In North America, hipparion horses consisted of four distinct genera, Hipparion, Neohipparion, Nannippus, and Cormohipparion, known from the medial Miocene to the late Pliocene, ca. 15.5–2 ma ago. The speciation mode is determined for 10 of the 16 valid hipparion species; 50% (5) seem to have resulted from anagenesis and 50% (5) from cladogenesis. Roughly contemporaneous species of the four hipparion genera seem to proceed through similar evolutionary stages of cranial and dental characters. The mean hipparion species longevity is 3.3 ma (N = 16); the mean generic longevity is 8.4 ma (N = 4). Only two species become extinct in less than 2 ma; extinction occurs at a rate of ca. 470 μmacarthurs for species with durations between 2 and 5 ma. For selected dental characters, mean evolutionary rates (for the 16 species) were between .08 darwins (d) and .03 d with crown height evolving most rapidly. Despite the commonly held notion that fossil horses evolved rapidly, these data suggest normal or average (horotelic) evolution for hipparions, which is not surprising because they were already established in the grazing adaptive zone during their history.

Diving investigations of Recent comatulid crinoids at Lizard Island, Great Barrier Reef, indicate that, contrary to long-held notions, crinoids are subject to predation, principally by fishes of several families. Predation usually occurs as sublethal damage to the visceral mass and arms, from which the crinoids usually recover by regeneration. Aspects of the life habits, morphology, biochemistry, and physiology of comatulids are postulated to be adaptations that enable comatulid crinoids to resist predation. Comatulid versatility in coping with predation may account in large measure for their evolutionary success in the face of the late Mesozoic teleost radiation. Frequency of damage and repair in fossil crinoids can be used as a measure of predation pressure in order to assess the impact of predation during the Phanerozoic evolution of crinoids. Paleozoic stalked crinoids made a heavy investment in skeletal armor, while in contrast, comatulids reduced the calyx and became mobile. The relationship between these phyletic trends and predation pressure can now be critically examined.

An international conference on “Fossil Arthropods as Living Animals” was held at the Royal Society of Edinburgh on October 25–27, 1984. Major international conferences devoted to fossil arthropods are rare events. The previous one, which was held in Oslo in July 1973 as a NATO Advanced Study Institute, resulted in a volume on “Evolution and Morphology of the Trilobita, Trilobitoidea and Merostomata” (Martinsson 1975). Participants in the Edinburgh meeting heard 28 contributions covering all aspects of arthropod paleobiology. Most are published as a special issue of the Transactions of the Royal Society of Edinburgh: Earth Sciences (Waterston 1985). All four major arthropod groups are treated (trilobites, crustaceans, chelicerates, and uniramians), as well as trace fossils and some groups of problematic status.

Patterns of temporal variation of six characters in a Miocene stickleback (Gasterosteus doryssus) are presented. Most pairs of characters tend to be correlated, but these correlations account for only about 26% of the observed variation, and thus the characters are studied separately. All character state frequencies exhibit temporal heterogeneity, and their means have temporal trends. Regardless of these overall trends, reversals of the trends cause end members of four time series not to differ significantly from each other. We argue that most observed temporal variation represents intrapopulation evolutionary change. Although most time series have somewhat stepped patterns, complete stasis is absent. Gradual variation is predominant, and in one case is nearly linear, but very rapid evolutionary bursts are also seen. The most rapid stratigraphic change appears to represent a local extinction, followed by recolonization by a differentiated population, but another episode of rapid change probably represents evolution in situ by means of standard neo-Darwinian mechanisms, without involvement of “macromutations” or true saltation. The irregular patterns and great magnitude of phenotypic change that are observed indicate that conventional paleontological samples may miss important evolutionary phenomena and are not comparable to shorter-term evolution in extant populations.

An evaluation of the history of polysyringian species (filibranchs and eulamellibranchs) reveals that the huge early Pliocene bivalve fauna of the Mediterranean Basin (MB) and North Sea Basin (NSB) suffered heavy extinction during late Pliocene and early Pleistocene time. This is evidenced by low survivorship of early Pliocene species in the Recent and by a decline in species richness of the two basins from 323 known early Pliocene species to 198 living species.

