Adaptation has been defined and recognized by two different criteria: historical genesis (features built by natural selection for their present role) and current utility (features now enhancing fitness no matter how they arose). Biologists have often failed to recognize the potential confusion between these different definitions because we have tended to view natural selection as so dominant among evolutionary mechanisms that historical process and current product become one. Yet if many features of organisms are non-adapted, but available for useful cooptation in descendants, then an important concept has no name in our lexicon (and unnamed ideas generally remain unconsidered): features that now enhance fitness but were not built by natural selection for their current role. We propose that such features be called exaptations and that adaptation be restricted, as Darwin suggested, to features built by selection for their current role. We present several examples of exaptation, indicating where a failure to conceptualize such an idea limited the range of hypotheses previously available. We explore several consequences of exaptation and propose a terminological solution to the problem of preadaptation.

Bones of recent mammals in the Amboseli Basin, southern Kenya, exhibit distinctive weathering characteristics that can be related to the time since death and to the local conditions of temperature, humidity and soil chemistry. A categorization of weathering characteristics into six stages, recognizable on descriptive criteria, provides a basis for investigation of weathering rates and processes. The time necessary to achieve each successive weathering stage has been calibrated using known-age carcasses. Most bones decompose beyond recognition in 10 to 15 yr. Bones of animals under 100 kg and juveniles appear to weather more rapidly than bones of large animals or adults. Small-scale rather than widespread environmental factors seem to have greatest influence on weathering characteristics and rates. Bone weathering is potentially valuable as evidence for the period of time represented in recent or fossil bone assemblages, including those on archeological sites, and may also be an important tool in censusing populations of animals in modern ecosystems.

We believe that punctuational change dominates the history of life: evolution is concentrated in very rapid events of speciation (geologically instantaneous, even if tolerably continuous in ecological time). Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction. Phyletic gradualism is very rare and too slow, in any case, to produce the major events of evolution. Evolutionary trends are not the product of slow, directional transformation within lineages; they represent the differential success of certain species within a clade—speciation may be random with respect to the direction of a trend (Wright's rule).

As an a priori bias, phyletic gradualism has precluded any fair assessment of evolutionary tempos and modes. It could not be refuted by empirical catalogues constructed in its light because it excluded contrary information as the artificial result of an imperfect fossil record. With the model of punctuated equilibria, an unbiased distribution of evolutionary tempos can be established by treating stasis as data and by recording the pattern of change for all species in an assemblage. This distribution of tempos can lead to strong inferences about modes. If, as we predict, the punctuational tempo is prevalent, then speciation—not phyletic evolution—must be the dominant mode of evolution.

We argue that virtually none of the examples brought forward to refute our model can stand as support for phyletic gradualism; many are so weak and ambiguous that they only reflect the persistent bias for gradualism still deeply embedded in paleontological thought. Of the few stronger cases, we concentrate on Gingerich's data for Hyopsodus and argue that it provides an excellent example of species selection under our model. We then review the data of several studies that have supported our model since we published it five years ago. The record of human evolution seems to provide a particularly good example: no gradualism has been detected within any hominid taxon, and many are long-ranging; the trend to larger brains arises from differential success of essentially static taxa. The data of molecular genetics support our assumption that large genetic changes often accompany the process of speciation.

Phyletic gradualism was an a priori assertion from the start—it was never “seen” in the rocks; it expressed the cultural and political biases of 19th century liberalism. Huxley advised Darwin to eschew it as an “unnecessary difficulty.” We think that it has now become an empirical fallacy. A punctuational view of change may have wide validity at all levels of evolutionary processes. At the very least, it deserves consideration as an alternate way of interpreting the history of life.

We present a quantitative method for describing how heterochronic changes in ontogeny relate to phyletic trends. This is a step towards creating a unified view of developmental biology and evolutionary ecology in the study of morphological evolution. Using this representation, we obtain a greatly simplified and logical scheme of classification. We believe that this scheme will be particularly useful in studying the data of paleontology and comparative morphology and in the analysis of processes leading to adaptive radiation. We illustrate this scheme by examples drawn from the literature and our own work.

