust of the earth,
the force of the impact sends debris weighing 60 times the meteorite's
weight into the atmosphere. A cloud of debris remains suspended
in the atmosphere for at least two years, blocking sunlight and
plunging the biosphere into a long, dark winter. In his 1997 book,
T. rex and the Crater of Doom, Walter
Alvarez imagines the moment of this devastating impact:
zone where bedrock was melted or vaporized, no living thing could
have survived. Even out to a few hundred kilometers from ground
zero, the destruction of life must have been nearly total. Sterilized
by the intense light from shock-compressed air and from the fireball
of rock vapor, crushed when pores and cracks in rock were slammed
shut by the passing shock wave, and bombarded by the falling debris
of the ejecta blanket, little or nothing was left alive in this
central area… Soon the earth's surface itself became an enormous
broiler - cooking, charring, igniting, immolating all trees and
animals, which were not sheltered under rocks or in holes… Entire
forests were ignited, and continent-sized wildfires swept across
sucked oxygen out of the soot-filled atmosphere. Earthquakes registering
magnitude 13 on the Richter scale caused landslides as well as tsunamis
-- some over a kilometer in height - that flooded coastal regions.
Any creature fortunate enough to survive the initial fireball would
have been threatened by showers of debris, acid rain, and years of
impenetrable darkness. Alvarez tells us, "We wonder how anything
could survive this environmental apocalypse. Yet there were survivors,
and their descendents populate the world today."
When the impact ejecta settled to Earth and light returned, the earth
became oppressively hot. The release of carbon dioxide from incinerated
limestone at the impact site turned the planet into a gigantic greenhouse.
Alvarez estimates that it may have been thousands of years before
enough carbon dioxide was removed from the atmosphere to bring temperatures
back to normal ranges. The meteorite may have impacted a subterranean
deposit of gypsum (calcium sulfate). Once ejected into the atmosphere,
the sulfate would have combined with water to create acid rain. Acidic
shallow water environments would have become uninhabitable.
Debating the Theory
bold suggestion that an impact brought about the K-T extinction
was not a new idea. In 1973, University of Chicago Nobel Laureate
Harold Urey hypothesized that the Cretaceous ended with a collision
with a comet. But his idea was widely ignored and, eight years later,
when the Alvarez team actually announced evidence for such an extraordinary
event, their theory still met considerable resistance.
Paleontologists offered as a major criticism of the Impact Theory
the fact that the fossil record provided no hard evidence for a
sudden catastrophic extinction. Furthermore, paleobotanists such
as Leo Hickey of Yale University commented that the patterns of
differential survival and extinction were inconsistent with the
notion of a catastrophic extinction. For instance, tropical plants,
which would be expected to suffer the most during several years
of global darkness, actually were only moderately affected by the
extinction. In contrast, temperate plants were devastated, in spite
of the fact that temperate plants can suffer through periods of
reduced sunlight better than tropical floras.
At the 1985 meeting of the Society of Vertebrate Paleontologists,
a survey conducted by The New York Times revealed that a
meager 4% of paleontologists were convinced that a meteorite impact
had brought about the mass extinction, although 90% were receptive
to the theory that an impact had indeed occurred at the K-T boundary.
In short, paleontologists were convinced by the Alvarez report that
an impact had occurred, but they doubted the link between the meteorite
crash and the global extinction.
Robert Bakker, a dinosaur expert with the University of Colorado
Museum, attacked the Alvarez theory, exclaiming,
arrogance of these people is simply unbelievable. They know next
to nothing about how real animals evolve, live, and become extinct.
But, despite their ignorance, the geochemists feel that all you
have to do is crank up some fancy machine and you've revolutionized
science. The real reasons for the dinosaur extinctions have to do
with temperature and sea level changes, the spread of diseases by
migration and other complex events. In effect, they're saying this:
we high-tech people have all the answers, and you paleontologists
are just primitive rockhounds."
Bakker and his colleagues felt strongly that the fossil record told
a story that was terribly inconsistent with the scenario implicit
in the Alvarez model. But Bakker expressed a sentiment held by many
in the scientific community: that paleontology had remained a low-tech
endeavor. Paleontologists were still "digging in the dirt,"
while researchers in other disciplines had embraced novel technologies
and become more eager to collaborate with scientists outside their
These sentiments were summed up harshly by Luis Alvarez, who, frustrated
by the resistance he encountered, lamented, "I don't like to
say bad things about paleontologists, but they're really not very
good scientists. They're more like stamp collectors… I can say these
things about some of our opponents because this is my last hurrah,
and I have to tell the truth. I don't want to hold these guys up to
too much scorn. But, they deserve some scorn because they're publishing
Within the few years that followed the emergence of the impact theory,
the Alvarez team faced two types of opponents. First, there were the
paleontologists, who, for the most part, were willing to accept that
an impact had occurred, but were not convinced that the event, no
matter how catastrophic, could explain the patterns of extinction
at the eleventh hour of the Cretaceous. While they did not doubt the
environmental repercussions of an impact would have been devastating
enough to trigger a mass extinction, they felt the list of survivors
after the K-T event was irreconcilable with the climatological and
environmental consequences of a meteorite impact.
