Both intensive analysis of Gryphaea and an extensive survey of Jurassic bivalves in Europe supports the punctuated equilibria model of speciation, with the exception of phyletic size increase, which appears within the limits of the data to be gradualistic and affects at least a significant minority of species. The size increase takes place both within species and between successive species in a lineage. In Liassic Gryphaea, a combination of allometry in the ontogenetic development of the ancestral species and paedomorphosis led also to changes of shape up the sequence. The production of evolutionary trends by species selection is not supported.

The favoured interpretation of phyletic size increase is a gradual delay in maturation time consequent upon a change in the organisms' adaptive strategy from the r-selected to the K-selected mode and is backed up by evidence on changing population numbers and distribution. An extinction/speciation model is proposed for neritic organisms based on fluctuating sea levels. Times of low sea level or regression correspond with times of high stress and hence high extinction among stenotopic organisms and increased rates of allopatric speciation, with r-selection as the dominant mode. Times of high sea level or transgression correspond with low extinction and speciation rates and increased freedom of migration. K-selection is the dominant mode and often leads to phyletic size increase provided the environment remains stable for a sufficiently long period.

Like living herbivorous lizards, chelonians, birds, and mammals, plant-eating dinosaurs probably relied on a symbiotic gut microflora, housed in a hindgut fermentation chamber, to break down plant cell wall constituents. Large body sizes in most herbivorous dinosaurs resulted in low mass-specific metabolic rates and low rates of digesta passage through the gut; the effects of large body size were probably enhanced by the low metabolic rates of large dinosaurs as compared with large mammals. The long residence time of digesta in the gut permitted long exposure of refractory plant materials to the microflora, probably enabling even those dinosaurs with unsophisticated dentitions to survive on fodder with high fiber content. Large herbivorous dinosaurs probably fed on plants whose allelochemical defenses were geared more toward reducing digestibility than attacking the herbivore's metabolism directly, obviating the need for a foregut fermentation chamber and permitting these large herbivores to take advantage of the energetic benefits of hindgut fermentation for digestion of low-quality fodder. Differences in dentitions among the groups of herbivorous dinosaurs may correlate with differences in standard metabolic rate, activity level, body size, or food quality, or combinations of these factors, but the relative importance of each is difficult to assess. Because the mass of the fermentation contents was probably large in big herbivorous dinosaurs, the heat of fermentation may have been a significant source of thermoregulatory heat for these reptiles.

This summer's meeting season saw five gatherings devoted specifically to the topic of extinction: Chicago's Field Museum symposium in May (see Diamond 1983); a field conference to examine the Cretaceous-Tertiary transition in Montana; a symposium on molluscan extinction at the American Malacological Union meeting in Seattle; a special session at the annual meeting of the Society for the Study of Evolution in St. Louis; and a research conference on the campus of Northern Arizona University in Flagstaff (see also Lewin 1983a,b,c). Several recent books consider the topic (Ehrlich and Ehrlich 1981; Nitecki 1981; Silver and Schultz 1982; Martin and Klein, 1984) and plans are underway to publish the papers presented at the Field Museum, Seattle, and Flagstaff meetings. Our review of the topic is based principally on the themes explored at the St. Louis and Flagstaff meetings.

The presumed affinities of the Terminal Neoproterozoic Ediacara biota have been much debated. However, even in the absence of concrete evidence for phylogenetic affinity, numerical paleoecological approaches can be effectively used to make inferences about organismal biology, the nature of biotic interactions, and life history. Here, we examine the population structure of three Ediacaran rangeomorph taxa (Fractofusus, Beothukis, and Pectinifrons), and one non-rangeomorph taxon (Thectardis) across five fossil surfaces around the Avalon Peninsula, Newfoundland, through analysis of size-frequency distributions using Bayesian Information Criterion (BIC). Best-supported models resolve communities of all studied Ediacaran taxa at Mistaken Point as single cohorts with wide variance. This result is best explained in terms of a “continuous reproduction” model, whereby Ediacaran organisms reproduce aseasonally, so that multiple size modes are absent from preserved communities. Modern benthic invertebrates (both as a whole and within specific taxonomic groups) in deeper-water settings reproduce both seasonally and aseasonally; distinguishing between biological (i.e., continuous reproductive strategies) and environmental (lack of a seasonal trigger) causes for this pattern is therefore difficult. However, we hypothesize that the observed population structure could reflect the lack of a trigger for reproduction in deepwater settings (i.e., seasonal flux of organic matter), until the explosive appearance of mesozooplankton near the base of the Cambrian.

