Benthic foraminifera have attained gigantic sizes many times throughout geologic history. To understand the selective processes underlying foraminiferal gigantism, we used a computer-based, three-dimension, solid finite element model to analyze the mechanical strength of different discoid forms, including two of the largest living foraminifera—Cycloclypeus carpenteri Brady and Marginopora vertebralis Quoy and Gaimard. These two species enlarge by cyclic, planar growth, resulting in slightly biconvex (C. carpenteri) and slightly biconcave (M. vertebralis) forms. As the tests enlarge, the maximum stresses induced by a standard bending moment decrease in both species. Such stress-reducing growth plans apparently allow growth to extraordinarily large sizes and allow volume to increase with minimal lowering of the surface-to-volume ratio, a critical functional factor in ensuring increased surface area for photosynthesis by endosymbionts contained within the tests and for chemical exchange by the foraminifera with the external environment. Of the two species, M. vertebralis has a stronger construction and a lower surface-to-volume ratio. These features indicate optimal constructional solutions to environmental constraints (degree of turbulence and light availability) in their disparate natural habitats: M. vertebralis in the mechanically rigorous inter- to subtidal, and C. carpenteri on the light-minimal and hydraulically quieter, deeper sea beds. We conclude that morphological design in larger foraminifera is constrained by a biomechanical factor, and that gigantism and biomechanical optimization are demonstrably related.

Synchronous episodes of phenotypic change in five lineages of the endemic land snail genus Mandarina of the oceanic Bonin (Ogasawara) Islands through 40,000 years were documented by morphological analyses of radiocarbon-dated fossil specimens. Adult shell traits of these lineages show mostly synchronous patterns of change with longer intervals of stability and rapid shifts to other states of stability. Although there are intermediates between these different phenotypic states, the net interval of the phenotypic shift was relatively short, i.e., no longer than 7000 years. Extinction of a lineage and speciation occurred during the intervals of the rapid phenotypic shift. This period of abrupt change corresponds to the period of lowest sea level and end of the last glacial period. This implies that climatic change is one of the causes of these episodes of simultaneous rapid phenotypic shifts, speciation, and extinction in different lineages. These findings suggest that major phenotypic changes in phyletic evolution, separation of different species, and extinction of lineages may occur synchronously in many lineages during an extremely short interval.

Nonmammalian cynodonts represent a speciose and ecologically diverse group with a fossil record stretching from the late Permian until the Cretaceous. Because of their role as major components of Triassic terrestrial ecosystems and as the direct ancestors of mammals, cynodonts are an important group for understanding Mesozoic tetrapod diversity. We examine patterns of nonmammalian cynodont species richness and the quality of their fossil record. A supertree of cynodonts is constructed from recently published trees and time calibrated using a Bayesian approach. While this approach pushes the root of Cynodontia back to the earliest Guadalupian, the origins of Cynognathia and Probainognathia are close to their first appearance in the fossil record. Taxic, subsampled, and phylogenetic diversity estimates support a major cynodont radiation following the end-Permian mass extinction, but conflicting signals are observed at the end of the Triassic. The taxic diversity estimate shows high diversity in the Rhaetian and a drop across the Triassic/Jurassic boundary, while the phylogenetic diversity indicates an earlier extinction between the Norian and Rhaetian. The difference is attributed to the prevalence of taxa based solely on teeth in the Rhaetian, which are not included in the phylogenetic diversity estimate. Examining the completeness of cynodont specimens through geological time does not support a decrease in preservation potential; although the median completeness score decreases in the Late Triassic, the range of values remains consistent. Instead, the poor completeness scores are attributed to a shift in sampling and taxonomic practices: an increased prevalence in microvertebrate sampling and the naming of fragmentary material.

Rancho La Brea is unquestionably the most famous fossil deposit in North America, and its fauna occupies a key position in vertebrate paleontology as the basis for the Rancholabrean Land Mammal Age. The 23 acres now known as Hancock Park yielded the bulk of fossil specimens from this locality. A minimum of 518 taxa have been identified to date including 79 algae (all but one of which are diatoms), 72 vascular plants, 9 nonmarine mollusks (13 taxa of marine mollusks were carried to the site by early Indians), 135 arthropods, 27 lower vertebrates, 138 birds, and 58 mammals. A detailed list of the biota, too lengthy for inclusion here, is available from the authors on request.