The second group of opponents was a faction of geochemists who argued
that the iridium spike in the Italian and Danish sections could be
explained by an alternative hypothesis. These researchers suggested
that massive volcanic activity responsible for the Deccan lava flows
in India during the late Cretaceous may have ejected trillions of
tons of greenhouse gases (as well as high concentrations of platinum
metals, including iridium) into the atmosphere. The environmental
consequences of volcanic activity would have been quite similar to
the effects of an impact. Perhaps, those geochemists argued, the effects
of such a vast outpouring of lava would have been slightly less catastrophic
and abrupt than a collision with an asteroid the size of San Francisco.
Deccan Flood Traps
In the mid-1980s,
a team of Indian geochemists, supervised by N. Bhandari of the Physical
Research Laboratory in Ahmedabad, discovered an iridium spike in
the lava beds of the Deccan Traps, corresponding to exactly the
time of the K-T boundary. At roughly the same time, French geologist
Vincent Courtillot used the magnetic reversal timescale to conclude
that the Deccan volcanic eruptions started at least one million
years before the K-T boundary and were sustained for about 1 million
years after it. If Deccan volcanism was the chief cause of the extinction,
then it is unlikely that the extinction was as rapid as the Alvarez
team suggested. However, dinosaur fossils are recovered from within
the sediment layers that sandwich the Deccan lava flows, providing
strong evidence that dinosaurs endured at least part of the volcanic
Furthermore, the iridium levels detected by the Bhandari team was
detected in a layer of sedimentary rock deposited after the major
period of volcanism had ended. In simple terms, the iridium spike
did not occur at the same time as the volcanism that these researchers
claimed was responsible for the mass extinction. Iridium is not
detected in the majority of the sedimentary layers of the Deccan
Traps, and wherever it is detected, its concentration is relatively
low compared to what one would expect in sediments so close to the
site of volcanic activity.
The advocates of the Deccan Traps volcanism theory simply asked
the Alvarez group: Where is the smoking gun? A 10-kilometer meteorite,
they argued, could not hit the earth and wipe out half of all species
on the planet without leaving some sort of evidence of its existence.
Show us, they demanded, the crater.
team anticipated that they would have to address this issue when
they published their first paper in Science in 1980. They end the
paper with these words:
is a 2/3 probability that the object fell in the ocean. Since the
probable diameter of the object, 10 km, is twice the typical oceanic
depth, a crater would be produced on the ocean bottom and pulverized
rock could be ejected. However, in this event we are unlikely to
find the crater, since bathymetric information is not sufficiently
detailed and since a substantial portion of the pre-Tertiary ocean
has been subducted."
How does one search for a crater that may not even exist any more?
In 1989, Alan Hildebrand, then with the Geological Survey of Canada,
found a rather peculiar 50-centimeter-thick layer of sediments in
Massi de la Selle, Haiti. The sediments contained both shocked quartz
grains (minerals that have been deformed under conditions of extreme
pressure) and tektites (terrestrial rocks believed to have been ejected
into the upper atmosphere after an impact and, after orbiting the
earth, settled back to the surface). The thickness of this sedimentary
layer suggested that "ground zero" was probably quite close
employed in the petroleum industry had been studying the basement
rocks of the Caribbean for decades. These researchers measure variations
in the strength of the earth's gravitational field in different parts
of the earth's crust. Gravity anomalies
are zones where the local gravitational field may be slightly stronger
than in surrounding regions. The presence of these anomalies often
suggests a broad area of more dense rock because the strength of a
local gravitational field is proportional to its mass. Geophysicists
often search for gravity anomalies in order to pinpoint the location
of oil-bearing rocks.
Hildebrand discovered an early report by geophysicists Glen Penfield
and Antonio Camargo that had been largely ignored at the time of its
publication in 1981. Penfield and Camargo detected circular gravity
anomalies in the northern Yucatán Peninsula centered near the town
of Chicxulub. Their findings suggested the existence of a belt of
dense, compacted rocks buried beneath a kilometer of sediments. Penfield
and Camargo concluded their 1981 presentation at the meeting of the
Society of Exploration Geophysicists with these words: "We would
like to note the proximity of this feature in time to the hypothetical
Cretaceous-Tertiary boundary event responsible for the emplacement
of iridium-enriched clays on a global scale and invite investigation
of this feature in the light of the meteorite impact-climatic alteration
hypothesis for the late Cretaceous extinctions."
the Penfield-Camargo lecture occurred the same exact week when most
researchers studying the K-T boundary had convened in Snowbird, Utah
to debate the Alvarezes. Thus, on almost the same day the most vehement
critics of the Alvarezes argued that there was no crater to substantiate
the theory of a meteorite impact, Penfield and Camargo presented the
first evidence for a crater.
Amazingly, only several months after the Alvarez team announced the
elevated iridium in the Gubbio clay, Penfield and Camargo had discovered
the smoking gun: a crater of appropriate age, large enough to have
been produced by impact with a 10-kilometer wide object. And, more
amazingly, nobody seemed to have noticed.