Tertiary and Recent marine gastropods include in their ranks a complement of mechanically sturdy forms unknown in earlier epochs. Open coiling, planispiral coiling, and umbilici detract from shell sturdiness, and were commoner among Paleozoic and Early Mesozoic gastropods than among younger forms. Strong external sculpture, narrow elongate apertures, and apertural dentition promote resistance to crushing predation and are primarily associated with post-Jurassic mesogastropods, neogastropods, and neritaceans. The ability to remodel the interior of the shell, developed primarily in gastropods with a non-nacreous shell structure, has contributed greatly to the acquisition of these antipredatory features.

The substantial increase of snail-shell sturdiness beginning in the Early Cretaceous has accompanied, and was perhaps in response to, the evolution of powerful, relatively small, shell-destroying predators such as teleosts, stomatopods, and decapod crustaceans. A simultaneous intensification of grazing, also involving skeletal destruction, brought with it other fundamental changes in benthic community structure in the Late Mesozoic, including a trend toward infaunalization and the disappearance or environmental restriction of sessile animals which cannot reattach once they are dislodged. The rise and diversification of angiosperms and the animals dependent on them for food coincides with these and other Mesozoic events in the marine benthos and plankton.

The new predators and prey which evolved in conjunction with the Mesozoic reorganization persisted through episodes of extinction and biological crisis. Possibly, continental breakup and the wide extent of climatic belts during the Late Mesozoic contributed to the conditions favorable to the evolution of skeleton-destroying consumers. This tendency may have been exaggerated by an increase in shelled food supply resulting from the occupation of new adaptive zones by infaunal bivalves and by shell-inhabiting hermit crabs.

Marine communities have not remained in equilibrium over their entire geological history. Biotic revolutions made certain modes of life obsolete and resulted in other adaptive zones becoming newly occupied.

Data on numbers of marine families within 91 metazoan classes known from the Phanerozoic fossil record are analyzed. The distribution of the 2800 fossil families among the classes is very uneven, with most belonging to a small minority of classes. Similarly, the stratigraphic distribution of the classes is very uneven, with most first appearing early in the Paleozoic and with many of the smaller classes becoming extinct before the end of that era. However, despite this unevenness, a Q-mode factor analysis indicates that the structure of these data is rather simple. Only three factors are needed to account for more than 90% of the data. These factors are interpreted as reflecting the three great “evolutionary faunas” of the Phanerozoic marine record: a trilobite-dominated Cambrian fauna, a brachiopod-dominated later Paleozoic fauna, and a mollusc-dominated Mesozoic-Cenozoic, or “modern,” fauna. Lesser factors relate to slow taxonomic turnover within the major faunas through time and to unique aspects of particular taxa and times.

Each of the three major faunas seems to have its own characteristic diversity so that its expansion or contraction appears as being intimately associated with a particular phase in the history of total marine diversity. The Cambrian fauna expands rapidly during the Early Cambrian radiations and maintains dominance during the Middle to Late Cambrian “equilibrium.” The Paleozoic fauna then ascends to dominance during the Ordovician radiations, which increase diversity dramatically; this new fauna then maintains dominance throughout the long interval of apparent equilibrium that lasts until the end of the Paleozoic Era. The modern fauna, which slowly increases in importance during the Paleozoic Era, quickly rises to dominance with the Late Permian extinctions and maintains that status during the general rise in diversity to the apparent maximum in the Neogene. The increase in diversity associated with the expansion of each new fauna appears to coincide with an approximately exponential decline of the previously dominant fauna, suggesting possible displacement of each evolutionary fauna by its successor.