During the middle Cretaceous, exceptional ecologic conditions developed in the Tarfayan Basin (SW Morocco) marked by the deposition of marls and limestones, rich in organic matter. The ostracod fauna of the Cenomanian to Coniacian consisted of a few ornamentally stable species. Towards the close of the Turonian, Oertliella? tarfayaensis became dimorphic at the top of its range, the new morph with smooth lateral zones being the undoubted ancestor of Oertliella? chouberti. The multivariate morphometric evolutionary pattern for O.? tarfayaensis fluctuates about a stationary value in a manner which could reflect evolutionary stasis. The passage from the ancestral to the descendant species took between 1 × 105 and 2 × 105 yr. At the multivariate level, the transition could have taken place by random genetic drift with, or without, selective effects. At the univariate level, there is no clear evidence for either of these mechanisms. The phenotype is regionally stable which suggests that a non-genetic origin of the change is unlikely. An alternative model for the speciation event is allopatric speciation in a peripheral isolate and immigration. The Late Turonian morph may have resulted from environmentally cued polymorphism with elimination of the original morph due to a long term ecologic change, possibly connected with an increase in silica in the environment.

Silurian crinoids reached maximum abundance on shallow shelf bottoms of skeletal-rich carbonate. In-place crinoid preservation in the Wenlock Limestone of Dudley, England, provides a community model for these occurrences, showing a pattern among crinoids of high taxonomic diversity, conspecific clustering, relatively robust morphology, and numerical dominance of other invertebrates. Densities of about 40 crinoid individuals per m2 occupied a feeding level between 10 cm and 1 m above the bottom and may have effected trophic control over other fauna. Beneath the crinoids lived a second level of bryozoa, a third level of tabulates, and a sparse fauna of brachiopods and other solitary organisms. Crinoids also exercised significant biologic control of substrate. In contrast to the Dudley assemblage, Silurian communities of terrigenous, soft-bottom environments contained crinoids of low abundance, low taxonomic diversity, and small size. In the terrigenous communities, crinoids appear to have had no major effect on either trophic structure or substrate. High sedimentation rates limited the role of crinoids in such communities, and the dominant organisms were deposit feeders and solitary, low-level suspension feeders.

Understanding temporal patterns in biodiversity is an enduring question in paleontology. Compared with studies of taxonomic diversity, long-term perspectives on ecological diversity are rare, particularly in terrestrial systems. Yet ecological diversity is critical for the maintenance of biodiversity, especially during times of major perturbations. Here, we explore the ecological diversity of Cretaceous herbivorous dinosaurs leading up to the K-Pg extinction, using dental and jaw morphological disparity as a proxy. We test the hypothesis that a decline in ecological diversity could have facilitated their rapid extinction 66 Ma. We apply three disparity metrics that together capture different aspects of morphospace occupation and show how this approach is key to understanding patterns of morphological evolution. We find no evidence of declining disparity in herbivorous dinosaurs as a whole—suggesting that dinosaur ecological diversity remained high during the last 10 Myr of their existence. Clades show different disparity trends through the Cretaceous, but none except sauropods exhibits a long-term decline. Herbivorous dinosaurs show two disparity peaks characterized by different processes; in the Early Cretaceous by expansion in morphospace and in the Campanian by morphospace packing. These trends were only revealed by using a combination of disparity metrics, demonstrating how this approach can offer novel insights into macroevolutionary processes underlying patterns of disparity and ecological diversity.

The Ordovician was a time of drastic biological and geological change. Previous work has suggested that there was a dramatic increase in global diversity during this time, but also has indicated that regional dynamics and dynamics in specific environments might have been different. Here, we contrast two paleocontinents that have different geological histories through the Ordovician, namely Laurentia and Baltica. The first was situated close to the equator throughout the whole Ordovician, while the latter has traversed tens of latitudes during the same time. We predict that Baltica, which was under long-term environmental change, would show greater average and interval-to-interval origination and extinction rates than Laurentia. In addition, we are interested in the role of the environment in which taxa originated, specifically, the patterns of onshore–offshore dynamics of diversification, where onshore and offshore areas represent high-energy and low-energy environments, respectively. Here, we predict that high-energy environments might be more conducive for originations.