Several kinds of evidence indicate that cooling rather than the areal effect of eustatic sea-level lowering was the primary cause of the excessive extinction: (1) heavy Plio-Pleistocene extinction of mollusks was not global but concentrated around the margins of the northern Atlantic—an ocean fringed by polar ice caps; (2) taxa of tropical affinities were most severely affected; (3) heavy extinction occurred in the MB in areas not marked by facies change; (4) in the MB the onset of extinctions coincided with the onset elsewhere, but because of tectonic activity, water depths in the MB were not under tight eustatic control; (5) 14 species present in both the MB and NSB during early Pliocene time are now restricted to waters south of the NSB; and (6) the majority of species common to the two basins during the early Pliocene (eurythermal species) have survived to the present.

Molluscan data support palynological evidence that the climate in the MB was warmer and less seasonal in early Pliocene time than today, when latitudinal temperature gradients are steeper. Molluscan evidence indicates that the North Sea is exceptional in being less seasonal (though cooler) today than in early Pliocene time, and we attribute this anomaly to the local effects of the Gulf Stream, which was strengthened in mid-Pliocene time by the uplift of the Isthmus of Panama.

The heavy extinction in the MB and NSB about 3.2–3.0 ma ago approximately coincided with the earliest deposition of glacial tills in Iceland and with isotopic shifts in the tests of planktonic foraminifers preserved in deep-sea cores. Additional heavy extinction probably coincided with a pulse of severe cooling in late Pliocene time, 2.5–2.4 ma ago. Heavy extinction of mollusks in the MB and NSB continued into the early Pleistocene but not into the middle and late Pleistocene, apparently because by this time it was primarily only eurythermal species that survived. Today the molluscan faunas of the MB and NSB are unusually eurythermal; few species are restricted to a single biogeographic province.

Nature's discontinuities occur both in the hierarchical structuring of genealogical individuals and in the distinct processes operating at different scales of time, here called tiers. Conventional evolutionary theory denies this structuring and attempts to render the larger scales as simple extrapolation from (or reduction to) the familiar and immediate—the struggle among organisms at ecological moments (conventional individuals at the first tier). I propose that we consider distinct processes at three separable tiers of time: ecological moments, normal geological time (trends during millions of years), and periodic mass extinctions.

I designate as “the paradox of the first tier” our failure to find progress in life's history, when conventional theory (first tier processes acting on organisms) expects it as a consequence of competition under Darwin's metaphor of the wedge. I suggest a resolution of the paradox: whatever accumulates at the first tier is sufficiently reversed, undone, or overriden by processes of the higher tiers. In particular, punctuated equilibrium at the second tier produces trends for suites of reasons unrelated to the adaptive benefits of organisms (conventional progress). Mass extinction at the third tier, a recurring process now recognized as more frequent, more rapid, more intense, and more different than we had imagined, works by different rules and may undo whatever the lower tiers had accumulated.

Recent ideas about the role of competition in the ecology and evolution of acorn barnacles are based partly on relationships between morphology and the outcome of competition for space. One hypothesis is that present distributions and patterns of evolutionary diversification and decline among acorn barnacles reflect the competitive exclusion and replacement of solid-walled forms by those with tubiferous skeletal structure. An alternate view is that large barnacles generally outcompete smaller ones, independent of differences in skeletal structure, with predation and disturbance favoring the ecological and evolutionary success of small barnacles. Field experiments and observations in the Gulf of California suggested that the small, solid-walled species, Chthamalus anisopoma, competitively exlcudes the larger, tubiferous Tetraclita stalactifera confinis on the lower part of the shore. Greater tolerance to aerial exposure appears to allow Tetraclita to occupy a high intertidal refuge above Chthamalus. A common denominator among several cases of competitive exclusion in acorn barnacles is the greater settlement density of the competitive dominant, not its morphology. Morphological considerations alone are insufficient to predict or explain the outcome of competition between barnacle species.

