Although there is extraordinary interest in the quantitative measurement of species diversity, published statements on the behavior of the components of species diversity are contradictory and lead to opposite conclusions. In this paper, we demonstrate that the confusion is due to two key oversights: (1) whether or not biological sampling is carried out within or between communities; and (2) determination of the statistical distribution underlying a biological community, which is crucial for the evaluation of all of the components of diversity measurement.

The problem of sampling “within” a population or community is basically distinct from the equivalent integration of structure and diversity measurement “between” differing multispecies populations. “Within-community sampling” is defined as a set of biological samples from a statistical population that has a particular statistical distribution or a constant value for the associated parameter(s). As the number of individuals increases along with the number of species, for a log series distribution, the diversity measures of Shannon's H, log series or Fisher's α, and Simpson's Index 1/λ remain constant while the evenness measures of Buzas-Gibson's E and Pielou's J decrease. For a log-normal distribution, J will remain constant while E decreases and α, 1/λ, and H increase. No single measure of evenness remains constant over all statistical distributions, so if constancy as a type of independence is required, the appropriate distribution must first be determined. Each species ensemble is mathematically fixed by the applicable statistical distribution.

In contrast, “between-community sampling” is defined as a set of biological samples from different statistical distributions and/or the same distribution with differing parametric values. If sampling is between communities and S increases while the number of individuals remains constant, then all the other measures considered here increase. The exception is the broken stick, for which E remains constant while H, J, α, and 1/λ increase.

Herein we propose and justify the use of the log-series distribution (with regression on the information decomposition equation) as a null model for determination of community structure and demonstrate that the community structure of a Miocene bed at Calvert Cliffs, Maryland, is a log series by use of this new unified methodology.

Stage-level range data for 983 Jurassic and Cretaceous ammonite genera, distributed within 83 families, were analyzed by assigning absolute ages to stages or substages. We found ammonite genera to have a mean generic range of 7.3 Myr/ammonite genus. Using a similar methodology, mean generic range per family was also computed. The distribution of long-ranging genera (arbitrarily chosen as those ammonite genera ranging for 12 Myr or more) among families was found to be nonrandom. Instead, long-ranging genera were found to be concentrated in a few families, resulting in significant heterogeneity in the distribution of generic longevities within families (taxotely sensu Raup and Marshall 1980). Although some of the long-ranging genera were found to be morphologically simpler than shorter-ranging genera, others were equally or even more complex, indicating that longevity among ammonite genera is not merely a taxonomic artifact, dependent on degree of differentiable conch morphology. Those Cretaceous families composed of a large number of long-ranging genera were also among the leaders in mean species longevity per family, based on species-level range data for Cretaceous ammonites of the Great Valley Sequence of California. Many of the long-ranging genera and species possess a similar morphologic attribute: siphuncular tubes (connecting rings) of small diameter but high wall thickness.

Lagerstätten from the Precambrian–Cambrian transition have traditionally been a relatively untapped resource for understanding the paleoecology of the “Cambrian explosion.” This quantitative paleoecological study is based on 10,238 fossil specimens belonging to 100 animal species, 11 phyla, and 15 ecological categories from the lower Cambrian (Series 2, Stage 3) Chengjiang biota (Mafang locality near Haikou, Yunnan Province, China). Fossils were systematically collected within a 2.5-meter-thick sequence divided into ten stratigraphic intervals. Each interval represents an induced time-averaged assemblage of various event (obrution) beds of unknown duration. Overall, the different fossil assemblages are taxonomically and ecologically similar, suggesting the presence of a single community type recurring throughout the Mafang section. The Mafang community is dominated by epibenthic vagile hunters or scavengers, sessile suspension feeders, and infaunal vagile hunters or scavengers represented primarily by arthropods, brachiopods, and priapulids, respectively. Most species have low abundance and low occurrence frequencies, whereas a few species are numerically abundant and occur frequently. Overall, in structure and ecology the Mafang community is comparable to the Middle Cambrian (Series 3, Stage 5) Burgess Shale biota (Walcott Quarry, Yoho National Park, British Columbia, Canada). This suggests that, despite variations in species identity within taxonomic and ecological groups, the structure and ecology of Cambrian Burgess Shale-type communities remained relatively stable until at least the Middle Cambrian (Series 3, Stage 5) in subtidal to relatively deep-water offshore settings in siliciclastic soft-substrate environments.