It turned out that petroleum geologists working for the Mexican oil
giant Petroleos Mexicanos had discovered the circular Chicxulub (a
Mayan word meaning "red devil") structure in the 1950s.
In search of oil reservoirs, Petroleos Mexicanos researchers drilled
into the structure and collected rock cores. In 1980, immediately
after the publication of the Alvarez paper in Science,
Penfield wrote to Walter Alvarez to inform him of the existence of
this structure. He never received a response.
Despite the evidence, the debate is far from over. Did the impact
at Chicxulub cause a global mass extinction? Or were the horrific
environmental consequences localized in North America, the region
that logically would have been most directly affected by the impact
debris, tsunamis, seismic waves, and acid rain in the wake of the
Trusting the Fossil Record
key issue in the debate between the proponents of the Impact Theory
and paleontologists is whether the extinction was sudden or gradual.
Many paleontologists are confident the extinction was gradual, and
feel that even if the impact had occurred and did have globally
catastrophic effects, it was not the principal factor responsible
for the decline in species diversity in the late Cretaceous.
accurately can we tell whether an extinction in the fossil record
is gradual or sudden? At the 1981 Snowbird I conference, Phil Signor
and Jere Lipps of the University of California at Berkeley suggested
data on species diversity extracted from the fossil record might
be much harder to accurately interpret than most paleontologists
had previously assumed.
Signor and Lipps demonstrated that the number of unique ammonite
genera known from the fossil record declines sharply at the end
of the Cretaceous. However, the area of sedimentary rocks across
the globe was also decreasing. Thus, there are fewer sedimentary
rocks of Late Cretaceous origin in which ammonites could have been
deposited and preserved. These authors demonstrated that ammonite
genus diversity throughout the entire Mesozoic correlates
with the number of square kilometers of sedimentary rock available
to be sampled for each geological period.
Next, consider the dilemma of a paleontologist who is searching
for the fossils of a rare dinosaur. The question this researcher
must address is whether this species went extinct suddenly at the
final moments of the Mesozoic or whether it died out gradually in
a slow demise that lasted throughout the Maastrichtian. If this
particular species was widespread during the Cretaceous, then it
should theoretically be abundant in the fossil record; we would
expect to find it at virtually every stratigraphic horizon. If the
fossil persisted until the bitter end of the Cretaceous, then we
would expect to find it in the sediments just below the Impact Event
boundary. However, if the species was very rare (as are most dinosaur
fossils), then the probability of finding it in the youngest Maastrichtian
sediments is rather low, even if it did live up until the very end.
Simply stated, geologists are unlikely to determine the true time
at which a species went extinct unless the species is very common
in the record. Signor and Lipps demonstrated that a sudden extinction,
like that predicted by the Alvarez team, might indeed appear gradual
to a paleontologist sifting through Late Cretaceous deposits.
On the closing page of his 1998 book, Night Comes to the Cretaceous,
James Lawrence Powell writes,
we have gone about as far as science can go in corroborating the
notion that the impact of a meteorite caused the extinction of the
dinosaurs. But as always, answering one set of questions raises
others, and we are left pondering the true role of impact. As even
its bitterest opponents have to admit, the Alvarez theory has brought
geology not only a new set of questions, but also a greatly improved
set of sampling techniques and analytical methods for answering
them. Paleontologists collect much larger samples and subject them
to statistical tests. Today geologists know how to find and identify
terrestrial craters. These are the hallmarks of a fertile theory."
Indeed, the Alvarez theory, which once seemed so far-fetched and contrived
as to provoke ridicule and scorn, now has become universally praised.
And, beyond a doubt, the debate that followed the first discovery
of the iridium spike revolutionized the way paleontologists study
the history of life.
In Part II of this series (to be published in April 2002), we explore
further theories and new evidence for possible mechanisms that bring
us closer to understanding the causes of mass extinctions.
Alvarez, W. T.
Rex and the Crater of Doom. Princeton: Princeton University
Alvarez, L. W., Alvarez, W., Asaro, F., and Michel, H. V. "Extraterrestrial
Cause for the Cretaceous-Tertiary Extinction." Science,
v. 208 (1980), p. 1095-1108.
Archibald, J. D. Dinosaur Extinction and the End of an Era: What
the Fossils Say. New York: Columbia University Press, 1996.
MacLeod, N. and G. Keller. Cretaceous-Tertiary Mass Extinctions:
Biotic and Environmental Changes. New York: W. W. Norton & Company,
Officer, C.B. The Great Dinosaur Extinction Controversy. Reading:
Powell, J.L. Night Comes to the Cretaceous: Dinosaur Extinction
and the Transformation of Modern Geology. New York: W.H. Freeman,
Ward, P.D. The End of Evolution: on Mass Extinctions and the Preservation
of Biodiversity. New York: Bantam Books, 1994.
of Young Investigators. 2002. Volume Five.
Copyright © 2002 by David Weinreb and JYI. All rights reserved.