Two speciation events occurred in the Eocene lineage beginning with Echinocythereis isabenana Oertli (Aragon, Spain). After a long period of stasis in ornament and shape/size, this species underwent a relatively rapid decrease in size, accompanied by ornamental changes. The transition required about 30,000 yr. The new species E. aragonensis Oertli is significantly smaller (20% decrease in size). The lateral papillation is denser and less regular, the papillae being significantly smaller than in the ancestral form. This ornament gradually yields to increasing frequencies of individuals with reduced papillae superimposed on a progressively better developed network of filaments. This change, which is regionally valid, is ultimately replaced by fully expressed reticulations, on which minute pustules may occur; this development typifies the third species, E. posterior Oertli. Some ornamental characteristics of E. aragonensis are anticipated in rare individuals of E. isabenana. Ornamental features of E. posterior appear in late larvae of E. isabenana. Generally, E. aragonensis and E. posterior do not differ significantly in size. A trend toward smaller carapaces in the upper samples of E. posterior could be related to ecophenotypic effects.

All species are highly polymorphic for both shape and ornament; the morphs are not sharply bounded and are mostly of uncertain evolutionary status. The transition from isabenana to aragonensis is heralded by a sharp rise in the coefficient of variation for length measures of the carapace. There is a pronounced multivariate morphometric discontinuity between isabenana and the descendant forms, which are largely multivariately homogeneous. The isabenana-aragonensis transition could have occurred by selection or by random genetic drift. Both models can be accommodated by the data; however, the weight of evidence favors the former hypothesis. Evolution in the Echinocythereis lineage seems to have occurred by two different mechanisms. The first transition was rather rapid, with many disjunct features. The second was slow, the changes being more of degree than of kind.

For some higher taxa, species can be identified in the fossil record with a high degree of reliability. The great geological durations of species indicate that phyletic evolution is normally so slow that little change occurs within a lineage during 105–107 generations. Failure to recognize sibling species in the fossil record has no bearing on this conclusion because they embody virtually no morphological change. Although slowness is the rule, we have no more precise assessment of morphological rates of phyletic evolution for any major taxon. Morphological data that have been assembled to assess rates of phyletic evolution have been meager, unrepresentative, and predominantly reflective of nothing more than body size. Net selection pressures within long segments of phylogeny—even ones that embrace large amounts of evolution—are infinitesimal and seemingly unsustainable against random fluctuations. This suggests that natural selection operates in a highly episodic fashion.

Rates of adaptive radiation and extinction at the species level can be estimated for many taxa and, from them, rates of speciation in adaptive radiation. Species selection should universally tend to increase rate of speciation and decrease rate of extinction, yet these rates are positively correlated in the animal world, apparently because they are linked by common controls: both rate of speciation and rate of extinction seem to increase with level of stereotypical behavior and to decrease with dispersal ability. Only a few “supertaxa” have been able to combine high rates of speciation with moderate rates of extinction.

The theoretical framework developed here delineates the biological and paleontological circumstances under which the observed durations and diversities of fossil taxa will be significantly affected by three time-dependent sampling biases. The pull of the Recent and the loss of fossiliferous sediments shorten the observed durations of ancient taxa, as compared to the observed durations of Recent taxa. By contrast, the duration truncation mechanism lengthens the observed durations of ancient, as compared to more recent, taxa. Both the pull of the Recent and loss of fossiliferous sediments decrease the observed diversity of ancient, as compared to contemporary, communities.

This theoretical framework is built on four parameters: the extinction and origination rates q and p; the fossilization rate, v; and the fossil loss rate, h. I derive formulas for the observed durations and diversities of fossil taxa in terms of these parameters and develop a method of estimating these parameters. The estimation procedure is applied to the bivalve families, and I provide statistical evidence for the conclusion that q – p < 0 for bivalve families, and hence that bivalve diversity has been increasing through time. The theory predicts that a certain fraction of an extinct fauna will be represented by only one fossil, and hence will have an observed duration of zero. As the record becomes poorer, this fraction increases. For bivalves I assume that families known from only one stage have an observed duration of zero, and exploit this assumption to estimate h and v/p.

To evaluate the errors made in applying the model to the bivalves, I estimate several parameters in two different ways and find the estimates to be consistent. Additionally, I argue that the errors caused by the simplifying assumptions of the model are less than the sampling errors inherent in estimating p, q, h, and v.

Diversity and faunal turnover of North American land mammals are calibrated against the magnetic polarity time scale for million-year intervals for the latest Eocene through late Oligocene. A major, gradual Late Eocene decline in diversity, caused mostly by an extended period of extinction of archaic forms, seems to be related to the worldwide crisis known as the “Terminal Eocene Event.” Along with other evidence of gradual changes in deep-sea microfossils, this evidence argues against a catastrophic explanation for late Eocene extinctions.

