From _Skeptic_ vol. 1, no. 3, Fall 1992, pp. 38-47.
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PUNCTUATED EQUILIBRIUM AT TWENTY: A PALEONTOLOGICAL PERSPECTIVE
By Donald R. Prothero, Ph.D.
_"It was twenty years ago today, Sgt. Pepper taught the band to
play . . ."_
In many ways Niles Eldredge and Stephen Jay Gould taught paleontology to
play twenty years ago, publishing a paper that helped revitalize the
science. Long associated in the public minds with musty old bones,
paleontology had the well-deserved reputation of being a stagnant
backwater among the sciences.
Before the seventies, most college paleontology classes were little
more than rote memorization of fossil names and anatomy. In his preface
to the 1972 book _Models in Paleobiology_ (where the punctuated
equilibrium paper first appeared), Tom Schopf pointed out that a typical
dissertation in paleontology consisted of describing some new fossils,
with little thought about their broader theoretical implications, or
about the possibilities for asking novel questions of the fossil record.
Virtually all the paleontology textbooks of the time (such as the
classic text by Moore, Lalicker and Fischer, first published in 1952)
were simply compendia of fossils, and the broader theoretical issues
were confined to few sketchy introductory chapters. The meetings of the
Paleontological Society at the Geological Society of America convention
were dominated by descriptive papers ("a new fauna from X" or "a new
species of Y"), with only occasional broader theoretical papers that
appealed to anyone other than the narrow specialist. This approach was
called _idiographic_ by Gould (1980a), since it focuses on studying the
objects for their own sake. Others sneered and called it "stamp-
In the late sixties and early seventies, however, this situation
changed radically. Perhaps the student activism of the sixties
penetrated paleontology, or maybe the emphasis on ecology and holistic
viewpoints were influential. In any case, a new generation of "young
Turks" who finished their Ph.D.'s in the late sixties led a revolution
that shook up the musty old profession. They emphasized thinking of
_fossils as organisms_, rather than dead objects to be described,
catalogued, and put away in a museum drawer. In their papers and books,
they applied ideas from modern biology--ecology, speciation theory,
diversity and variation, population genetics, and many other concepts--to
the fossil record. Although they recognized the limitations of the
fossil record, they also found many instances where biological models
lent new perspectives on long-studied fossils. Gould (1980a) called this
the _nomothetic_ approach, since it seeks to find general, law-like
properties among all the idiographic details.
In 1971, David Raup and Steve Stanley published a radical new
textbook entitled _Principles of Paleontology_. Unlike any paleontology
text before (or since), it had no descriptions of fossil invertebrates;
it was entirely focused on the theoretical issues of how we interpret
the fossil record, and what we can (and can't) learn from it. In 1972
Tom Schopf edited _Models in Paleobiology_ (mentioned above), which
contained a number of influential papers emphasizing new conceptual
approaches to the fossil record. By 1975 Tom Schopf and Ralph Johnson
had founded the journal _Paleobiology_, which carried only papers of
general theoretical interest; descriptive papers stuck to the venerable
_Journal of Paleontology_. Since that time, the program of the
Paleontological Society meetings has been packed with mind-boggling (and
sometimes numbing) theoretical papers; abstracts of papers aimed at
narrow specialists are rejected. Ultimately, the Paleontological Society
recognized the influence of the generation of "young Turks" by
establishing the Charles Schuchert Award for the outstanding
paleontologist under the age of 40.
Although the original "young Turks" are now middle-aged, a new
generation of paleontologists that they have trained or influenced
dominates the profession. (My first freshman paleontology class in 1973
was taught using the brand-new Raup and Stanley text for the first time
in my professor's career). _Paleobiology_ has been joined by _Historical
Biology_, _Lethaia_, _Palaios_, and other journals which emphasize
papers of broad theoretical interest. More importantly, paleontology is
no longer an intellectual backwater. Paleontological data and ideas are
shaking up evolutionary theory. The controversy over mass extinctions
(and whether they are periodic or extraterrestrially caused) has been
written up in several best-sellers, made the cover of _Time_ magazine,
and stimulated the public debate about modern extinctions due to
environmental destruction by humans. Dinosaurs are the hottest fad for
kids of a certain age, although this rarely translates into careers in
paleontology. (Like many paleontologists, however, I'm one of those kids
who got hooked on dinosaurs at age 4 and never grew up). Paleontology
has always gotten front-page billing for amazing idiographic wonders
like giant dinosaurs, but now general, nomothetic ideas from
paleontology are also influencing the rest of the scientific community.
