Thinking of Biology
Biotic replacements extinction or
clade interaction?
Have the changes which lead us
from one geological state to another
been, on a lang average, uniform in
their intensity, or have they con-
sisted of epochs of paroxysmal and
catastrophic action, interposed hetween periods of comparative tranquillity? These two opioions will
probahly for some time divide the
geological world ioto two seets,
which may perhaps he designated as
the Uniformitarians and the
Catastrophists (WheweIl1832).
F
rom Charles Darwin's time in
the mid-nineteenth centuey
until approximately 18 years
aga, historical replacements of major groups of organisms were generally thought to he the result of competitive interactions. In reeent years,
however, there has been an increasing ernphasis on abiotic factors to
such an extent that biotic relationships have been de-ernphasized or
even considered unimportant. This
change represents a fundamental shift
in the attitudes of many biologists and
paleontologists. Previously, I have observed (Briggs 1994) that rhe present
era is one of neocatastrophism, in
which sudden carasrrophic events are
often considered to be responsible
for major biotic replacements.
Neocarastrophism is the recapitulation of an idea, catasrrophism, rhat
was prevalent in the lare eighteenth
and early ninereenth cenruries. It
began with Swiss naturalist Charles
Bonnet (1720-1793), who was the
first to use the term "evolution" in a
biological sense. Bonnet defined evolution as aseries of catastrophic
events followed by new "creations."
Each new biota represented a step
upward in the progression. Bonnet's
theory was adopted by the French
anatomist and paleontologist Georges
Cuvier (1769-1832), who identified a
series of great catastrophes, the most
by John C. Briggs
May 1998
recent being the biblical Deluge. After each catastrophe, Cuvier reasoned, the earth was repopulated by
organisms that had somehow survived the crisis. The new species that
subsequendy appeared were supposed to have come from previously
unknown parts of the world. The
English geologist William Buckland
(1784-1856) argued that each catastrophe was worldwide and was followed by an entirely new creation.
At around the same time, the French
naturalist Alcide D'Orbigny (18021857), who divided fossil-bearing
beds irrto stages of deposit, stated
that each stage represented an independent biota made by a special act
of creation.
A general shitt away from catastrophism and toward uniformitarianism rook place in the mid-nineteenth century. Uniformitarianism
arose from the work of the Scottish
geologist James Hutton (17261797), who had proposed the uniformitarian theory of geology. This
idea, which holds that rocks are
formed by gradual processes, was
accepted only after lengthy debate.
Its approval was ensured by the British geologist Charles Lyell (17971875), who pointed out that historie
changes in the earth's surface could
be explained by contemporary processes. Lyell was followed by Charles
Darwin (1809-1882), who stressed
the significance of uniformitarianism for his evolutionary theory.
Although signs of a revival of
catastrophism began to appear before 1980, the shitt from uniformitarianism toward neocatastrophism
began in that year with Alvarez er
a1.'s (1980) proposal that an asteroid impact was responsible for a
great "mass extinction" that took
place at ehe Cretaceous-Tertiary
(K/T) boundary, approximately 65
million years (Ma) ago. This starding news received extensive cover-
age in both the scientific and popular presses. The subsequent periodicity theory of Raup and Sepkoski
(1984), which suggested that extraterrestrial bodies hit the earth every
26 Ma on average, created more interest in the idea of catastrophism.
Finally, the advocacy of volcanism
as an alternative to an extraterrestrial impact (Officer and Drake 1985)
gave catastrophism a new respectability and stimulated an enormous
amount of research on catastrophic
events and their possible biological
consequences.
Although several paleolltologists
who had worked with fossils from
the KIf boundary protested the widespread assumption that the extinction had occurred in a sudden, catasrrophic manner (Clemens et al.
1981, Hickey 1981, Archibald and
Clemens 1984), they were paid litde
heed. In fact, many scientists began
to argue that such catastrophes
(calIed mass extinctions) had important evolutionary advantages. For
example, Gould and Calloway (1980)
compared the evolutionary histories
of the brachiopods with those of the
pelecypods and conduded that the
pelecypods had gained superiority
over the brachiopods because the
brachiopods were hit harder by the
Permian-Triassic (P/T) extinction.