A three-phase kinetic model with time-specific perturbations is used to describe large-scale patterns in the diversification of Phanerozoic marine families. The basic model assumes that the Cambrian, Paleozoic, and Modern evolutionary faunas each diversified logistically as a consequence of early exponential growth and of later slowing of growth as the ecosystems became filled; it also assumes interaction among the evolutionary faunas such that expansion of the combined diversities of all three faunas above any single fauna's equilibrium caused that fauna's diversity to begin to decline. This basic model adequately describes the diversification of the evolutionary faunas through the Paleozoic Era as well as the asymmetrical rise and fall of background extinction rates through the entire Phanerozoic. Declines in diversity and changes in faunal dominance associated with mass extinctions can be accommodated in the model with short-term accelerations in extinction rates or declines in equilibria. Such accelerations, or perturbations, cause diversity to decline exponentially and then to rebound sigmoidally following release. The amount of decline is dependent on the magnitude and duration of the perturbation, the timing of the perturbation with respect to the diversification of the system, and the system's initial per-taxon rates of diversification and turnover. When applied to the three-phase model, such perturbations describe the changes in diversity and faunal dominance during and after major mass extinctions, the long-term rise in total diversity following the Late Permian and Norian mass extinctions, and the peculiar diversification and then decline of the remnants of the Paleozoic fauna during the Mesozoic and Cenozoic Eras. The good fit of this model to data on Phanerozoic familial diversity suggests that many of the large-scale patterns of diversification seen in the marine fossil record of animal families are simple consequences of nonlinear interrelationships among a small number of parameters that are intrinsic to the evolutionary faunas and are largely (but not completely) invariant through time.

The modern synthesis, as an exclusive proposition, has broken down on both of its fundamental claims: extrapolationism (gradual allelic substitution as a model for all evolutionary change) and nearly exclusive reliance on selection leading to adaptation. Evolution is a hierarchical process with complementary, but different, modes of change at its three major levels: variation within populations, speciation, and patterns of macroevolution. Speciation is not always an extension of gradual, adaptive allelic substitution to greater effect, but may represent, as Goldschmidt argued, a different style of genetic change—rapid reorganization of the genome, perhaps non-adaptive. Macroevolutionary trends do not arise from the gradual, adaptive transformation of populations, but usually from a higher-order selection operating upon groups of species, while the individual species themselves generally do not change following their geologically instantaneous origin. I refer to these two discontinuities in the evolutionary hierarchy as the Goldschmidt break (between change in populations and speciation) and the Wright break (between speciation and trends as differential success among species).

A new and general evolutionary theory will embody this notion of hierarchy and stress a variety of themes either ignored or explicitly rejected by the modern synthesis: punctuational change at all levels, important non-adaptive change at all levels, control of evolution not only by selection, but equally by constraints of history, development and architecture—thus restoring to evolutionary theory a concept of organism.

A simple equilibrial model for the growth and maintenance of Phanerozoic global marine taxonomic diversity can be constructed from considerations of the behavior of origination and extinction rates with respect to diversity. An initial postulate that total rate of diversification is proportional to number of taxa extant leads to an exponential model for early phases of diversification. This model appears to describe adequately the “explosive” diversification of known metazoan orders across the Precambrian-Cambrian Boundary, suggesting that no special event, other than the initial appearance of Metazoa, is necessary to explain this phenomenon. As numbers of taxa increase, the rate of diversification should become “diversity dependent.” Ecological factors should cause the per taxon rate of origination to decline and the per taxon rate of extinction to increase. If these relationships are modeled as simple linear functions, a logistic description of the behavior of taxonomic diversity through time results. This model appears remarkably consistent with the known pattern of Phanerozoic marine ordinal diversity as a whole. Analysis of observed rates of ordinal origination also indicates these are to a large extent diversity dependent; however, diversity dependence is not immediately evident in rates of ordinal extinction. Possible explanations for this pattern are derived from considerations of the size of higher taxa and from simulations of their diversification. These suggest that both the standing diversity and the pattern of origination of orders may adequately reflect the behavior of species diversity through time; however, correspondence between rates of ordinal and species extinction may deteriorate with progressive loss of information resulting from incomplete sampling of the fossil record.