The earliest and most influential of all was punctuated equilibria.
The Birth of "Punk Eek"
Since his 1942 classic _Systematics and the Origin of Species_, Ernst
Mayr has led the biological community in research in speciation theory.
In 1954, Mayr proposed the _allopatric speciation model_. According to
this idea, new species usually do not arise within the main body of a
population, because the genetic exchange between organisms rapidly
swamps any new variations. Instead, small subpopulations which are
genetically isolated from the main population are more likely to change,
because an evolutionary novelty has a much better chance of dominating a
small population than a large one.
This can be due to several factors. Many small populations,
particularly those founded by a small number of settlers on an island,
show the _founder effect_. The founders were a small subsample of the
mainland population which may have had unusual gene frequencies (simply
by accident of sampling), and all of their descendants will carry those
genes. The founder effect need not be confined to islands, however. The
Amish and Mennonites, who live among the rest of the American population
but rarely interbreed for religious reasons, have many unusual genes.
Another possible cause is _genetic drift_. If a high percentage of
genes are invisible to natural selection (as much research now shows),
then they can randomly mutate without being weeded out. Ultimately, this
random walk of mutation (or "genetic drift") can produce something which
may have a selective advantage--or may be deleterious. Either way, it
has a much better chance of becoming dominant in a small population that
is genetically isolated from its ancestors.
These populations are said to be _allopatric_, or living in "another
homeland."If their isolation is long enough, they become so genetically
different that when they are reintroduced or reinvade their original
homeland (become _sympatric_), they can no longer interbreed with the
ancestors; they have become a new species. This new species may die out
quickly, or it may drive its ancestor to extinction, or both may persist
side-by-side, typically by exploiting slightly different ecological
niches. In paleontological terms, the allopatric speciation model
predicts that species arise rapidly (a few hundred to a thousand years,
but instantaneous in a geological sense) on the periphery of their range
(where they are rarely fossilized). It predicts that the main population
(most likely to be fossilized) will show little or no change, but will
be suddenly invaded by new species with no apparent transitions between
Despite the harsh words of critics and derogatory labels (such as
"evolution by jerks" or "punk eek"), the original formulation of
punctuated equilibria in 1972 was remarkably modest. As recounted by
Eldredge (1985a) and Gould (1992), they were originally both graduate
students at the American Museum of Natural History in New York. At the
turn of the century the American Museum had once dominated vertebrate
paleontology, but they came there to study with Norman Newell, who had
an interest in evolutionary problems in fossil invertebrates. Both
Eldredge and Gould found that tracing evolution in their chosen
organisms (trilobites and land snails, respectively) was difficult; most
of their fossils showed no change through thousands to millions of years
of strata. In 1971, Niles Eldredge published a paper in _Evolution_
which attempted to explain this apparent lack of change. Their joint
paper published the next year in the Schopf volume, however, has been
the focus of all the controversy.
Since the allopatric model had been dominant in biology for decades
before Eldredge and Gould, it seems surprising that paleontologists
ignored its implications for the fossil record. Some of this may have
been inherent conservatism, or ignorance of biology, but it also had
deeper philosophical roots. As Eldredge and Gould (1972) pointed out,
paleontologists were raised in a tradition inherited from Darwin known
as _phyletic gradualism_, which sought out the gradual transitions
between species in the fossil record. They viewed species as part of a
continuum of gradual change in anatomical characteristics through time.
The classic metaphor showed each species as part of a bell-shaped
frequency curve, with the mean shifting gradually up through time
(_Figure 1_). Each species was thus an arbitrary slice through a
continual lineage, and paleontologists agonized for years as to whether
these arbitrary slices should be designated species. Indeed, this debate
had its own label: "the species problem in paleontology."
Even their detractors concede that Eldredge and Gould were the first
to point out that modern speciation theory would not predict gradual
transitions over millions of years, but instead the sudden appearance of
new species in the fossil record _punctuated_ by long periods of species
stability, or equilibrium. Eldredge and Gould not only showed that
paleontologists had been out-of-step with biologists for decades, but
also that they had unconsciously trying to force the fossil record into
the gradualistic mode. The few supposed examples of gradual evolution
were featured in the journals and textbooks, but paleontologists had
long been mum about their "dirty little trade secret:" most species
appear suddenly in the fossil record and show no appreciable change for
millions of years until their extinction.