A combination of historical, functional, and biomechanical constraints has shaped the masticatory apparatus of fossil and extant xenarthrans. Among the more notable features are the teeth: hypselodont; commonly reduced in size, complexity, and number; separated by short diastema; and composed of osteodentine. Enamel is absent, as are the cuspal patterns of other mammals. A comprehensive revision of teeth and other features of the masticatory apparatus of xenarthrans reveals that previous generalizations underestimate the morphological diversity and adaptive possibilities developed within the clade. The great diversity of forms suggests several such possibilities ranging from specialized myrmecophagous species to carrion feeders or predators among animalivores; selective browsers to bulk grazers among herbivores; and omnivores. In some cases xenarthrans represent less extreme versions of patterns developed in other major clades of mammals (marsupials, afrotheres, euarchontoglires, and laurasiatheres) clearly predetermined by a tribosphenic dental morphology, whereas in others they represent unique novelties indicative of particular biological roles. The combination of tooth features that characterize xenarthrans might be seen as the key innovation for the ecologic diversity developed at least since the Oligocene, breaking the mold of the tribosphenic condition that constrained the evolution of the other major clades of mammals.

Marsupial mammals are characterized by a pattern of dental replacement thought to be unique. The apparent primitive therian pattern is two functional generations of teeth at the incisor, canine, and premolar loci, and a series of molar teeth, which by definition are never replaced. In marsupials, the incisor, canine, and first and second premolar positions possess only a single functional generation. Recently this pattern of dental development has been hypothesized to be a synapomorphy of metatherians, and has been used to diagnose taxa in the fossil record. Further, the suppression of the first generation of teeth has been linked to the marsupial mode of reproduction, through the mechanical suppression of odontogenesis during the period of fixation of marsupials, and has been used to reconstruct the mode of reproduction of fossil organisms. Here we show that dental development occurs throughout the period of fixation; therefore, the hypothesis that odontogenesis is mechanically suppressed during this period is refuted. Further, we present comparative data on dental replacement in eutherians and demonstrate that suppression of tooth replacement is fairly common in diverse groups of placental mammals. We conclude that reproductive mode is neither a necessary nor a sufficient explanation for the loss of tooth replacement in marsupials. We explore possible alternative explanations for the loss of replacement in therians, but we argue that no single hypothesis is adequate to explain the full range of observed patterns.

The intersection of paleontology and biomechanics can be reciprocally illuminating, helping to improve paleobiological knowledge of extinct species and furthering our understanding of the generality of biomechanical principles derived from study of extant species. However, working with data gleaned primarily from the fossil record has its challenges. Building on decades of prior research, we outline and critically discuss a complete workflow for biomechanical analysis of extinct species, using locomotor biomechanics in the Triassic theropod dinosaur Coelophysis as a case study. We progress from the digital capture of fossil bone morphology to creating rigged skeletal models, to reconstructing musculature and soft tissue volumes, to the development of computational musculoskeletal models, and finally to the execution of biomechanical simulations. Using a three-dimensional musculoskeletal model comprising 33 muscles, a static inverse simulation of the mid-stance of running shows that Coelophysis probably used more upright (extended) hindlimb postures and was likely capable of withstanding a vertical ground reaction force of magnitude more than 2.5 times body weight. We identify muscle force-generating capacity as a key source of uncertainty in the simulations, highlighting the need for more refined methods of estimating intrinsic muscle parameters such as fiber length. Our approach emphasizes the explicit application of quantitative techniques and physics-based principles, which helps maximize results robustness and reproducibility. Although we focus on one specific taxon and question, many of the techniques and philosophies explored here have much generality to them, so they can be applied in biomechanical investigation of other extinct organisms.

Size distributions, based on molar length, are presented for various groups of living and fossil primates. I examine (1) the extent to which particular taxonomic groups with distinctive morphological and behavioral attributes are characterized by distinctive size ranges or distributions and (2) the way in which size ranges and distributions are affected by the presence or absence of other primates within the same geographical area and time. Distinctive behavioral and ecological “adaptive zones” are characterized by distinctive size ranges and behavioral and morphological parallelism or convergence among living and fossil primate taxa usually results in similar size distributions.