Gould (1984) suggested that mass
extinctions might be the primary,
indispensable seed of major changes
and shifts in life's history .
Along this same Hne, it has been
generally assumed that mass extinctions are a good thing because the
survival of better adapted organisms
would lead to an overall improvement in the general fitness level (Raup
1984a). Eldredge (1987) suggested
that without extinctions to free up
ecological niches, life would still be
confined to a primitive state somewhere on the seafloor. Stanley (1987)
stated that "[hJad the dinosaurs sur389
appeared. Vermeij (1991) provived, there is no question that
posed that the Pliocene invasion
we would not walk the earth
today. Mammals would still reof Pacific molluscan species into
main small and unobtrusive, not
the Atlantic Oeean was successt
unlike the rodents of the modern
ful because of an extinction in
world" (p. 8). The idea that
~
that oeean that occurred immeF
major extinctions convey evoludiately before the invasion.
tionary benefits was also supVermeij (1991) described this
ported by Hsü (1986), who subproposal as a "hypothesis of ecostituted evolution by means of
logical opportunity." Similarly,
Oiversity
global extinctions for Darwin's
in their work on fossil turdes
and Wallace's mechanism of Figure 1. Two distinct patterns of replacement: Rosenzweig and McCord (1991)
natural selection. Aceording to (left) the double-wedge type and (right) the mass- eoncluded that the replacement
Hsü, evolutionary advances extinction type. In the double-wedge type, one of the Amphichelydia by the
would take plaee as survivors clade waxes while the other wanes. In the mass- Cryptodira took place largely as
adapted to the spaees created by extinction type, an extinction terminates one the result of physical events at
group, allowing another to expand. The extinc- the KrT boundary. They called
extinction events.
tion event is indicated by the dashed line and
Did mass extinctions really asterisk. The width of each pattern indicates the process an "incumbent-reconfer evolutionary benefits? relative clade diversity. After Benton (1991).
placement model." All of these
Mass extinetions have been charproposals-whether a mass-exacterized as having a "noncontinetion pattern, a stationary hystructive selectivity" (Raup 1984b, macroevolution Benton (1987) con- pothesis, a turnover-pulse hypothesis,
Jablonski 1989). That is, the traits cluded that two distinct patterns of a preemptive competition, a hypoththat enhance survival during such biotic replaeement may be reeog- esis of ecological opportunity, or an
extinetions (e.g., broad geographie nized in the geologie reeord (Figure incumbent-replacementmodel-place
range at the clade level) need have 1): a double-wedge type, and a mass- great emphasis on the importanee of
little correlation with traits that pro- extinction type. The double-wedge physical events in evolutionary hismote survival during background pattern is familiar from many spin dIe tory. However, I will argue that bitim es. Therefore, mass extinetions diagrams published by paleontolo- otie replacements often attributed to
are unlikely to increase the long- gists, in which spin dIe width repre- mass extinctions and other physieal
term adaptation of biota. The high sents the numbers of speeies, genera, events may aetually represent the
level of species loss caused by mass or families at different times in the effects of competition.
extinctions and the extended times history of a clade. The waning of one
required forcommunity recovery (5- cladc while the other waxcs may Clade replacements
10 Ma) emphasize their destructive suggest a eompetition between the
nature (Jablonski 1994). Far from two. By contrast, the mass-extinc- The problem of decline in or extinepromoting evolutionary benefits, tion pattern shows one clade disap- tion of an entire dade is fundamenmass extinctions probably eaused pearing geologically instantaneously tally different from that of extincevolutionary setbaeks (Briggs 1995). and the other radiating afterward. tion at the speeies level. Clades
This coneept is supported by the To describe the double-wedge pat- comprise higher taxonomie groups
model 01 Kitchell and Carr (1985), tern, Hallam (1990, 1994) used the that usually include many species,
which predicts that taxonomie turn- term "displacement eompetition," genera, and often families. When two
over is gene rally delayed by extinc- and for the physieal extinetion pat- da des of organisms oecupy the same
!ion episodes, instead of than being tern he used the term "preemptive habitat and the long-term diversity
precipitated by them.