Large cats, canids, bears, and hyenas create distinctive types of damage when they gnaw bones. This paper describes the diagnostic characteristics of damage done by each taxon to femora and tibiae of herbivores whose body weights are 300 kg or more. Pleistocene and Recent fossil collections that include gnawed bones might provide data on the presence of carnivores whose own remains are not found in the collections. Information might also be gained about predator and scavenger utilization of prey carcasses, often a reflection of prey vulnerability or availability in past communities.

The environmental and biotic history of the late Quaternary represents a critical junction between ecology, global change studies, and pre-Quaternary paleobiology. Late Quaternary records indicate the modes and mechanisms of environmental variation and biotic responses at timescales of 101–104 years. Climatic changes of the late Quaternary have occurred continuously across a wide range of temporal scales, with the magnitude of change generally increasing with time span. Responses of terrestrial plant populations have ranged from tolerance in situ to moderate shifts in habitat to migration and/or extinction, depending on magnitudes and rates of environmental change. Species assemblages have been disaggregated and recombined, forming a changing array of vegetation patterns on the landscape. These patterns of change are characteristic of terrestrial plants and animals but may not be representative of all other life-forms or habitats. Complexity of response, particularly extent of species recombination, depends in part on the nature of the underlying environmental gradients and how they change through time. Environmental gradients in certain habitats may change in relatively simple fashion, allowing long-term persistence of species associations and spatial patterns. Consideration of late Quaternary climatic changes indicates that both the rate and magnitude of climatic changes anticipated for the coming century are unprecedented, presenting unique challenges to the biota of the planet.

Based on inspection of gnawing damage done to bones of modern prey animals, sets of typical damage types or patterns are recognized for certain elements. The presence of these damage patterns suggests carnivore activity even when bones exhibit no identifiable tooth marks or other obvious sign of gnawing. Observations are made of these damage types on bones of Pleistocene and Recent North American mammals, including Bison, proboscidean, Alces, Equus, Cervus, and Rangifer. Damage to the following elements is briefly described: antlers, vertebrae, scapulae, humeri, ulnae, radii, femora, tibiae, metapodials, and innominates.

Two alternative hypotheses have been advanced to explain the demise of about half of the mammalian genera exceeding 5 kg in body mass in the later Pleistocene. One hypothesis invokes climatic change and resulting habitat transformations. This fails to predict the increased likelihood of extinctions with increasing body size, greater severity in both North and South America than in Eurasia or Australia, lack of simultaneous extinctions in Africa and tropical Asia, and the absence of extinctions at the end of previous glacial periods. The other hypothesis invokes human predation as the primary cause. This fails to explain the simultaneous extinctions of a number of mammalian and avian species not obviously vulnerable to human overkill. I propose a “keystone herbivore” hypothesis, based on the ecology of extant African species of megaherbivore, (i.e., animals exceeding 1,000 kg in body mass). Due to their invulnerability to non-human predation on adults, these species attain saturation densities at which they may radically transform vegetation structure and composition. African elephant can change closed woodland or thicket into open grassy savanna, and create open gaps colonized by rapidly-regenerating trees in forests. Grazing white rhinoceros and hippopotamus transform tall grasslands into lawns of more nutritious short grasses. The elimination of megaherbivores elsewhere in the world by human hunters at the end of the Pleistocene would have promoted reverse changes in vegetation. The conversion of the open parklike woodlands and mosaic grasslands typical of much of North America during the Pleistocene to the more uniform forests and prairie grasslands we find today could be a consequence. Such habitat changes would have been detrimental to the distribution and abundance of smaller herbivores dependent upon the nutrient-rich and spatially diverse vegetation created by megaherbivore impact. At the same time these species would have become more vulnerable to human predation. The elimination of megaherbivore influence is the major factor differentiating habitat changes at the end of the terminal Pleistocene glaciation from those occurring at previous glacial-interglacial transitions.