Barstovian (medial Miocene) mammalian faunas from the Texas Gulf Coastal Plain contained four apparently sympatric species of rhinoceroses: the common forms Aphelops megalodus and Teleoceras medicornutus, a dwarf Teleoceras, and a dwarf Peraceras. Previous work has suggested positive allometry in tooth area with respect to body size in several groups of mammals, i.e., larger mammals have relatively more tooth area. However, dwarfing lineages were shown to have relatively more tooth area for their body size. Our data show no significant allometry in post-canine tooth area of either artiodactyls or ceratomorphs. Similarly, dwarf rhinoceroses and hippopotami show no more tooth area than would be predicted for their size. Limbs are proportionately longer and more robust in larger living ceratomorphs (rhinos and tapirs) than predicted by previous authors. Limb proportions of both dwarf rhinoceroses and dwarf hippopotami are even more robust than in their living relatives.

The high rhinoceros diversity reflects the overall high diversity of Barstovian faunas from the Texas Gulf Coastal Plain. The first appearance of several High Plains mammals in these faunas indicates “ecotone”-like conditions as faunal composition changed. Study of living continental dwarfs shows that there is commonly an ecological separation between browsing forest dwarfs and their larger forebears, which are frequently savannah grazers. This suggests that the dwarf rhinoceroses might have been forest browsers which were sympatric with the larger grazing rhinos of the High Plains during the Barstovian invasion. The continental dwarf model also suggests that insular dwarfism may be explained by the browsing food resources that predominate on islands.

The estimation of origination and extinction rates and their temporal variation is central to understanding diversity patterns and the evolutionary history of clades. The fossil record provides the only direct evidence of extinction and biodiversity changes through time and has long been used to infer the dynamics of diversity changes in deep time. The software PyRate implements a Bayesian framework to analyze fossil occurrence data to estimate the rates of preservation, origination, and extinction while incorporating several sources of uncertainty. Building upon this framework, we present a suite of methodological advances including more complex and realistic models of preservation and the first likelihood-based test to compare the fit across different models. Further, we develop a new reversible jump Markov chain Monte Carlo algorithm to estimate origination and extinction rates and their temporal variation, which provides more reliable results and includes an explicit estimation of the number and temporal placement of statistically significant rate changes. Finally, we implement a new C++ library that speeds up the analyses by orders of magnitude, therefore facilitating the application of the PyRate methods to large data sets. We demonstrate the new functionalities through extensive simulations and with the analysis of a large data set of Cenozoic marine mammals. We compare our analytical framework against two widely used alternative methods to infer origination and extinction rates, revealing that PyRate decisively outperforms them across a range of simulated data sets. Our analyses indicate that explicit statistical model testing, which is often neglected in fossil-based macroevolutionary analyses, is crucial to obtain accurate and robust results.

Paleoecological analyses that test for spatial or temporal variation in diversity must consider not only sampling and preservation bias, but also the effects of temporal scale (i.e., time-averaging). The species-time relationship (STR) describes how species diversity increases with the elapsed time of observation, but its consequences for assessing the effects of time-averaging on diversity of fossil assemblages remain poorly explored. Here, we use a neutral, dispersal-limited model of metacommunity dynamics, with parameters estimated from living assemblages of 31 molluscan data sets, to model the effects of within-habitat time-averaging on the mean composition and multivariate dispersion of assemblages, on diversity at point (single station) and habitat scales (pooled multiple stations), and on beta diversity. We hold sample size constant in STRs to isolate the effects of time-averaging from sampling effects. With increasing within-habitat time-averaging, stochastic switching in the identity of species in living (dispersal-limited) assemblages (1) decreases the proportional abundance of abundant species, reducing the steepness of the rank-abundance distribution, and (2) increases the proportional richness of rare, temporally short-lived species that immigrate from the neutral metacommunity with many rare species. These two effects together (1) can shift the mean composition away from the non-averaged (dispersal-limited) assemblages toward averaged assemblages that are less limited by dispersal, resembling that of the metacommunity; (2) allow the point and habitat diversity to increase toward metacommunity diversity under a given sample size (i.e., the diversity in averaged assemblages is inflated relative to non-averaged assemblages); and (3) reduce beta diversity because species unique to individual stations become shared by other stations when limited by a larger but static species pool. Surprisingly, these scale-dependent changes occur at fixed sample sizes and can become significant after only a few decades or centuries of time-averaging, and are accomplished without invoking ecological succession, environmental changes, or selective postmortem preservation. Time-averaging results in less inflation of diversity at habitat than at point scales; paleoecological studies should thus analyze data at multiple spatial scales, including that of the habitat where multiple bulk samples have been pooled in order to minimize time-averaging effects. The diversity of assemblages that have accumulated over 1000 years at point and habitat scales is expected to be inflated by an average of 2.1 and 1.6, respectively. This degree of inflation is slightly higher than that observed in molluscan death assemblages at these same spatial scales (1.8 and 1.3). Thus, neutral metacommunity models provide useful quantitative constraints on directional but predictable effects of time-averaging. They provide minimal estimates for the rate of increase in diversity with time-averaging because they assume no change in environmental conditions and in the composition of the metacommunity within the window of averaging.