Faunal stability characterized the rest of the Oligocene except for a wave of extinctions in the mid-Oligocene (Chadronian-Orellan boundary, about 32.4 ma). This mid-Oligocene event is sudden and severe, occurring in less than 200,000 yr, based on estimates from sedimentation rates calibrated from magnetic polarity interval boundaries. The mid-Oligocene event is found in many paleoclimatic records, but not in all of them. It may be related to the completion of the Circum-Antarctic Current and to increased mid-Oligocene glaciation.

Delayed maturation and growth to large sizes are only advantageous under stable environmental conditions where food resources are limited. Specialization to algal symbiosis is also highly advantageous under those conditions if sunlight is available. The coevolution of these two characteristics has occurred many times in many foraminiferal lineages. These traits are sometimes associated with increased embryon size and suppression of sexual reproduction, which are also characteristics most advantageous under stable environmental conditions. Specialization for these traits, ensuring success in warm, shallow, stable, oligotrophic environments, often dooms the species or lineage to extinction when conditions change.

Phytogeographic analysis of three Early Carboniferous intervals (Tournaisian–early Visean, Visean, and late Visean–early Namurian A) indicates a high level of phytogeographic differentiation in the beginning of the Early Carboniferous that decreases toward the end of this period. Climatic amelioration (warmer or wetter conditions) in the north middle and high latitudes, caused by the collision of Laurussia and Gondwana at the end of the Early Carboniferous, may be responsible for this decrease in phytogeographic provinciality. Toward the end of the Early Carboniferous, a large number of equatorial genera expand their ranges northward, and the average generic diversity of assemblages in the north high latitudes (Siberia) also rises. Both support the hypothesis of climatic amelioration. Northward migration of equatorial forms and the appearance of new genera endemic to Siberia both contributed to the rise in Siberian diversity. Although this trend is not statistically significant, additional evidence of diversity increase in northern high latitudes tied to climatic amelioration comes from the northernmost limit of diverse (≥ 10 genera) assemblages, which rises from 20°N at the beginning to 55°N at the end of the Early Carboniferous. Global plant diversity assessed at the generic level remained constant during the Early Carboniferous. The increase in Siberian diversity was offset by a decrease in equatorial diversity, perhaps due to the loss of pronounced latitudinal climatic gradients between north-middle and equatorial latitudes.

The impact of environment, interspecific competition, and, to a lesser extent history, on the structure of the guild of large predatory mammals is explored in one fossil and four Recent communities. Two aspects are emphasized: (1) the number of species within each guild and (2) the extent of locomotor convergence as inferred from morphology among the constituent species. Locomotor behavior reflects habitat choice, hunting mode, and escape strategy, all of which appear to be important avenues of adaptive divergence among coexisting predators.

Locomotor behavior in extinct and extant predators is determined from body weight and five measured characteristics of the postcranial skeleton, including ungual shape, elbow shape, and limb proportions. Results indicate that levels of morphologic and inferred ecologic similarity between large predators are higher in the tropical grassland guild of East Africa than in the equivalent guilds of either tropical or temperate forest. This may be due to the great density and diversity of terrestrial herbivores in the productive grasslands. The fossil guild, from the Late Chadron–Orellan (Oligocene) of North America, appears most similar to the tropical forest guilds, but the predators seem to have been slower and more robust than their modern counterparts. Since the Orellan represents an early stage in the evolution of large, fissiped carnivores, both ancestry and time could have influenced Orellan guild structure.

Morphological continuity in the fossil record is the principal evidence favoring evolution as a historical explanation for the diversity of life. Continuity is usually discussed on scales broader than the species level. Patterns of morphological variation characteristic of living species are useful in recognizing species on time planes in the fossil record, but the fossil record is rarely complete enough temporally or geographically to preserve more than a fraction of species living in a given interval. Transitions between known species are even rarer. Where transitions are preserved, new species appear to arise through anagenesis (transformation of an ancestral stock producing a modified descendant) and through cladogenesis (subdivision of an ancestral lineage where one or more descendants differ from the ancestral stock). Evolutionary species are often necessarily bounded arbitrarily in the dimension of time.