When the punctuated equilibrium paper first came out, reactions were
mixed. Since 1972 there have been many traditional paleontologists who
denied its importance, and trotted out their favorite example of gradual
evolution. Many of these "classic" examples were restudied in critical
detail, and turned out to be ambiguous, or actually demonstrated
punctuated equilibria better than gradualism.
There were a host of more trivial objections and misunderstandings,
which have been discussed by Gould and Eldredge (1977) and Gould (1992).
Most studies fell short because they focused on a single lineage
(neglecting faunal variation) from a single section (neglecting
geographic variation), often showing change in only one characteristic
(neglecting morphological variation), which had not been analyzed by
rigorous statistical methods. Other cases failed because they were on
the wrong time scale to be relevant to the debate, or too poorly dated
to know anything about change through time.
For example, one of the main proponents of gradualism, Philip
Gingerich (1976, 1980, 1987), showed just two or three examples of
supposed gradual evolution in early Eocene (about 50-55 million years
old) mammals from the Bighorn Basin of northwestern Wyoming. But a
detailed examination of the _entire mammal fauna_ (monographed by Bown,
1979, and Gingerich, 1989) shows that most of the rest of the species do
not change gradually through time. Also, studies on specific lineages in
restricted areas cannot account for the possibility that a gradual
transition may actually reflect the migration of a clinally varying
population across a region through time. This was documented by
Schankler (1981), who showed that some of Gingerich's patterns from the
northern Bighorn Basin did not even hold up in the southern Bighorn
Basin, just a few dozen miles away!
As Gould and Eldredge (1977) pointed out in their five-year
retrospective on the debate, it's easy to pick one specific example of
either gradualism or punctuation, but the important issue is one of
generality. Which pattern is dominant among the species in the fossil
record, since both are known to occur? If you sample all the members of
a given fauna, which pattern is most common? In the twenty years since
the paper, more and more case studies have been generated, and by now a
pattern seems to be emerging (Gould, 1992; Stanley, 1992).
It is now clear that among microscopic protistans, gradualism does
seem to prevail (Hayami and Ozawa, 1975; Scott, 1982; Arnold, 1983;
Malmgren and Kennett, 1981; Malmgren et al., 1983; Wei and Kennett,
1988, on foraminiferans; Kellogg and Hays, 1975; Kellogg, 1983; Lazarus
et al., 1985; Lazarus, 1986, on radiolarians, and Sorhannus et al.,
1988; Fenner et al., 1989; Sorhannus, 1990, on diatoms). As discussed by
Gould and Eldredge (1977) and Lazarus (1983), this may be due to the
fact that most of these organisms are either asexual clones, or show
alternation of of sexual and asexual generations. Each cloning lineage
is distinct and many never interbreed with other lineages, so the issue
of gene exchange and homogenization may be moot. They do not fit the
genetic models that biologists developed from complex sexual organisms
such as insects and birds. In addition, they live in enormous (trillions
of individuals) populations that span entire oceanic water masses, so
they do not form many small, isolated populations (Prothero and Lazarus,
1980; Lazarus, 1983; Lazarus and Prothero, 1984). Finally, many of the
morphological variants that we call species may in fact be the same
genetic lineage which responds to different environmental conditions
with different anatomical features. This is called _ecophenotypic_
variation, and appears to be very common in planktonic microfossils.
Perhaps much of the morphological change seen in microfossils does not
reflect any underlying genetic change, but is simply an ecophenotypic
response to the changing environment (Lazarus, 1983).