Biotic invasions are a common feature of the fossil record, yet remarkably little is known about them, given their enormous potential to reveal the processes that regulate local and regional diversity over long time scales. We used additive diversity partitioning to examine how diversity structure changed as a result of a marine biotic invasion in tropical, shallow and deep subtidal environments spanning approximately 4 Myr in the Late Ordovician. The biotic invasion increased richness in the regional ecosystem by nearly 40%. Within-habitat turnover diversity accounts for most of the increase in richness, with between-habitat turnover diversity contributing a lesser amount. Increases in these components of diversity were accommodated by increased packing of species along a depth gradient and increased habitat heterogeneity. Diversity metrics that incorporate taxon abundance (Shannon information, Simpson's D) show similar patterns and reveal that many invading taxa were locally abundant and widespread in their occurrence. Extinction of incumbent taxa did not foster the invasion; rather the invasion appears to be linked to a regional or global warming event. Taken together, these observations indicate that these Late Ordovician marine communities were open to invasion and not saturated with species. Moreover, the increase in species diversity caused by the invasion was not ephemeral; instead it lasted for at least 1 Myr. Similar studies of other biotic invasions in the fossil record are necessary to determine (1) the factors, such as extinction of incumbents or resource limitation, that may facilitate or inhibit invasion in ancient ecosystems; (2) how local and regional ecosystems respond to invasion; and (3) the extent to which biotic invasions play a substantial role in ecosystem change through geologic time.

In the past decade paleobiologists have applied the techniques of both ecological and historical biogeography, although vicariance/cladistic approaches have as yet had minimal impact. The traditional focus of paleobiogeographic study has been the province, a statistical entity defined by clusters of range endpoints of individual taxa. The study of such provinces has been useful in inferring past continental positions (although ambiguities remain that must be resolved using independent geological criteria) and in understanding the role of past global geographies in regulating biotic diversity through changes in the numbers and extent of provinces. This approach can be complemented by the treatment of geographic ranges of taxa as irreducible or emergent traits with far-reaching evolutionary effects upward and downward within a genealogical hierarchy. Temperature tolerances in benthic marine organisms appear to be by-products of selection for enzyme structures imparting favorable activity levels within the normal temperature range rather than direct products of selection for resistance to temperature extremes. Thus geographic range endpoints, which are also influenced by dispersal capability and the resulting scale of gene flow among disjunct populations, are not direct products of selection. However, the magnitudes of geographic ranges of species and clades behave as emergent properties and significantly influence taxonomic survivorship during background and mass extinctions in ways that are not extrapolations of effects at lower hierarchical levels. Biogeography shapes macroevolutionary patterns of origination and extinction during times of normal, background extinction and mass extinction. Preferential extinction among regions or among endemic rather than widespread clades can result in strong biases in the nature of the survivors of mass extinctions, with taxa being lost not because of selection against attributes of individual organisms but because of higher-order patterns of geographic selectivity.