competition." But the latter term may of one gradually incrcases while that
Nevertheless, additional support be inappropriate beeause when a suc- of the other dec1ines, the general
for the evolutionary importanee of cessor moves into a vacated habitat conclusion is that a competitive, or
physical events came from the "sta- there is no eompetition, ooly passive negative, interaction has taken place
(Sepkoski and Hulver 1985). But such
tionary hypothesis, " which predieted succeSSlOlL
that evolution would eease in the
In many of the cases in which the a double-wedge pattern eould conabsence of change in abiotic param- double-wedge (competition) pattern ceivably be produced by differential
eters (Stenseth and Maynard Smith had been assumed, Benton (1987) responses to physical change, by dif1984). Vrba (1985) then proposed a went on to point out, the evidenee ferential predation pressure, or by
"turnover-pulsehypothesis" thatstated was equivoeal and could actually chance alone (Benton 1991).
that "[s]peciation does not oeeur un- represent mass-extinction patterns.
Other complieations may affect
less foreed (initiated) by ehanges in the Thus, he argued that macroevolu- the interpretation of a clade's hisphysieal environment. Similarly, tionary models involving competi- tory. For example, incomplete samforeing by the physieal environment ti on as a major driving factor are pling could suggest a gradualloss of
is required to produee extinctions open to question.
diversity when a sharp extinction
and most migratory events" (p. 264).
Recently, two more examples of had actually raken place (the SignorFinally, in a comprchensive re- evolutionary radiation as the appar- Lipps EHect; Signor "nd Lipps 1982).
view of progress and competition in ent result of extinction events have Conversely, an apparent catastrophic
390
BioScience Vol. 48 No. 5
extinction pattern can often be produced by disconformities in the fossil record. For example, Kauffman
(1984) noted that over 90% of exposed Krr boundary sequences in
the marine record had major disconformities or intercalations of nonmarine sediments. The bigger the
marine stratigraphie gap across the
boundary, the greater the apparent
boundary catastrophe.
In view of the complications that
may affect the apparent history of a
given dade, the question of whether
physical change or biotic interactions were responsible for evolutionary replacements is probably best
approached by studying individual
cases. A number of replacements involving major biotic groups have
beenidentified (Benton 1987, 1991).
These replacements represent longterm patterns that often involved only
a partial replacement-that is, one
in which the declining group did not
become extinct. For most of the ca ses,
Benton concluded that support for
competition as the causative factor
was inadequate. Although support
for a mass extinction as the primary
cause was also lacking, Benton concluded that three of the patterns represented mass extinctions followed
by opportunistic replacement: brachiopods giving way to bivalves,
synapsids giving way to archosaurs,
and dinosaurs giving way to mammals. I will show, however, that in
a11 three cases, there appears to be
ample evidenee for competitive interaction.
Brachiopods versus bivalves
Gould and Calloway (1980) lound
that the P/T extinction sharply reduced brachiopod diversity and that
they did not subsequently recover
their former abundance. At the same
time, the bivalves also suffered notable losses. However, whereas the
bivalves recovered rapidly and proceeded to dominate the warm, shallow habitats of the world's oceans,
the brachiopods were relegated to
cooler, deeper waters. This relationship was considered by Gould and
Calloway to have resulted from a
mass-extinction pattern with opportunistic replacement. However, the
bivalves were weil equipped to ga in
the ascendancy over brachiopods:
May 1998
Figure2. Model ofdiversitytrends
in brachiopods (solid line) and
bivalves (dotted line). The y-axis
indicates generic diversity, and
the x-axis indicates model (not
actual) time. The sharp drop in
hoth groups represents the extinction at the end of the Permian. After Sepkoski (1996).
600
~
i!!
~ 400
Cl
200
They move aboilt actively
instead of being attached to
a substrate, they pump water through their gills far more efficiendy, and they re ach reproductive
maturity more quickly (Vermeij
1987). These trairs indicate a superior competitive ability that could
weil have ensured bivalve dominance
even WithOllt the P/T extinction
(Briggs 1995).