Our new analyses show that the global Ordovician spike in genus richness from the Dapingian to the Darriwilian Stage resulted from a very high origination rate at the Dapingian/Darriwilian boundary, while the extinction rate remained low. We found substantial interval-to-interval variation in the origination and extinction rates in Baltica and Laurentia, but the probabilities of origination and extinction are somewhat higher in Baltica than Laurentia. Onshore and offshore areas have largely indistinguishable origination and extinction rates, in contradiction to our predictions. The global spike in origination rates at the Dapingian/Darriwilian boundary is apparent in Baltica, Laurentia, and onshore and offshore areas, and abundant variability in diversification rates is apparent over other time intervals for these paleocontinents and paleoenvironments. This observation hints at global mechanisms for the spike in origination rates at the Dapingian/Darriwilian boundary but a domination of more regional and local mechanisms over other time intervals in the Ordovician.

In this study we focused on the dynamics of encrusting assemblages preserved on brachiopod hosts collected from upper Frasnian and lower Famennian deposits of the Central Devonian Field, Russia. Because the encrusted brachiopods come from deposits bracketing the Frasnian/Famennian (F/F) boundary, the results also shed some light on ecological differences in encrusting communities before and after the Frasnian–Famennian (F-F) event. To explore the diversity dynamics of encrusting assemblages, we analyzed more than 1300 brachiopod valves (substrates) from two localities. Taxon accumulation plots and shareholder quorum subsampling (SQS) routines indicated that a reasonably small sample of brachiopod host valves (n=50) is sufficient to capture the majority of the encrusting genera recorded at a given site. The richness of encrusters per substrate declined simultaneously with the number of encrusting taxa in the lower Famennian, accompanied by a decrease in epibiont abundance, with a comparable decrease in mean encrustation intensity (percentage of bioclasts encrusted by one or more epibionts). Epibiont abundance and occupancy roughly mirror each other. Strikingly, few ecological characteristics are correlated with substrate size, possibly reflecting random settlement of larvae. Evenness, which is negatively correlated with substrate size, shows greater within-stage variability among samples than between Frasnian and Famennian intervals and may indicate the instability of early Famennian biocenoses following the faunal turnover. The occurrence distribution of encrusters points to nonrandom associations and exclusions among several encrusting taxa. However, abundance and occupancy of microconchids remained relatively stable throughout the sampled time interval. The notable decline in abundance (~60%) and relatively minor decline in diversity (~30%) suggest jointly that encrusting communities experienced ecological collapse rather than a major mass extinction event. The differences between the upper Frasnian and lower Famennian encrusting assemblages may thus record a turnover associated with the F-F event.

One of the longest-lived, noncolonial animals on the planet today is a bivalve that attains life spans in excess of 500 years and lives in a cold, seasonally food-limited setting. Separating the influence of temperature and food availability on life span in modern settings is difficult, as these two conditions covary. The life spans of fossil animals can provide insights into the role of environment in the evolution of extreme longevity that are not available from studies of modern taxa. We examine bivalves from the unique, nonanalogue, warm and high-latitude setting of Seymour Island, Antarctica, during the greenhouse intervals of the Late Cretaceous and Paleogene. Despite significant sampling limitations, we find that all 11 species examined are both slow growing and long-lived, especially when compared with modern bivalves living in similar temperature settings. While cool temperatures have long been thought to be a key factor in promoting longevity, our findings suggest an important role for caloric restriction brought about by the low and seasonal light regime of the high latitudes. Our life-history data, spanning three different families, emphasize that longevity is in part governed by environmental rather than solely phylogenetic or ecologic factors. Such findings have implications for both modern and ancient latitudinal diversity gradients, as a common correlate of slow growth and long life is delayed reproduction, which limits the potential for evolutionary change. While life spans of modern bivalves are well studied, data on life spans of fossil bivalves are sparse and largely anecdotal. Life histories of organisms from deep time can not only elucidate the controls on life span but also add a new dimension to our understanding of macroevolutionary patterns.