Species diversity among fossil invertebrates of the Phanerozoic is highly correlated with volume and area of sedimentary rocks. The correlations are statistically significant at the 1% level. The relationship holds even in regions (such as Canada) where the area and volume of rock do not increase through time. These results are interpreted as indicating that the apparent number of species is strongly dependent on sampling and that many of the changes in diversity seen in the Phanerozoic are artifactual. Consequently, there is no compelling evidence for a general increase in the number of invertebrate species from Paleozoic to Recent. This conclusion applies primarily to marine organisms. Diversity may have been in dynamic equilibrium throughout much of this time.

A few intervals of the Phanerozoic have consistently fewer invertebrate species than would be predicted from the amount of sedimentary rock available for study. The Silurian, Permian, and Cretaceous stand out in this regard. This may result either from lower than normal diversity during these periods or from an unusual abundance of unfossiliferous rocks (evaporites, red beds, etc.).

The kinetic model of taxonomic diversity predicts that the long-term diversification of taxa within any large and essentially closed ecological system should approximate a logistic process controlled by changes in origination and extinction rates with changing numbers of taxa. This model is tested with a new compilation of numbers of metazoan families known from Paleozoic stages (including stage-level subdivisions of the Cambrian). These data indicate the occurrence of two intervals of logistic diversification within the Paleozoic. The first interval, spanning the Vendian and Cambrian, includes an approximately exponential increase in families across the Precambrian-Cambrian Boundary and a “pseudo-equilibrium” through the Middle and Late Cambrian, caused by diversity-dependent decrease in origination rate and increase in extinction rate. The second interval begins with a rapid re-diversification in the Ordovician, which leads to a tripling of familial diversity during a span of 50 Myr; by the end of the Ordovician diversity attains a new dynamic equilibrium that is maintained, except for several extinction events, for nearly 200 Myr until near the end of the Paleozoic. A “two-phase” kinetic model is constructed to describe this heterogeneous pattern of early Phanerozoic diversification. The model adequately describes the “multiple equilibria,” the asymmetrical history of the “Cambrian fauna,” the extremely slow initial diversification of the later “Paleozoic fauna,” and the combined patterns of origination and extinction in both faunas. It is suggested that this entire pattern of diversification reflects the early success of ecologically generalized taxa and their later replacement by more specialized taxa.

When the Bering Strait between Alaska and Siberia opened about 3.5 Ma during the early Pliocene, cool-temperate and polar marine species were able to move between the North Pacific and Arctic-Atlantic basins. In order to investigate the extent, pattern, and dynamics of this trans-Arctic interchange, I reviewed the Recent and fossil distributions of post-Miocene shell-bearing Mollusca in each of five northern regions: (1) the northeastern Atlantic (Lofoten Islands to the eastern entrance of the English Channel and the northern entrance of the Irish Sea), (2) northwestern Atlantic (southern Labrador to Cape Cod), (3) northeastern Pacific (Bering Strait to Puget Sound), (4) northwestern Pacific (Bering Strait to Hokkaido and the northern Sea of Japan), and (5) Arctic (areas north of the Lofoten Islands, southern Labrador, and Bering Strait).

I have identified 295 molluscan species that either took part in the interchange or are descended from taxa that did. Of these, 261 are of Pacific origin, whereas only 34 are of Arctic-Atlantic origin. Various analyses of the pattern of invasion confirm earlier work, indicating that there is a strong bias in favor of species with a Pacific origin.

A geographical analysis of invaders implies that, although trans-Arctic interchange contributed to a homogenization of the biotas of the northern oceans, significant barriers to dispersal exist and have existed for trans-Arctic invaders within the Arctic-Atlantic basin. Nevertheless, trans-Arctic invaders in the Atlantic have significantly broader geographical ranges than do taxa with a pre-Pliocene history in the Atlantic.

Among the possible explanations for the asymmetry of trans-Arctic invasion, two hypotheses were explicitly tested. The null hypothesis of diversity states that the number of invaders from a biota is proportional to the total number of species in that biota. Estimates of Recent molluscan diversity show that the North Pacific is 1.5 to 2.7 times richer than is the Arctic-Atlantic, depending on how faunistic comparisons are made. This difference in diversity is much smaller than is the asymmetry of trans-Arctic invasion in favor of Pacific species. Rough estimates of regional Pliocene diversity suggest that differences in diversity during the Pliocene were smaller than they are in the Recent fauna. The null hypothesis was therefore rejected.