Additional data bearing on this
qllestion were analyzed by Sepkoski
(1996), who usecl coupled logistic
equations to represent the diversity
trends of brachiopods and hivalves
before and after the P/T extinction
(Figure 2). Sepkoski's analysis indicated that a brachiopod decrease,
accompanied by a bivalve increase,
had been weil established before the
extinction. Both groups dedined at
the end of the Permian, but after a
recovery period the previous relationship was resumed. Sepkoski suggested that the diversity histories of
the two grollps were consistent with
the hypothesis that the brachiopods
had been displaced as a result of
competition with bivalves.
The history of the replacement of
brachiopods by bivalves is similar to
that of other marine replacements
for which mass extinctions were not
implicated. The stalked crinoids,
which were once common on the
continental shelves, became restricted
to waters deeper than 100 m after
the Jllrassic (Meyer and Macurda
1977). Their shell habitat was taken
over by their relatives, the mobile,
active feather stars. Similarly,
glypheoid crustaceans, which were
once widespread in the shallow waters of the early Mesozoic, are now
represented by a single deep-water
species (Forest et a1. 1976). Its modern descendants, the lobsters and
crabs, now occupy the shallow waters. In addition, the oldest stock of
stomatopod crustaceans, the once
widespread family Bathysquillidae,
100
200
300
400
Time
is now found at depths from 200 m
to 1500 rn (Manning et al. 1990).
Their former shallow water habitat
is now occupied by more recent stomatopod families.
Many other marine examples of
replacements of primitive dades by
more advanced groups could be cited.
Such onshore-to-offshore evolutionary progressions, in which major new
dades appeared first in near-shore
settings and then expanded to offshore settings, are common in the
history of marine life (Jablonski et
at. 1983); however, none can be attributed primarily to a catastrophic
extinction.
Synapsids versus archosaurs
The qllestion of how the synapsids
(mammal-like reptiles) came to be
replaced by the archosaurs (especially the dinosaurs) is perplexing
becallse litde evidence is available to
indicate wh at might have happened
in the history of that relationship.
Charig (1984) ernphasized that at
the beginning of the Triassic, the
large land vertebrates were nearly all
therapsids (a group of synapsids),
but by the end of the Triassic, they
were nearly all archosaurs. He suggested that the competitive success
of the latter was due to improvements in locomotion and a large increase in size. New feeding adaptations may also have aided the
archosaur radiation (Zawiskie 1986).
What did happen to the synapsids
during the 40 Ma of the Triassic?
Although the Norian Stage (Table 1)
is generally referred to as the stage
during which the greatest Triassic
extinction took place, Benton (1990)
concluded that thefe were three separate extinctions, none dearly more
extensive than the other two. Taken
together, these extinctions, which
391
Table 1. The stages of the Late Triassic.
Stage
Rhaetian
Norian
Carnian
Age (Mal
209.5-208
224.4-209.5
235-223.4
were protracted, apparently occupied most of the Late Triassic.
New studies of the earliest dinosaurs have shown that aIl three main
lineages (i.e., the theropods, sauropodomorphs, and ornithischians)
were probably present during the
Carnian Stage of the Late Triassic.
At that time, dinosaurs were small in
size and represented less than 6 % of
the tetrapod diversity (Ben ton 1993).
Benton suggested that one of the
three extinctions took place near the
Carnian-Norian boundary, approximately 220 Ma ago. He linked this
event with the formation of the
Manicouagan crater in Quebec, suggesting that an asteroid impact had
caused the extinction. But as
Fastovsky and Weishampel (1996)
have noted, the Quebee erater is
somewhat older than or intermediate in age between the two Late Triassic extinctions, so a linkage does
not appear to be possible.
There are two further objections
to the asteroid-impact scenario. First,
the evidence that a major extinction
occurred approximately 220 Ma is
based on conjecture. Studies of family diversity in the marine environment have not provided evidence of
this extinction (Sepkoski 1992), and
evidence of a sudden decline in terrestrial diversity also appears to be
lacking. Seeond, stratigraphie ranges
of the two major therapsid groups
(i.e., the dicynodonts andcynodonts)
show the first terminating at the end
of the Triassie and the second extending well into the Jurassie (Carroll
1988). During the Norian, each
group apparently underwent a
gradual, noncatastrophic decline.