Microfossil bonebeds are multi-individual accumulations of disarticulated and dissociated vertebrate hardparts dominated by elements in the millimeter to centimeter size range (≤75% of bioclasts ≤5 cm maximum dimension). Modes of accumulation are often difficult to decipher from reports in the literature, although predatory (scatological) and fluvial/hydraulic origins are typically proposed. We studied the sedimentology and taphonomy of 27 microfossil bonebeds in the Campanian Judith River Formation of Montana in order to reconstruct formative histories. Sixteen of the bonebeds examined are hosted by fine-grained facies that accumulated in low-energy aquatic settings (pond/lake microfossil bonebeds). Eleven of the bonebeds are embedded in sandstones that accumulated in ancient fluvial settings (channel-hosted microfossil bonebeds). In lieu of invoking separate pathways to accumulation based on facies distinctions, we present a model that links the accumulation of bioclasts in the two facies. We propose that vertebrate material initially accumulates to fossiliferous levels in ponds/lakes and is later reworked and redeposited as channel-hosted assemblages. This interpretation is grounded in reasonable expectations of lacustrine and fluvial depositional systems and supported by taphonomic data. Moreover, it is consistent with faunal data that indicate that channel-hosted assemblages and pond/lake assemblages are similar with regard to presence/absence and rank-order abundance of taxa.

This revised model of bonebed formation has significant implications for studies of vertebrate paleoecology that hinge on analyses of faunal data recovered from vertebrate microfossil assemblages. Pond/lake microfossil bonebeds in the Judith River record are preserved in situ at the scale of the local paleoenvironment, with no indication of postmortem transport into or out of the life habitat. Moreover, they are time-averaged samples of their source communities, which increases the likelihood of capturing both ecologically abundant species and more rare or transient members of the paleocommunity. These attributes make pond/lake microfossil bonebeds excellent targets for paleoecological studies that seek to reconstruct overall community membership and structure. In contrast, channel-hosted microfossil bonebeds in the Judith River record are out of place from a paleoenvironmental perspective because they are reworked from preexisting pond/lake assemblages and redeposited in younger channel facies. However, despite a history of exhumation and redeposition, channel-hosted microfossil bonebeds are preserved in relatively close spatial proximity to original source beds. This taphonomic reconstruction is counter to the commonly held view that microfossil bonebeds are biased samples that have experienced long-distance transport and significant hydrodynamic sorting.

In phytogeographic data sets, the number of assemblages or floras from each interval may provide a test of the influence of sampling intensity on land-plant diversity. Using a data set of Silurian and Devonian compression-impression plant genera from Laurussia and the Acadian terrain, regression of five measures of land-plant diversity (total diversity, mean genus richness of floras, median assemblage diversity, most diverse assemblage, and standing diversity at interval boundaries) against the number assemblages or floras from thirteen intervals suggests that sampling bias influences all of the diversity measures to some extent, including within-habitat measures. The standing diversity of land plants at interval boundaries, the measure least influenced by sampling (r = 0.65, p = 0.05), increased steadily from the Middle Silurian to the late Givetian/early–middle Frasnian boundary, fell sharply in the early–middle Frasnian and remained low throughout the late Frasnian–middle Famennian. Standing diversity rose dramatically in the late Famennian and Strunian (latest Devonian): the Frasnian–Famennian extinction event may have affected land plants. The standing diversity of Silurian and Devonian microspore genera at interval boundaries mirrors that of compression-impression genera: neither record supports a land-plant diversity equilibrium during the Devonian.

Hierarchies in natural science are ranked and nested structures such that units at each rank include parts that are units at lower ranks. Hierarchies are able to render complexity tractable, by homogenizing units into collectives and by ordering collectives in ranks of increasing inclusiveness. Hierarchies contrast with positional structures, such as phylogenetic trees, for in trees all positions are occupied by the same sort of entity—there are no ranked collectives—and positions are specified by the order of appearance or precedence of the entities. In hierarchies, interactions within ranks are most important; in trees, sequences of events along branches are of primary concern. As a result hierarchies do, and trees do not, display emergent properties.

The value of the hierarchical structure can be lost when ranks are misspecified. A common error is the use of only a fraction of entities that actually occur in a rank, as when genes are considered as a rank below cells, disregarding the remaining cell contents and rendering the nature of cellular organization moot. Misspecification is also common when attributes or processes are used in ranks without indications of the physical entities to which they refer, thus losing track of the proper composition and ranking of the collectives.