Geographic range and taxonomic duration are known to be positively correlated in a number of biologic groups; this is usually attributed to the influence of range upon duration rather than the other way about. Here we analyze two distinct components of this correlation within species and genera of marine invertebrates and microfossils by partitioning the total duration into two parts: the time it takes a taxon to attain its maximum geographic range, and the time a taxon persists after attaining its peak range. We find that the longer it takes a taxon to attain its maximum geographic range, the wider is that range. We also find that the broader the maximum range, the greater is the duration after this maximum is attained. These two correlations are equally strong on average. There is thus a reciprocal relationship between duration and geographic range, and there is no compelling evidence that range generally determines duration more or less than duration determines range.

We compare the preservation (taphonomic grade) and age of Chione (bivalve) and foraminifera from modern siliciclastic tidal flat sediments of Bahia la Choya, Sonora, Mexico (northern Gulf of California). Disarticulated shells of Chione collected from the sediment-water interface of Choya Bay exhibit a substantial range in taphonomic grade and age, several hundred years to ∼80–125 ka based on Accelerator Mass Spectrometer 14C dates and D-Alloisoleucine/L-Isoleucine values. There is not, however, a one-to-one correspondence between age and taphonomic alteration of Chione: old (or young) valves may be highly altered or they may be relatively pristine. In contrast to Chione, most foraminiferal tests at Choya Bay are quite pristine, which suggests a quite young age, but tests are surprisingly old (up to ∼2,000 calendar years based on Accelerator Mass Spectrometer 14C dates).

We suggest that following seasonal pulses in reproduction, some foraminiferal tests are rapidly incorporated into a subsurface shell layer by “Conveyor Belt” deposit feeders and preserved there, while the rest of the reproductive pulse rapidly dissolves. Ultimately, some of these buried tests, along with Chione, are transported back to the surface by biological activity and storms. The much greater range of taphonomic grades and ages among Chione shells suggests that they, unlike foraminifera, are sufficiently large and preservable (low surface/volume ratio and chemical reactivity) to undergo many cycles of degradation, burial, and exhumation before complete destruction. The age of foraminiferal tests indicates that time-averaging of microfossil assemblages at Choya Bay is much more insidious than would be expected considering the relatively pristine state of the tests alone.

Based on our studies, the lower limit of temporal resolution of shallow shelf microfossil assemblages appears to be ∼1000 years. We caution, however, that each depositional setting (taphofacies) should be evaluated on a case-by-case basis before gross generalizations are made. Indeed, the discrepancy between age and taphonomic grade of fossil assemblages at Choya Bay suggests that neither hardpart size or taphonomic grade are infallible indicators of test preservability or likely temporal resolution of the host assemblage, and that the dynamics of hardpart input and loss must also be evaluated.

When assessing the timing of branching events in a phylogeny, the most important tools currently recognized are a reliable molecular phylogeny and a continuous, relatively complete fossil record. Coralline algae (Rhodophyta, Corallinales, and Sporolithales) constitute an ideal group for this endeavor because of their excellent fossil record and their consistent phylogenetic reconstructions. We present the evolutionary history of the corallines following a novel, combined approach using their fossil record, molecular phylogeny (based on the 18S rDNA gene sequences of 39 coralline species), and molecular clocks. The order of appearance of the major monophyletic taxa of corallines in the fossil record perfectly matches the sequence of branching events in the phylogeny. We were able to demonstrate the robustness of the node ages in the phylogeny based on molecular clocks by performing an analysis of confidence intervals and maximum temporal ranges of three monophyletic groups of corallines (the families Sporolithaceae and Hapalidiaceae, as well as the subfamily Lithophylloideae). The results demonstrate that their first occurrences are close to their observed appearances, a clear indicator of a very complete stratigraphic record. These chronological data are used to confidently constrain the ages of the remaining branching events in the phylogeny using molecular clocks.

The kill curve for Phanerozoic marine species is used to investigate large-body impact as a cause of species extinction. Current estimates of Phanerozoic impact rates are combined with the kill curve to produce an impact-kill curve, which predicts extinction levels from crater diameter, on the working assumption that impacts are responsible for all “pulsed” extinctions. By definition, pulsed extinction includes the approximately 60% of Phanerozoic extinctions that occurred in short-lived events having extinction rates greater than 5%. The resulting impact-kill curve is credible, thus justifying more thorough testing of the impact-extinction hypothesis. Such testing is possible but requires an exhaustive analysis of radiometric dating of Phanerozoic impact events.