Orthogenesis and punctuated equilibrium lie at opposite poles in a spectrum of speciation modes. Orthogenesis, highly constrained anagenesis, is probably rare. Cladogenesis appears to differ little from anagenesis once ancestral stocks are segregated. Limited evidence suggests that morphological differentiation during cladogenesis postdates genetic isolation. Hence punctuated equilibrium may be rare as well. Patterns of gradual change over time indicate that morphological evolution is reasonably viewed as continuous within and between species. Rates of evolution vary greatly (continuity does not require constancy). Rate distributions are truncated and biased by limits of stratigraphic completeness and time resolution: moderate to high rates of morphological evolution and species turnover are rarely recorded by fossils. Species durations are poorly characterized, but they appear to be so variable that there is no suggestion of periodicity. Species longevity is unpredictable. The episodic nature of faunal turnover suggests that extrinsic environmental factors rather than intrinsic homeostatic factors govern evolution at the species level.

The locomotory stresses acting at the midshaft of the third metacarpal of Equus are estimated from a geometrical model of the manus during peak loading, which is derived from in vivo data. The stress results compare favorably with those from in vivo strain gage experiments. The model is then modified to account for the anatomical and inferred functional differences in the feet of the two extinct, tridactyl equids Merychippus and Mesohippus, incorporating assumptions from earlier functional studies. Estimates of the stresses in the fossil metacarpals are derived, and fall within the range of values obtained in vivo for limb bones of a wide size range of mammals. The results indicate that the third metacarpal of Merychippus was the most highly stressed and that of Mesohippus the least, with stresses for Equus falling between. Thus the anatomical and functional changes in the manus between Mesohippus and Equus are not simply a result of the difference in body size.

Morphologic analysis was made on early whorls of two Paleozoic and 43 Mesozoic ammonoid species to obtain basic data on early life history. A positive linear relationship between protoconch and ammonitella sizes is recognized among the species examined. Total rotation angle of the ammonitella (ammonitella angle) seems to be independent of ammonitella size but has a significant negative linear relation with whorl expansion rate. This supports the model that the embryonic shell of ammonoids consists of gas-filled protoconch and the succeeding body whorl up to the primary varix. Growth analysis of siphuncle diameter in the early post-embryonic stage also indicates that the shell growth rates of the species hatched from large-sized eggs were smaller than those of species hatched from small-sized eggs.

The evolutionary pattern of speciation and extinction in any biologic group may be described by a variety of mathematical models. These models provide a framework for describing the history of taxonomic diversity (clade shape) and other aspects of larger evolutionary patterns. The simplest model assumes time homogeneity: that is, speciation and extinction probabilities are constant through time and within taxonomic groups. In some cases the homogeneous model provides a good fit to real world paleontological data, but in other cases the model serves only as a null hypothesis that must be rejected before more complex models can be applied. In cases where the homogeneous model does not fit the data, time-inhomogeneous models can be formulated that specify change, regular or episodic, in speciation and extinction probabilities. An appendix provides a list of the most useful equations based on the homogeneous model.

Immunologically reactive protein substances were extracted from bone samples of an American mastodon (Mammut americanum), 10,200 yr old by radiocarbon dating, and from muscle samples of three woolly mammoths (Mammuthus primigenius), 10,000, 40,000 and 53,000 yr old, respectively. The mastodon samples contained significant quantities of the amino acids hydroxylysine and hydroxyproline, both of which are usually found in collagens and not in albumins. Using these products and other comparable extracts, as well as sera and purified collagens from modern elephants and other living mammals, as test antigens, immunological comparisons were carried out with the following antisera: rabbit anti-mastodon bone; chicken anti-mammoth muscle; chicken anti-elephant muscle; rabbit anti-elephant albumin and rabbit anti-elephant collagen, as well as with rabbit antisera to purified albumins and collagens of other mammals. For the first time, mastodon bone was found to have elephant-like proteins, which elicited antibodies that reacted strongly with collagen and serum proteins of extant elephants. Mammoth muscle strongly reacted with anti-elephant collagen and anti-elephant albumin, but the concentrations of the recoverable mammoth collagen and albumin decreased with increasing chronological age of the mammoth specimens. Nevertheless, in the immunological comparisons, the mammoth was closer to Asian (Elephas maximus) and African (Loxodonta africana) elephants than to the mastodon; in turn, the mastodon was closer to these elephantid species than to mammals outside the order Proboscidea.