Ecology is thought to be of crucial importance in determining taxonomic turnover at geological time scales, yet general links between ecology and biodiversity dynamics are still poorly explored in deep time. Here we analyze the relationships between the environmental affinities of Triassic–Jurassic marine benthic genera and their biodiversity dynamics, using a large, taxonomically vetted data set of Triassic–Jurassic taxonomic occurrences.

On the basis of binomial probabilities of proportional occurrence counts, we identify environmental affinities of genera for (1) carbonate versus siliciclastic substrates, (2) onshore versus offshore depositional environments, (3) reefs versus level-bottom communities, and (4) tropical versus non-tropical latitudinal zones. Genera with affinities for carbonates, onshore environments, and reefs have higher turnover rates than genera with affinities for siliciclastic, offshore, and level-bottom settings. Differences in faunal turnover are largely due to differences in origination rates. Whereas previous studies have highlighted the direct influence of physical and biological factors in exploring environmental controls on evolutionary rates, our analyses show that the patterns can largely be explained by the partitioning of higher taxa with different evolutionary tempos among environments. The relatively slowly evolving bivalves are concentrated in siliciclastic rocks and in level-bottom communities. Furthermore, separate analyses on bivalves did not produce significant differences in turnover rates between environmental settings. The relationship between biodiversity dynamics and environments in our data set is thus governed by the partitioning of higher taxa within environmental categories and not directly due to greater chances of origination in particular settings. As this partitioning probably has ecological reasons rather than being a simple sampling artifact, we propose an indirect environmental control on evolutionary rates.

Affinities for latitudinal zones are not linked to systematically different turnover rates, possibly because of paleoclimatic fluctuations and latitudinal migrations of taxa. However, the strong extinction spike of tropical genera in the Rhaetian calls for an important paleoclimatic component in the end-Triassic mass extinction.

Being of especially high quality, the Neogene fossil record of planktonic foraminifera offers special opportunities for assessing patterns of extinction and speciation. A variety of metrics indicates that within this group the mean duration of lineages has been much shorter (rate of extinction has been higher) for the globorotaliid clade than for the globigerinid clade. Furthermore, in the globorotaliid clade rates of extinction and speciation have not been closely linked to changes in diversity, but rather have been relatively high even at times when diversity has undergone little change. Thus, the globorotaliid clade has undergone more rapid evolutionary turnover than the globigerinid clade. Data for living species reveal that neither geographic range nor temperature tolerance is the primary factor controlling lineage duration. On the other hand, there is evidence that lineages marked by low abundance (small population size) are relatively short-lived. The reason that globorotaliid lineages have generally survived for shorter intervals, on the average, may be that their populations have been less abundant and less stable. Usually they live deeper in the water column, where food is often sparse, and many flourish only in areas of upwelling. Furthermore, the globorotaliids lack symbiotic algae for nutritional support. The same ecological factors may have accelerated speciation in the globorotaliid clade, by causing species to be patchily distributed. Thus, population size and structure have been more important than geographic range in determining rates of extinction and speciation. This parallels the situation for Neogene marine bivalves.

For planktonic foraminifera, as for Western Atlantic Bivalvia, the normal pattern of extinction was reversed in late Pliocene time, apparently in response to climatic cooling. The globigerinids suffered a sudden pulse of extinction. The deeper dwelling globorotaliids fared better; probably many of their species benefited from elevation of the seasonal thermocline into the photic zone. At the same time, rate of speciation declined in the globorotaliid clade, which supports the idea, inferred from the evolutionary history of marine bivalves, that an increase in the size and stability of populations should depress both rate of extinction and rate of speciation.