Among more complex organisms, however, the opposite consensus had
developed. As paleontologists had known for over a century, most species
are stable for millions of years, and change so rapidly that we rarely
witness it in the fossil record. Of the hundreds of studies that have
been reviewed elsewhere (Gould and Eldredge, 1977, 1986; Gould, 1992), a
few stand out (Stanley, 1992). Cheetham (1986) and Stanley and Yang
(1987) examined all the available lineages of their respective groups
(bryozoans and bivalves) through long intervals of time, using
multivariate analysis of multiple character states. Both concluded that
most of their species were static through millions of years, with rare
but rapid episodes of speciation. Williamson (1981, 1985) examined the
details of evolution of molluscs in Lake Turkana, Kenya, and showed that
there were multiple examples of rapid speciation and prolonged stasis,
but no gradualism. Barnosky (1987) reviewed a great number of different
lineages of mammals, from mammoths to shrews and rodents, that lived
during the last two million years of the Ice Ages. He found a few
examples of gradualism, but many more which showed stasis and
My own research (Prothero and Shubin, 1983; Prothero, 1992; Prothero,
Heaton, and Stanley, in press) examined all the mammals with a
reasonably complete record from the Eocene-Oligocene (about 30-35
million years ago) beds of the Big Badlands of South Dakota and related
areas in Wyoming and Nebraska (_Figure 2_). This study not only sampled
every available lineage without bias, but also had much better time
control from magnetic stratigraphy (Prothero and Swisher, 1992) and
wider geographic coverage than the studies by Gingerich cited above.
With one exception (gradual dwarfing in the oreodont _Miniochoerus_), we
found that all of the Badlands mammals were static through millions of
years, or speciated abruptly (if they changed at all).
Contrary to claims by Hoffman (1989, 1992) that the punctuated
equilibrium model is either trivial, false, or irrelevant, it has been
one of the most stimulating and provocative hypotheses in paleobiology.
Witness the enormous literature it generated, or the fact that there
have been several recent symposia on the subject (e.g., Somit and
Peterson, 1992) and twenty-year retrospectives at national
paleontological meetings in Chicago in July, 1992, and Cincinnati in
October, 1992. Although a survey of the programs of recent meetings
would show fewer studies about evolutionary patterns than a decade ago,
there are still many new studies with major new insights being published
Stasis, Landscapes, and Polyhedra
If the punctuated equilibrium model had merely shown that the
biological species models could be applied to the fossil record, then
there would have been little controversy outside paleontology. The
fossil record would have just provided further data for biological
orthodoxy, as paleontologists such as Simpson (1944, 1953) did during
the Neo-Darwinian synthesis of the 1950s (Gould, 1983; Eldredge, 1985b).
In the 1960s, evolutionary biologists often took an extreme
panselectionist position. Natural selection was said to be constantly
acting on every tiny feature of an organism, weeding out even the
smallest imperfection. Species are arbitrary entities which constantly
track environmental change, like a ball rolling across hilly terrain.
Indeed, the popular metaphor of the time was the "adaptive landscape."
Species were always trying to reach the "adaptive peaks" of the
"landscape" and were continually modified in response to the shifting of
the peaks beneath them.
The discovery of stasis in most species for millions of years was an
fact that biologists did not expect (as even Mayr, 1992, concedes). At
first, they dismissed it as genetic homeostasis or stabilizing selection
(Charlesworth et al., 1983; Levinton, 1983; Lande, 1985). But such
models are only appropriate on scales of a few generations, or at most a
few thousand years. No environment is so constant that stabilizing
selection can act for millions of years. This type of explanation is
typical of reductionist evolutionary genetics (e.g., Dawkins, 1976),
which treats organisms as conduits for genes, and even defines evolution
as "change in gene frequencies through time." As Mayr (1992) points out,
such reductionism is now slowly going out of vogue, as biologists
realize that organisms are integrated wholes, with many different genes
interacting in complex ways.
More impressive are demonstrations of species stability _in spite of
well documented environmental change_. The fluctuations of glacial-
interglacial cycles during the last three million years of the Ice Ages
are about as extreme a climactic change as our planet experiences. Yet
studies from land mammals (White and Harris, 1977; Barnosky, 1987) to
microscopic marine ostracodes (Cronin, 1985, 1987) document extreme
stability in most species in spite of these changes. Rather than adapt
to new environments, species migrate back and forth in response to them.
My own research on the Eocene-Oligocene transition about 34 million
years ago (Prothero and Berggren, 1992) documents a similar phenomenon.