Substantial geographic coverage in paleontological study is essential in testing evolutionary models of phyletic gradualism and punctuated equilibrium. We present a multivariate morphometric study of the late Neogene planktonic foraminiferal clade Globoconella using specimens from four Deep Sea Drilling Project sites (DSDP 284, 207A, 208, and 588) along a latitudinal traverse in the southwest Pacific.

During the Late Miocene (7 Ma to 5 Ma), populations of the ancestral species Globorotalia (Globoconella) conomiozea formed a geographic cline showing continuous morphological variation from the temperate sites (DSDP 284 and 207A) to the warm subtropical sites (DSDP 208 and 588). Populations living to the south had higher conical angle and fewer chambers in the final whorl compared to the northern populations. Nevertheless, populations across the entire cline exhibited a coherent, directional trend towards having larger conical angle and fewer chambers through time. At the Miocene/Pliocene boundary, the intensification of the Tasman Front (Subtropical Divergence) possibly isolated the peripheral populations in the warm subtropics from the central stocks of the temperate water masses. The evolutionary trends became decoupled: the central populations gradually lost their keel and transformed into G. (G.) sphericomiozea, while the peripheral populations in the warm subtropical areas retained their keel and evolved into a flattened species, G. (G.) pliozea.

The gradual transformation of G. (G.) conomiozea terminalis (a form retaining a keel) into G. (G.) sphericomiozea (a form lacking a keel) occurred during an interval of about 0.2 m.y., with all measured morphologic variables showing continuous and steady changes. The evolution of the central populations follows the model of phyletic gradualism. In peripheral populations, the origin of the descendant species G. (G.) pliozea from the ancestor G. (G.) conomiozea terminalis occurred very rapidly within an interval of less than 0.01 m.y. The population size of G. (G.) pliozea was small at the incipient stage at about 5.05 Ma, but increased rapidly to become dominant during the next 0.2 m.y. when the ancestral species G. (G.) conomiozea terminalis became locally extinct. Following speciation, G. (G.) pliozea exhibited morphological stasis for about 0.6 m.y., until the central stock form G. (G.) puncticulata migrated back to the warm subtropics; during the next 0.5 m.y. of their sympatry, there is no sign of hybridization between these two sister species. The evolution of G. (G.) pliozea follows the model of punctuated equilibrium.

The evolution of the Globoconella clade shows both phyletic gradualism and punctuated equilibrium. These two “alternative” evolutionary models complement each other rather than being mutually exclusive. Both models are indispensable towards providing a complete picture of the evolution of Globoconella.

Quantification of Paleozoic, Triassic, Jurassic, and Cretaceous ammonoid shell ornamentation shows that commonality and roughness of ornamentation increased throughout the geologic range of the ammonoids. The two major hypotheses concerning the function of ammonoid shell ornamentation are that 1) ornament served a protective (defensive) function against shell breakage by predators, and 2) it increased hydrodynamic efficiency of the shell during swimming. The heavily ribbed and spined ammonoid shells of the late Mesozoic have ornamentation too coarse to have served any hydrodynamic purpose. The increasing proportion of such shells during the Jurassic and Cretaceous may have been in response to increased numbers of late Mesozoic shell crushing predators and “better armed” ammonoid prey. This trend parallels adaptive trends of other invertebrate groups during the “Mesozoic marine revolution” as defined by Vermeij (1977).

Faunal stasis among Indo-Pacific reef-building corals is explained as the result of chronic environmental disruptions preventing evolutionary processes from approaching completion since the Late Pliocene. The model assumes shallow reefal habitats (<20 m) on the continental shelves are major sites of scleractinian evolution and explores ecological and evolutionary consequences of high-frequency sea-level fluctuations and their associated transgression-regression cycles. Because single generations, dominated by a few large clonal genotypes, may persist indefinitely, local populations may not have experienced enough generations to approach evolutionary equilibrium with their environments during the estimated average duration (≈3200 yr) of existence of shallow habitats. Persistent consequences of chronic evolutionary disturbance may be the extensive intraspecific variation so characteristic of the dominant genera of shallow Indo-Pacific corals and the apparent paucity of recently evolved endemic species. The same disturbances may have accelerated speciation rates among reefal organisms with much shorter generation times.