The chamber formation cycle in externally shelled, chambered cephalopods consists of mural ridge formation, secretion of the siphuncular connecting ring, septal calcification, and cameral liquid removal. Radiographic observation of the chamber formation cycle in specimens of Nautilus macromphalus allows direct observation of the various processes of the chamber formation cycle in a chambered cephalopod, and gives direct measures of rates. New chamber formation in N. macromphalus initiates when slightly more than half of the cameral liquid has been removed from the last formed chamber. At this volume, the liquid within the chamber drops from direct contact with the permeable connecting ring to a level where it is no longer in direct contact and must move onto the connecting ring due to wettable properties of the septal face and septal neck. This change from “coupled” to “decoupled” emptying coincides with the formation of a mural ridge at the rear of the body chamber, in front of the last formed septum. With completion of the mural ridge, the septal mantle moves forward from its position against the face of the last formed septum and attaches to the new mural ridge, where it begins calcifying a new septum in front of the newly created, liquid-filled space. Emptying of the new cameral liquid from this space commences when the calcifying septum has reached from one-third to two-thirds of its final thickness. The cessation of calcification of the septum coincides with a liquid volume in the new chamber of approximately 50%, at which point the cycle begins anew. During the chamber formation cycle apertural shell growth appears to be continuous. Since apertural shell growth is the prime factor leading to increased density in seawater, and hence decreased buoyancy, the period in the chamber formation cycle between the onset of septal calcification and the onset of emptying would be a time of greatly decreasing buoyancy. This is avoided by the removal of decoupled liquid from previously produced chambers. In this way constant neutral buoyancy is maintained. The time between chamber formation events in aquarium maintained N. macromphalus appears to be between 70 and 120 d.

Taxonomists working with late Cenozoic faunas tend to view living species as starting points for the evaluation of chronospecies (i.e., segments of evolutionary lineages subjectively designated as species) that extend backward in time from the Recent. This practice makes it possible to construct a survivorship curve for late Cenozoic chronospecies by evaluating all fossil lineages believed to have survived to the present day. A survivorship curve is produced by plotting the fraction of these lineages existing at any time that have not undergone enough phyletic evolution that their extant representatives are assigned to new species. This kind of surviviorship curve has been plotted for chronospecies of mammals using the beginning of the Würm, rather than the Recent, as an endpoint in order to avoid the effects of the Würm and post-Würm mass extinction. The survivorship curve reveals that all but a small fraction of established chronospecies have long durations relative to intervals of time during which distinctive higher taxa have arisen. Phyletic turnover of species has been remarkably slow. Most net evolutionary change must have been associated with saltational speciation. Even the large majority of genera must have arisen rapidly by one or more divergent speciation events. Estimates of rates of extinction suggest that the bottleneck effect, in which constriction of a lineage is followed by re-expansion as a distinct species, cannot be a major source of evolutionary change. These conclusions, based on the evaluation of mammalian phylogeny, seem also to apply to other taxa of animals, supporting the punctuational model of evolution. The long durations of hominid species imply that the evolution of humans, like that of other mammals, conforms to this model.

A large sample of the Pikermi and Samos ungulates was examined by microwear analysis using a light stereomicroscope (561 extinct and 809 extant comparative specimens). The results were used to infer the dietary adaptations of individual species and to evaluate the Pikermian Biome ungulate fauna. Many of the bovids have wear consistent with mixed feeding, although a few mesodont taxa apparently enjoyed an exclusive browsing and or grazing diet. The giraffids spanned the entire dietary spectrum of browsing, mixed feeding, and grazing, but most of the three-toed horses (Hippotherium) were hypsodont grazers. The colobine monkey Mesopithecus pentelici displays microwear consistent with a mixed fruit and leaf diet most likely including some hard objects. Similar results were obtained from prior scanning electron microscopy microwear studies at 500 times magnification and from the light microscope method at 35 times magnification for the same species. Results show that diet can differ between species that have very similar gross tooth morphology. Our results also suggest that the Pikermian Biome was most likely a woodland mosaic that provided a diversity of opportunities for species that depended on browsing as well as species that ate grass. The grasses were most likely C3 grasses that would grow in shaded areas of the woodland, glades, and margins of water. The ungulate component of the Pikermi and Samos fauna was more species-rich and more diverse in diet than the ungulates observed in modern African forests, woodlands, or savannas, yet dietarily most similar to the ungulates found in woodland elements of India and to some extent of Africa. It is unlikely that the Pikermi and Samos ungulates inhabited dense forests because we find no evidence for heavy fruit browsing. Conversely, a pure savanna is unlikely because many mixed feeders are present as well as browsers. Extant woodland African species are morphologically and trophically very similar to the African savanna species. Therefore the evolution of grazing and of hypsodont morphology for Africa may have evolved within the Plio-Pleistocene woodlands of Africa. Our results show that major dietary and morphologic ungulate evolution may take place within woodlands rather than as a consequence of species moving into savannas both during the late Miocene of Pikermi and Samos and during the Pleistocene–Recent of Central Africa.