Most of the mammals from the Badlands discussed above (Prothero and
Shubin, 1983; Prothero, 1992) show remarkable stability over an interval
of well documented climactic change (_Figure 2_). Evidence from
paleosols and land floras (Retallack, 1992) document a striking cooling
and drying event across this boundary, with a woodland vegetation
(greater than 1000 mm annual precipitation) replaced by a wooded
grassland (500 mm annual precipitation). According to Wolfe (1992), mean
annual temperature declined almost 13 degrees C, and the annual range of
temperature increased dramatically from 5 degrees C to about 25 degrees
C. Sedimentological evidence from eastern Wyoming (Evanoff et al., 1992)
shows an abrupt transition from moist floodplains to semi-arid
landscapes with abundant wind-blown volcaniclastic dust. Most of these
events took place over a few thousand years. This is certainly one of
the most severe climactic events since the extinction of the dinosaurs.
Late Eocene land snails (Evanoff et al., 1992) were large-shelled
subtropical taxa now typical of central Mexico, indicating a mean annual
range of temperature of 16.5 degrees C and annual precipitation of about
450 mm. In the early Oligocene, these were replaced by drought-tolerant
small-shelled taxa indicative of a warm-temperate open woodland with a
pronounced dry season. Reptiles and amphibians (Hutchison, 1992) show a
trend toward cooling and drying, with aquatic forms (crocodilians,
freshwater turtles, and salamanders) replaced by land tortoises; size
reduction in turtles also indicates increased aridity.
In spite of all these changes, however, only one lineage of fossil
mammal underwent a gradual change. All of the rest either remained
unchanged through the interval, or went extinct, with new species
replacing them. None showed the panselectionist prediction of gradually
evolving to track their changing environment.
If species are static through millions of years in spite of
environmental changes, then there must be some sort of homeostatic
mechanism that preserves this stability beyond what traditional
reductionist Neo-Darwinism once postulated. Mayr (1992) argues that it
is merely the integration of species as complex wholes, so that small-
scale changes are insufficient to upset the complex balance of
integrated genes. Others suggest that fundamental developmental
constraints play an important role in restricting the possible avenues
of change (Gould and Lewontin, 1979; Kauffman, 1983). Still others
suggest that there might be properties of species that may not have been
discovered yet by geneticists and evolutionary biologists, properties
which operate on scales of millions of generations and years (Vrba and
Instead of the "rolling ball" metaphor so favored by evolutionary
biologists, perhaps species are more like a _polyhedron_, which can roll
rapidly over from face to face, but resists change when it is sitting on
one of its stable faces (Gould, 1980b). Change only occurs when the
threshold necessary to tip it over has been exceeded, and then the
polyhedron will resist further change until that threshold is once again
reached. Between stable states (the faces), however, the transitions are
very rapid. This kind of phenomenon is very similar to catastrophe
theory (Schubert, 1992) and other theoretical models of discontinuous
change (Masters, 1992).
Species Sorting and Macroevolution
The other major implication of the idea that species are static for
millions of years is the implication for the reality of species.
Traditionally, species were considered the sum of all their component
populations, and all processes (such as selection) operated on the level
of individual and populations. But if species are not just arbitrary
slices of a continuum, but distinct entities with their own history of
"birth" (speciation) and "death" (extinction), then perhaps species have
characteristics that operate on a _hierarchical level above that of the
genes, the individual, or the population_. This concept of _hierarchy_
(species are made up of populations, populations are made of up
individuals, individuals are made up of genes, etc.) has important
implications for evolutionary biology (Gould and Eldredge, 1977; Gould,
1982a, 1982b; Vrba and Eldredge, 1984; Salthe, 1985; Eldredge, 1985b;
Gould, 1985; Vrba and Gould, 1986).
Although glimmerings of this idea were present in the original 1972
Eldredge and Gould paper, it first emerged explicitly in a brief paper
by Stanley (1975) followed by his stimulating and controversial book
Macroevolution (1979). Stanley called this concept "species selection,"
and it was the basis for a new round of debates for over a decade. Since
the original proposal, Vrba and Gould (1986) have since argued that it
should be called "species sorting," since the process is not really
analogous to natural selection on the level of individual populations.
In a nutshell, the argument postulates that species are real entities
which have characteristics that are more than the sum of the
characteristics of their component populations. When two or more species
come into competition, the differential survival which sorts out the
"winners" and "losers" may be due to these intrinsic species properties,
rather than natural selection on individuals or populations. The causes
of the survival of a given species cannot be reduced directly to the
survival of its component populations, but seems to be due to properties
which are species-specific.