There is lüde evidence, therefore, to
support the theory that the Norian
radiation of the dinosaurs represents
an opportunistic replacement following a mass extinction.
Dinosaurs versus mammals
The best-known, and most frequently
discussed, biotic succession is that in
which the mammals replaced the di-
392
nosaurs. This replacement has been
used co illusrrate a benefit of mass'
exrinction (Stanley 1987). In fact, it
is widely believed that humans are
here because thc dinosaurs were
eliminated in a great extinction event.
Many schoolchildren are familiar
wirh the story of a great comet coming out of the sky to hit the earth and
destroy the dinosa urs and most other
living things. Indeed, the educated
public believes that mammals had
made virtually no evolutionary
progress untiI the great feptiles were
gone. However, a look at the evidence shows that the mammals were
weIl established by the time the dinosaurs began their decline.
Mammals are a well-defined
monophyletic graup (a related assemblage with a common ancestor),
whose lineage diverged from other
amniotes more than 300 Ma ago.
The therapsid lineages began to show
advanced mammalian features in the
Late Triassie, more than 200 Ma aga.
One group of true mammals, the
monotremes (wh ich are found today
only in Australia) probably diverged
from the marsupial-placental clade
more than 150 Ma ago. The most
common Mesozoic mammals were
the multitubcrculates, wh ich are
known from the Upper Jurassie into
the Oligocene, aperiod of approximately 100 Ma.
The marsupials and the placentals
probably diverged from a common
ancestor in the Early Cretaceous
(Carroll1988). Three recent studies
provide essential information on the
evolution of the placental mammals.
First, Archibald (1996a) reported Lev
Nessov',s discovery of fossils of ungulate (hoofed) mammals from the
85-Ma-old Bissekty Formation in Uzbekistan. This clade includes the
modern orders Artiodactyla. Cetacea, Hyracoidea, Perissodactyla,
Proboscidea, and Sirenia. Second,
Hedges et al. (1996) repoeted molecular estimates of divergence times
of several other orders that indicated
ages of approximately 100 Ma.
Third, Cooper and Penny (1997) presen ted molecular data indicating that
at least 20 groups of modern mammals evolved in the Cretaceous. Together, these three studies show that
several orders of living placental
mammals were weIl established before the K/T boundary time of 65 Ma
ago. If so, a variety of advanced
mammals had developed and lived
with the dinosaurs foe more than 30
Ma. Moreover, by the time the large
dinosaurs became extinct, mammals
had been continuously evolving for
more than 200 Ma. With such a
radiation so weil under way, there
seems to be no reason to ass urne that
the extinction of the dinosaurs made
the subsequent evolution of mammals possible.
Indeed, it is not even dear that the
dinosaurs underwent a sudden extinction at the KfT boundary. AIthough some paleontologists have
argued for a sudden extinction of the
dinosaurs (Sheehan et a1. 1991), others have insisted that their demise
was gradual and was unlikelyto have
been caused by a catastrophic event.
For example, Sloan et a1. (1986)
found evidence of a decline in dinosaur diversity beginning approximately 7 Ma before the Krr boundary. The fossil material they
examined suggests that a competition with the rapidly evolving ungulates may have taken place. Moreover, for approximately the final 10
Ma of the Cretaceous, the number of
dinosaur genera decreased from approximately 32 to 19 (Clemens 1992).
Dinosaur eggshells found in the Early
Tertiary strata of the Nanxiong Basin in southeastern China appeared
to indicate a "periodical poisoning"
of the ecosystem beginning at the K!f
boundary (Stets et al. 1996). These
researchers concluded that the dinosaurs in that area had undergone a
stepwise extinction triggered by environmental stress.
The gradual extinction of the dinosaurs was probably due to a drastie drop in primary production caused
by a combination of environmental
events, such as sea level regression,
climatic change, volcanism, and possibly the effect of an asteroid impact
(Briggs 1995). Competitionfromsmall
ungulate herbivores may have been
an important factor in the dedine of
dinosaurs. Ungulate survival during
the environmental stresses of the KIT
boundary might have been enhanced
due to their individually small demands on primary production.