Punctuated equilibrium and phyletic gradualism are alternative hypotheses that purport to explain the tempo and mode of evolution. We evaluate the two hypotheses, as they apply to the fossil record, on both theoretical and empirical grounds. Hidden randomness in data increases as a function of greater aggregation, and the hypothesis of punctuated equilibrium should not be applied to those examples where randomness is likely to occur. False stasis can result from a sustained pattern of emigration and immigration, and geographic variation must be studied in order to posit an unambiguous case of punctuated equilibrium. We describe a statistical method based on the general linear model for testing the relative fit of the alternative hypotheses to any set of temporally ordered metric data. Our method is hierarchical in the sense that subsets of the total explained variance can themselves be explained. The size of the first molar of the primate Pelycodus and of the condylarth Hyopsodus are analyzed. There are 17 tests in the two data sets, and we discover 12 instances of gradualism, four of punctuation and one of equilibrium.

A survey of all Cerion taxa and geographic variants reveals that a distinctive shape—a narrowly spired “smokestack” shell more than three times higher than wide—occurs only in dwarfs and giants and never in the much more common populations of normal size. The smokestack shape evolved once in giants (in the newly described species C. excelsior from Great Inagua and Mayaguana—the largest of all Cerion), but at least seven and probably eight times independently in dwarfs. A study of complex allometric patterns in Cerion's ontogeny, and of covariance sets in growth, indicates that smokestacks can easily evolve in dwarfs and giants by the common route of relative increase in whorl number during an allometric phase that adds height but no width to the shell. (Giants simply add more whorls to a shell with whorls of normal size; dwarfs grow a normal number of whorls in a shell with small whorls and reduced maximum width.) This pathway is not open to Cerion of normal size, and the restriction of smokestacks to both extremes of the size range records a channel set by Cerion's invariant allometries and the geometry of spiral growth in general, not an immediate adaptation conferring special advantages via the elevated spire itself.

Pelagic marine mammals (cetaceans and pinnipeds) generally invaded the seas rapidly in response to new ecologic opportunities. After cetaceans initially appeared and radiated in the Eocene, they declined in the Oligocene, but radiated into many new adaptive types in the Miocene. Pinnipeds apparently evolved in the earliest Miocene, rapidly radiating into many adaptive types. We propose that the radiations, and declines, of species were responses to the availability of trophic resources in oceanic environments. These trophic resources are closely related to upwelling processes in the oceans. We suggest that increased upwelling intensity, due to climatic or tectonic events, permitted the initial invasions and radiations and that decreased intensity caused cetacean extinctions in the Oligocene.

The distribution of species abundance (number of individuals per species) is well documented. The distribution of species occurrence (number of localities per species), however, has received little attention. This study investigates the distribution of species occurrence for five large data sets. For modern benthic foraminifera, species occurrence is examined from the Atlantic continental margin of North America, where 875 species were recorded 10,017 times at 542 localities, the Gulf of Mexico, where 848 species were recorded 18,007 times at 426 localities, and the Caribbean, where 1149 species were recorded 6684 times at 268 localities. For Late Cretaceous molluscs, species occurrence is examined from the Gulf Coast where 716 species were recorded 6236 times at 166 localities and a subset of this data consisting of 643 species recorded 3851 times at 86 localities.

Logseries and lognormal distributions were fitted to these data sets. In most instances the logseries best predicts the distribution of species occurrence. The lognormal, however, also fits the data fairly well, and, in one instance, better. The use of these distributions allows the prediction of the number of species occurring once, twice, …, n times.

Species abundance data are also available for the molluscan data sets. They indicate that the most abundant species (greatest number of individuals) usually occur most frequently. In all data sets approximately half the species occur four or less times. The probability of noting the presence of rarely occurring species is small, and, consequently, such species must be used with extreme caution in studies requiring knowledge of the distribution of species in space and time.

Gould's Wonderful Life (1989) was a landmark in the investigation of the Cambrian radiation. Gould argued that a number of experimental body plans (“problematica”) had evolved only to become extinct, and that the Cambrian was a time of special fecundity in animal design. He focused attention on the meaning and significance of morphological disparity versus diversity, and provoked attempts to quantify disparity as an evolutionary metric. He used the Burgess Shale as a springboard to emphasize the important role of contingency in evolution, an idea that he reiterated for the next 13 years. These ideas set the agenda for much subsequent research. Since 1989 cladistic analyses have accommodated most of the problematic Cambrian taxa as stem groups of living taxa. Morphological disparity has been shown to be similar in Cambrian times as now. Konservat-Lagerstätten other than the Burgess Shale have yielded important new discoveries, particularly of arthropods and chordates, which have extended the range of recognized major clades still further back in time. The objective definition of a phylum remains controversial and may be impossible: it can be defined in terms of crown or total group, but the former reveals little about the Cambrian radiation. Divergence times of the major groups remain to be resolved, although molecular and fossil dates are coming closer. Although “superphyla” may have diverged deep in the Proterozoic, “explosive” evolution of these clades near the base of the Cambrian remains a possibility. The fossil record remains a critical source of data on the early evolution of multicellular organisms.