For example, the tendency of a group to speciate rapidly or slowly is
not a property of its component individuals. Organisms do not speciate,
species do. Elisabeth Vrba (1980, 1985) has suggested that the antelopes
of Africa provide an example of this. The relatively conservative impala
clade seems to have an intrinsically low rate of speciation. Only three
very similar species in one lineage are known for the last five million
years. By contrast, the wildebeest tribe has speciated profusely during
the same period of time, with multiple episodes of evolutionary
branching and extinction. Hansen (1978, 1982) argued that marine snails
without planktonic larvae speciated more rapidly than those who disperse
their larvae as plankton all over the ocean. The less mobile non-
planktotrophic snails are more likely to be genetically isolated than
species whose planktonic larvae spread their genes all over the ocean.
Since the larval condition is a property of the species, not merely of
its component individuals, it might represent an example of species
Two groups of South American burrowing rodents, the tuco-tucos (genus
_Ctenomys_) and the coruros (genus _Spalacopus_) both have evolved
adaptations for a burrowing, gopher-like existence (Vrba and Gould,
1986). Tuco-tucos are far more speciose the coruros, even though they
have the same ecology and home range. The difference lies in the fact
that tuco-tucos have very low gene flow, so they can speciate rapidly,
while coruros are genetically homogeneous. Other possible examples of
species sorting were reviewed by Gilinsky (1986).
Traditional Neo-Darwinists have failed to see any difference between
traditional natural selection and species sorting (Mayr, 1992; Hecht and
Hoffman, 1986; Hoffman and Hecht, 1986; Hoffman, 1982, 1984, 1989,
1992). In reading the literature, it is clear that the debaters are
talking past each other, since each has fundamentally different
perceptions of the world. Traditional Neo-Darwinists come from a
reductionist viewpoint that cannot see species as entities, even after
all the evidence that has accumulated. The opposing camp sees the world
as hierarchically ordered, with each level having its own reality. As
long as this fundamental difference in worldview underlies the argument,
neither side will convince the other, even with the clearest possible
More is at stake here than the reality of species, however. If
species sorting is real, then the processes operating on the level of
species (_macroevolutionary processes_) are not necessarily the same as
those operating on the level of individuals and populations
(_microevolutionary processes_). In other words, macroevolution may not
just be microevolution scaled up. After decades of experiments on fruit
flies, the most interesting evolutionary phenomena might only be studied
in the fossil record, or in the embryology lab. With publications,
prestige, and grant money on the line, the traditional research
community of evolutionary biologists do not want to find themselves
suddenly irrelevant to the most interesting issues in macroevolution. On
the other hand, paleontologists have begun to shed their subservience to
evolutionary biology (Gould, 1983), and assert the importance of the
fossil record for detecting phenomena that are too large in scale for
biologists to observe (Gould, 1982a, 1982b, 1985; Eldredge, 1985b).
Clearly, all of evolutionary biology is undergoing ferment and change.
To the paraphrase the old Chinese proverb, we indeed live in interesting
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Donald R. Prothero is Chairman and Associate Professor of Geology at
Occidental College, Los Angeles, California. He has been a Guggenheim
and NSF Fellow, a Fellow of the Linnean Society, and in 1991 received
the Schuchert Award of the Paleontological Society for outstanding
paleontologist under the age of 40. He has authored or co-edited seven
books, including _The Evolution of Perissodactyls_ (Oxford Univ. Press,
1989, co-edited with R.M. Schoch), _Interpreting the Stratigraphic
Record_ (W.H. Freeman, 1990), _Eocene-Oligocene Climactic and Biotic
Evolution_ (Princeton Univ. Press, 1992, co-edited with W.A. Berggren),
_Evolution of the Earth_ (McGraw-Hill, 1993, with R.H. Dott), _Paradise
Lost: The Eocene-Oligocene Transition_ (Columbia Univ. Press, 1993),
_Horns, Tusks, Hooves and Flippers: The Evolution of Hoofed Mammals and
Their Relatives_ (Princeton Univ. Press, 1993, with R.M. Schoch), and
_The Terrestrial Eocene-Oligocene Transition in North America_
(Cambridge Univ. Press, 1994, co-edited with R.J. Emry). He is a
Technical Editor of _Journal of Paleontology_ and Adjunct Editor of