The hypothesis of ecological opportunity states that the number of invaders to a region is proportional to the number of species that became extinct there. The post-Early Pliocene magnitude of extinction was lowest in the North Pacific, intermediate in the northeastern Atlantic, and probably highest in the northwestern Atlantic. The absolute number and faunistic importance of post-Early Pliocene invaders (including trans-Arctic species, as well as taxa previously confined to warm-temperate waters and western Atlantic species that previously occurred only in the eastern Atlantic) was lowest in the North Pacific, intermediate in the northeastern Atlantic, and highest in the northwestern Atlantic. Further support for the hypothesis of ecological opportunity comes from the finding that hard-bottom communities, especially those in the northwestern Atlantic, show a higher representation of molluscan species of Pacific origin, and are likely to have been more affected by climatic events, than were communities on unconsolidated sandy and muddy bottoms. Support for the hypothesis does not rule out other explanations for the observed asymmetry of trans-Arctic invasion.

A preliminary study of species-level evolution within lineages of trans-Arctic invaders indicates that anagenesis and cladogenesis have been more frequent among groups with Pacific origins than among those with Atlantic origins, and that the regions within the Arctic-Atlantic basin with the highest absolute number and faunistic representation of invaders (western Atlantic and Arctic) are the regions in which speciation has been least common among the invaders. The asymmetry of invasion is therefore distinct from the asymmetry of species-level evolution of invaders in the various northern marine regions.

Morphological and taxonomic diversity each provide insight into the expansion and contraction of major biological groups, while the nature of the relationship between these two aspects of diversity also has important implications for evolutionary mechanisms. In this paper, I compare morphological and taxonomic diversity within the classes Blastoidea and Trilobita, and within the trilobite clades Libristoma, Asaphina, Proetida, Phacopida, and Scutelluina. Blastoid morphology is quantified with homologous landmarks on the theca, and trilobite form is measured with a Fourier description of the cranidium. Morphological diversity is measured as the total variance among forms in morphological space (proportional to the mean squared distance among forms). Blastoid taxonomic diversity is based on published compilation of stratigraphic ranges of genera. The Zoological Record was used to determine the number of new species of trilobites described since the publication of the Treatise; temporal patterns in species richness are similar to those for generic richness based on the Treatise, suggesting a common underlying signal.

Morphological variety and taxonomic richness often increase together during the initial diversification of a clade. This pattern is consistent with diffusion through morphospace, although some form of adaptive radiation cannot be ruled out. Morphological diversity varies little throughout much of the history of Proetida, a pattern that may suggest major constraints on the magnitude and direction of evolution, and that agrees with the perception of Proetida as a morphologically conservative group. Two major patterns are seen during the decline of clades. In Blastoidea, Trilobita, Libristoma, and Asaphina, morphological diversity is maintained at substantial levels, and in fact continues to increase, even in the face of striking reductions in taxonomic richness. This pattern suggests continued diffusion through morphospace and taxonomic attrition that is effectively non-selective with respect to morphology. In Phacopida, Scutelluina, and to some extent in Proetida, morphological diversity decreases along with taxonomic diversity. This pattern suggests heterogeneities such as elevated extinction and/or reduced origination in certain regions of morphospace. As found previously for the echinoderm subphylum Blastozoa, all studied clades of trilobites except Proetida show maximal morphological diversity in the Mid–Late Ordovician and maximal taxonomic diversity sometime during the Ordovician, suggesting some degree of common control on diversification patterns in these groups.