Specimens of Nautilus species caught in the wild show a marked increase in oxygen isotopic composition between embryonic and postembryonic septa. The significance of this increase in terms of the early life history of Nautilus has been unclear. To help explain this pattern, we analyzed the isotopic composition of the septa of three specimens of Nautilus belauensis raised in aquariums under controlled temperature conditions. Our results indicate that both embryonic and postembryonic septa are secreted with the same temperature-dependent fractionation of aragonite relative to water as that of other aragonite-secreting molluscs (Grossman and Ku 1986). The δ18O values of the septa thus provide a reliable means of determining the water temperature in which the septa form. Calculated temperatures based on oxygen isotopic data from specimens caught in the wild reveal that embryonic development occurs at 22°-24° corresponding to a depth of 100-200 m depending on the location. The increase in δ18O in postembryonic septa reflects a migration into colder, deeper water after hatching. In Cretaceous nautilids, a systematic shift in δ18O is not present, indicating that these animals probably did not change their habitat after hatching. This is consistent with the likelihood that they lived in shallower environments than that of modern Nautilus.

Substrata for numerous epizoans in the Silurian Waldron Shale were provided by the toppled and overturned calyces of camerate crinoids. These were inhabited by a skeletonized fauna consisting of at least 25 species; including several bryozoans, tabulates, inarticulate brachiopods, worms, and echinoderms.

Many of the encrusting bryozoans exhibit skeletal overgrowths which appear to reflect competitive interactions for space. Coexistence of certain of the bryozoans involved in such interactions is suggested by skeletal features such as inflated growth forms and upturned margins at junctions between colonies (both representing attempts to avoid being overgrown). In addition, the occurrence of overgrowth reversals within individual encounters indicates contemporaneity of that pair. Species may be ranked according to their relative overgrowth success; however, this ranking is far from rigid as lower ranked species may on occasion overgrow more highly ranked species. Such nontransitive patterns appear similar to those described from Recent cryptic bryozoan faunas.

Functionally significant aspects of skull morphology were examined in Eocene miacids and in early members of the modern carnivore families to see if functional craniology might shed light on factors involved in the origin and early evolution of the modern carnivores. No key innovations are apparent at the beginning of the modern carnivore radiation, and an alternative hypothesis to account for that radiation is proposed: the radiation represents the filling of niches vacated by the extinction of several groups of archaic carnivores in the late Eocene. Differences that distinguish modern viverrids, canids, felids, and mustelids from each other today were less pronounced in the Oligocene, when these families first appeared, and provide no insights into possible ecological differences at the family level. However, body size differences among the early members of the modern carnivore families suggest that partitioning of prey resources by size may have been a factor in their initial radiation. Comparison of cranial morphology in Eocene miacids, early members of the modern carnivore families, and living carnivores allows reconstruction of the primitive conditions of carnivore skull morphology and determination of the pathways of morphological transformation that resulted in the diversity of skull morphology seen in modern carnivores.

Patterns of dental and cranial variation are analyzed in a unique sample of early Eocene equid Hyracotherium from one fossil quarry of late Wasatchian age in the Huerfano Basin of southern Colorado. The sample includes remains of two species differing principally in size: H. vasacciense and H. tapirinum. The larger species, H. tapirinum, is represented by 24 individual specimens that show marked bimodality in cranial size and robustness and in upper and lower canine size. These differences are attributed to sexual dimorphism. Males tend to be about 15% larger than females in cranial dimensions and 40% larger than females in canine dimensions suggesting, by comparison with modern ungulates, that early Eocene horses were probably polygynous. The appearance of Hyracotherium in North America and its rise to dominance in early Eocene mammalian communities coincides with development of an environmental mosaic including open park woodland and savanna habitats. The evolutionary success of Hyracotherium may reflect, in part, the adaptive superiority of a social structure in which females in open habitats are grouped for mutual protection from predators, for optimal foraging in patchy environments, or both.