Considerable evidence thus exists
to indicate that the case ofthe dinosaurs versus mammals may fit the
double-wedge pattern-that is, a
BiaScience Val. 48 No. 5
gradual die-off of the large dinosaurs due to eompetition as weil as
to environmental change. Onee the
Jarge dinosaurs werc gone, mammals
continued their impressive radiation.
Additional insight into thc replaeement proeess may be gained by eonsidering three other sets of vertebrates
as weil as some supplemental information about invertebrates and plants.
Other replacements
Figure 3. Historical
changes in the r,pecies
diversity of the major
groups 01 vascular
plants duringthe Phanerozoic. S. Silurian;
DEV, Devonian; MISS,
Mississippian; PENN,
Pennsylvanian; PERM,
Permian; 'fR, Triassic;
JUR, Jurassic; L.K.,
Lower Cretaceous;
U.K., Uppcr Crctaceous; Pg, Paleogene;
Ng. Neogene. After
800
·u'"
700
Q)
Q)
600
a.
500
0
400
-'"
~
Q)
.0
E
Angiosperms
?---'*--.....
300
=> 200
Gymnosperms
Z
100
0
Pteridophytes
DEV
MISS PENN PERM
TR
LX
U.K
Pg
Ng
The Mesozoic Amphichelydia were
a primitive group of turdes that were Niklas (1986).
unable to tuck their heads and necks
inside their shells. They were replaeed alty attributed to the K/T extinction of clade replaeement within the
in the Northcrn Hemisphere by the was the marsupials. In North strombaidean gastropods (Ray 1996)
Cryptodira, a group that had devised Ameriea, 11 marsupial species were indicated thatthc family Aporrhaidae
a method of retracting their hcads apparently redueed to one or zero evolved during the Late Triassie and
and necks by vertical flexure (Rosen- (Sheehanand Fastovsky 1992). How- became an important component of
zweig and McCord 1991). The latter ever, this reduction in marsupial di- the gastropod fauna. The family
are known from the Upper Creta- versity has been eorrelated with the Strombidac arosc from an aporrhaid
ceous ofNorth America and Asia. In introducrion of new placental mam- ancestralline in the Lare Cretaceous.
western North Ameriea, the replaee- mals (Savage 1988, Archibald 1996b). The K/T extinetion removed approxime nt of the Amphiehelydia pro- Cretaceous distributions of the two mately 76% of the aporrhaid genceeded gradually, at thc ratc of 8- groups suggest that a Laurasian an- era, but strombid diversity was unaf9% per 5 Ma, until theKffboundary. cestral mammal may have given rise fected. In the Tertiary, the aporrhaid
That extinetion event was assoeiatcd to marsupials in Westamerica and to genera continued their decline, while
with a 24 % replaeement. The KIr placenrals in Asia. With the advent the latter increased their diversity.
change thus appears to have aceeler- of the Bering connection in the Late The strombids gradually replaeed the
ated the turnover. The incumbent- Cretaceous, new placentals may have aporrhaids in the tropics, and the
replaeement hypothesis prediets that been introduced to Westameriea, to aporrhaids are now restricted to
the amphiehelydians were not out- the eventual detriment of rhe marsu- colder, deeper waters. Roy (1996)
eompeteJ by the cryptodires but that pials; a more efficient reproductive suggested that these historie patterns
the eryptodires simply took over the system in the placentals prohably were the result of three causes: biotie
amphichelydians' niehe as they died provided them wirh a selecrive ad- interaerions, KIT extincrion, and environmental change.
out. However, 1 think it likely that vantage (Li11egraven 1975).
the eryptodires outeompeted the
The stresses of the KIT boundary
As al ready noted, Vermeij (1991)
amphyehelydians because they were certainly affeeted the terrestrial ver- praposed that the suceess of the
better ahle to proteet themselves from tebrate fauna, but these stresses gen- Plioeene invasion of Paeifie mo11userally seern only to have aceelerated can speeies into the Atlantie Ocean
predators.