The Siwalik formations of northern Pakistan consist of deposits of ancient rivers that existed throughout the early Miocene through the late Pliocene. The formations are highly fossiliferous with a diverse array of terrestrial and freshwater vertebrates, which in combination with exceptional lateral exposure and good chronostratigraphic control allows a more detailed and temporally resolved study of the sediments and faunas than is typical in terrestrial deposits. Consequently the Siwaliks provide an opportunity to document temporal differences in species richness, turnover, and ecological structure in a terrestrial setting, and to investigate how such differences are related to changes in the fluvial system, vegetation, and climate. Here we focus on the interval between 10.7 and 5.7 Ma, a time of significant local tectonic and global climatic change. It is also the interval with the best temporal calibration of Siwalik faunas and most comprehensive data on species occurrences. A methodological focus of this paper is on controlling sampling biases that confound biological and ecological signals. Such biases include uneven sampling through time, differential preservation of larger animals and more durable skeletal elements, errors in age-dating imposed by uncertainties in correlation and paleomagnetic timescale calibrations, and uneven taxonomic treatment across groups. We attempt to control for them primarily by using a relative-abundance model to estimate limits for the first and last appearances from the occurrence data. This model also incorporates uncertainties in age estimates. Because of sampling limitations inherent in the terrestrial fossil record, our 100-Kyr temporal resolution may approach the finest possible level of resolution for studies of vertebrate faunal changes over periods of millions of years.

Approximately 40,000 specimens from surface and screenwash collections made at 555 localities form the basis of our study. Sixty percent of the localities have maximum and minimum age estimates differing by 100 Kyr or less, 82% by 200 Kyr or less. The fossils represent 115 mammalian species or lineages of ten orders: Insectivora, Scandentia, Primates, Tubulidentata, Proboscidea, Pholidota, Lagomorpha, Perissodactyla, Artiodactyla, and Rodentia. Important taxa omitted from this study include Carnivora, Elephantoidea, and Rhinocerotidae. Because different collecting methods were used for large and small species, they are treated separately in analyses. Small species include insectivores, tree shrews, rodents, lagomorphs, and small primates. They generally weigh less than 5 kg.

The sediments of the study interval were deposited by coexisting fluvial systems, with the larger emergent Nagri system being displaced between 10.1 and 9.0 Ma by an interfan Dhok Pathan system. In comparison to Nagri floodplains, Dhok Pathan floodplains were less well drained, with smaller rivers having more seasonally variable flow and more frequent avulsions. Paleosol sequences indicate reorganization of topography and drainage accompanying a transition to a more seasonal climate. A few paleosols may have formed under waterlogged, grassy woodlands, but most formed under drier conditions and more closed vegetation.

The oxygen isotopic record also indicates significant change in the patterns of precipitation beginning at 9.2 Ma, in what may have been a shift to a drier and more seasonal climate. The carbon isotope record demonstrates that after 8.1 Ma significant amounts of C4 grasses began to appear and that by 6.8 Ma floodplain habitats included extensive C4 grasslands. Plant communities with predominantly C3 plants were greatly diminished after 7.0 Ma, and those with predominantly C4 plants, which would have been open woodlands or grassy woodlands, appeared as early as 7.4 Ma.

Inferred first and last appearances show a constant, low level of faunal turnover throughout the interval 10.7–5.7-Ma, with three short periods of elevated turnover at 10.3, 7.8, and 7.3–7.0 Ma. The three pulses account for nearly 44% of all turnover. Throughout the late Miocene, species richness declined steadily, and diversity and richness indices together with data on body size imply that community ecological structure changed abruptly just after 10 Ma, and then again at 7.8 Ma. Between 10 and 7.8 Ma the large-mammal assemblages were strongly dominated by equids, with more balanced faunas before and after. The pattern of appearance and disappearance is selective with respect to inferred habits of the animals. Species appearing after 9.0 Ma are grazers or typical of more open habitats, whereas many species that disappear can be linked to more closed vegetation. We presume exceptions to this pattern were animals of the mixed C3/C4 communities or the wetter parts of the floodplain that did not persist into the latest Miocene. The pace of extinction accelerates once there is C4 vegetation on the floodplain.

The 10.3 Ma event primarily comprises disappearance of taxa that were both common and of long duration. The event does not correlate to any obvious local environmental or climatic event, and the pattern of species disappearance and appearance suggests that biotic interactions may have been more important than environmental change.

