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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-
collecting."
   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 
them.
   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. 

The Debate

   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 
punctuation. 
   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 
every year.

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 
Eldredge, 1984). 
   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 
sorting. 
   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 
examples.
   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 
times.

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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 
_Paleobiology_.