Anorher reptilian extinetion that changes, possibly due to eompeti- was due to a prior extinction in the
is unlikelyto be due ro a eatastrophie tion, that were already in progress. Atlantic (his hypothesis of ecologievent is that of the pterosaurs. The At thc spccics level, rhe survival rate eal opportunity). However, atthetimc
fossil reeord of pterosaurs extends of terrestrial vertebrates may have of the transarctie invasion, approxifor approximately 150 Ma; nearly been as mueh as 64%, with most of matcly 3.S Ma ago, the Arctie Oeean
90 species have been recorded. They the losses oecurring in the dinosaurs was iee free , and boreal (cold-temdid not survive beyond the end of the and marsupials (Archibald 199Gb). perate) conditions prevailed (Golikov
Cretaeeous, but the K/T boundary Among the terrestrial invertebrates, and Scarlato 1989). Approximately
marked the extinetion of only a few there is no evidence of a major KIT 2.9-2.4 Ma ago, the cooling intensimembers of a single lineage within extinction. Studies of Phanerozoic fied, and glaeiers ealved ieebergs into
an order that had been gradually diversity in inseet families (Laban- the sca (Raymo 1994). Most of the
diminishing throughout the period deira and Sepkoski 1993, Jarzem- boreal speeies were eliminated, and
(Carroll 1988). A more dramaric bowski and Ross 1996) do not indi- the modem Aretie fauna began to
decline oecurred at the end of rhe eare a decrease at the KlT boundary develop. This evidence means that
Jurassie, an event that eoincided with bur onry a gradual dec1ine throughout the Arlantic boreal species of Pacifie
the emergence of birds. The two the Cretaceous that has been linked origin had been in plaee several hungroups had lived in eompetition for with the rise of angiosperm plants dred thousand years hefore the cool(Labandeira and Sepkoski 1993).
ing (extinction) episode. Conseapproximately 80 Ma.
Another vertebrate group to have
The stresses of the K/T boundary quently, the extinction appears to
undergone a major loss that is gener- also affected marine fauna. A study have taken place after the invasion,
May 1998
393
not before, so it could not have facilitated the faunal invasion.
The fossil record of plant life also
provides useful comparative information about dade replacement.
Large-scale changes in higher plant
communities occurred during the
Phanerozoic (Figure 3; Niklas 1986).
The gymnosperms began their development in the mid-Paleozoic and for
approximately 100 Ma increased
their diversity along with the pteridophytes. In the latter part of the
Paleozoic, gymnosperms became
somewhat more diverse. Both groups
suffered .lmost equ.lly in rhe PIT
extinction, butthe gymnosperms subsequently continued their expansion,
whereas the pteridophytes played an
increasingly minor role. The angiosperms arose in the beginning of
the Cretaceous and quickly developed into the dominant plant group.
In each case, the more primitive group
that was eventually overtaken continued to prosper for approximately
100 Ma beyond the origin of its
successor. Both pteridophytes and
gymnosperms eventually began to
dedine, but these changes had become evident before the PIT and KIT
boundaries, respectively. The angiosperm increase continued unabated, despite a considerable loss
of Northern Hemisphere biomass in
the KfT crisis.
Conclusions
The present era of neocatastrophism
has provided an intellectual dimate
that is receptive to the multiplication
of theories that emphasize the influence of abiotic factors on evolutionary progress. Several of the theories
propose that without the stimulus of
physical change, evolutionary stagnation (stasis) would occur. These
ideas represent a philosophical shift
away horn the Darwinian concept of
evolution by rneans of competitive
inter action. Furthermore, it has been
commonly assumed that catastrophic
extinctions, which generally emanate
frorn physical disturbance, confer
evolutionary benefits. However, eonsiderable evidence now suggests that
such extinctions interrupted evolutionary trends and were so destructive that after each one it taok the
earth several million years to rebuild
its former biotic diversity.
394
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Acknowledgments
I wish to thank Kevin Padian, Greg
Tolley, and Rebecca Chasan for their
useful suggestions.
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lohn C. Briggs is a professor emeritus in
the Department oi Marine Science at the
University of South Florida and an adjunet professor in the Department of Marine Science and in the Museum ofNatural
History at the University of Georgia. His
address is Zo%gica/ Colleetion, Museum
of Natural History, University of Georgia, Athens, GA 30602. © 1998 American
Institute of Biologicaf Sciences.
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