The 7.8 Ma event is characterized solely by appearances, and that at 7.3 Ma by a combination of appearances and disappearances. These two latest Miocene events include more taxa that were shorter ranging and less common, a difference of mode that developed between approximately 9.0 and 8.5 Ma when many short-ranging and rare species began to make appearances. Both events also show a close temporal correlation to changes in floodplain deposition and vegetation. The 7.8 Ma event follows the widespread appearance of C4 vegetation and is coincident with the shift from equid-dominated to more evenly balanced large-mammal assemblages. The 7.3 to 7.0 Ma event starts with the first occurrence of C4-dominated floras and ends with the last occurrence of C3-dominated vegetation. Absence of a consistent relationship between depositional facies and the composition of faunal assemblages leads us to reject fluvial system dynamics as a major cause of faunal change. The close correlation of latest Miocene species turnover and ecological change to expansion of C4 plants on the floodplain, in association with oxygen isotopic and sedimentological evidence for increasingly drier and more seasonal climates, causes us to favor explanations based on climatic change for both latest Miocene pulses.

The Siwalik record supports neither “coordinated stasis” nor “turnover pulse” evolutionary models. The brief, irregularly spaced pulses of high turnover are characteristic of both the stasis and pulse models, but the high level of background turnover that eliminates 65–70% of the initial species shows there is no stasis in the Siwalik record. In addition, the steadily declining species richness and abrupt, uncoordinated changes in diversity do not fit either model.

A comparison is made between compilations of times of origination and extinction of fossil marine animal families published in 1982 and 1992. As a result of ten years of library research, half of the information in the compendia has changed: families have been added and deleted, low-resolution stratigraphic data have been improved, and intervals of origination and extinction have been altered. Despite these changes, apparent macroevolutionary patterns for the entire marine fauna have remained constant. Diversity curves compiled from the two data bases are very similar, with a goodness-of-fit of 99%; the principal difference is that the 1992 curve averages 13% higher than the older curve. Both numbers and percentages of origination and extinction also match well, with fits ranging from 83% to 95%. All major events of radiation and extinction are identical. Therefore, errors in large paleontological data bases and arbitrariness of included taxa are not necessarily impediments to the analysis of pattern in the fossil record, so long as the data are sufficiently numerous.

Disparity is a measure of the range or significance of morphology in a given sample of organisms, as opposed to diversity, which is expressed in terms of the number (and sometimes ranking) of taxa. At present there is no agreed definition of disparity, much less any consensus on how to measure it. Two possible categories of metric are considered here, one independent of any hypothesis of relationship (phenetics), the other constrained within an evolutionary framework (cladistics).

The Early Cambrian radiation was clearly a period of significant morphologic and taxonomic diversification. However, we question the interpretation of its first generation products as numerous body plans at the highest level. Four phenetic and two cladistic measures have been used to compare disparity among Cambrian arthropods with that in the living fauna. Phenetic methods assessing character-state variability and the amount of morphological attribute space occupied yield similar results for Cambrian and Recent arthropods. Assessments of disparity within a taxonomic framework rely on the identification of particular characters that delineate higher level body plans. This requires a phylogenetic interpretation, a cladistic investigation of hierarchical structure in the data. Both sets of arthropods fall within the same major clades, and within this cladistic framework the amount of character-state evolution in the two groups is comparable. None of these methods identifies markedly greater disparity among the Cambrian compared with the Recent taxa.

Although measures of disparity are applied here to a consideration of the Cambrian radiation, the metrics clearly have a much wider potential for estimating macroevolutionary trends independently from existing taxonomic frameworks. Geometric morphometry is ideal for measuring morphological variety at lower taxonomic levels, but it requires the recognition of homologous landmarks in all the forms under comparison, or the identification of entire homologous structures. Conventional phenetics has much wider application as it can operate on data coded as discrete homologous character states (this facility is also a requirement of cladistics), which are a more appropriate basis for comparing disparity in markedly dissimilar forms.