The hadrosaurids were a successful group of herbivorous dinosaurs. During the Late Cretaceous, 100 to 66 million years ago, hadrosaurids had high diversity, rapid speciation rates, and wide geographic distribution. Most hadrosaurids were large bodied and had similar postcranial skeletons. However, they show important innovations in the skull, including disparate crests that functioned as socio-sexual display structures, and a complex feeding apparatus, with specialized jaws bearing dental batteries. Little is known about the macroevolutionary processes that produced these evolutionary novelties. Here we provide novel perspectives using evolutionary rate and disparity analyses. Our results show that hadrosaurid cranial evolution was complex and dynamic, but their postcranial skeleton and body size were conservative. High cranial disparity was achieved through multiple bursts of phenotypic innovation. We highlight contrasting evolutionary trends within hadrosaurids between the disparate facial skeleton and crests, which both showed multiple high-rate shifts, and the feeding apparatus, which had low variance and high rates on a single phylogenetic branch leading to the diverse Saurolophidae. We reveal that rapid evolutionary rates were important for producing the high disparity of exaggerated crests and present novel evidence that the hadrosaurid diversification was linked to both a key adaptive innovation in the feeding apparatus and multiple bursts of innovation in socio-sexual displays.

The lack of tools for teasing genetic information out of the fossil record has been a source of frustration to both neontologists and paleontologists, both of whom would like to know more about what was happening genetically in association with such evolutionarily significant events as cladogenesis and extinction. The dearth of such information has been a factor contributing to the current schism between population geneticists and paleontologists over interpretations of historical patterns of evolution (Gould and Eldredge 1977; Stanley 1979; Charlesworth et al. 1982; Levinton 1983; Turner 1983), a schism which seems unbridgeable without at least some insight into paleontological patterns of genetic variation. Several recent papers, however, have documented a negative correlation between the level of fluctuating asymmetry (small, random, right-left differences between otherwise bilaterally symmetrical characters) and the level of heterozygosity in a variety of organisms. Whatever its underlying cause may be, this correlation raises a flicker of hope that a tool may exist for inferring whether some populations of fossil taxa were more variable genetically than others.

Genetic and morphologic differentiation in the bryozoan Schizoporella errata parallel each other over a distance of 102 km along the southern shore of Cape Cod. This is seen in the allele frequencies of two marker genes and in the length of the avicularium (a type of defensive polymorph of the bryozoan colony). These changes appear to be part of the continuing evolutionary adaptation to local environmental conditions, perhaps characteristic of marine species whose dispersal ability is very limited. The length scale of significant changes in allele frequencies at the two loci is as low as 10–11 km, and for avicularium length is 13 km.

The analysis of morphological disparity and of morphospace occupation through the macroevolutionary history of clades is now a major research program in paleobiology, and increasingly so in organismal and comparative biology. Most studies have focused on the relationship between taxonomic diversity and morphological disparity, and on ecological or developmental controls. However, the geographic context of diversification has remained understudied. Here we address geography quantitatively. Diversity, disparity, and paleogeographic dispersion are used to describe the evolutionary history of an extinct echinoderm clade, the class Stylophora (cornutes, mitrates), from the Middle Cambrian to the Middle Devonian (about 128 Myr subdivided into 12 stratigraphic intervals). Taxonomic diversity is estimated from a representative sample including 73.3% of described species and 92.4% of described genera. Stylophoran morphology is quantified on the basis of seven morphometric parameters derived from image analysis of homologous skeletal regions. Three separate principal coordinates analyses (PCO) are performed for thecal outlines, plates from the lower thecal surface, and plates from the upper thecal surface, respectively. PCO scores from these three separate analyses are then used as variables for a single, global, meta-PCO. For each time interval, disparity is calculated as the sum of variance in the multidimensional morphospace defined by the meta-PCO axes. For each time interval, a semiquantitative index of paleogeographic dispersion is calculated, reflecting both global (continental) and local (regional) aspects of dispersion.

Morphospace occupation of cornutes and mitrates is partly overlapping, suggesting some morphologic convergences between the two main stylophoran clades, probably correlated to similar modes of life (e.g., symmetrical cornutes and primitive mitrocystitids). Hierarchical clustering allowed the identification of three main morphological sets (subdivided into 11 subsets) within the global stylophoran morphospace. These morphological sets are used to analyze the spatiotemporal variations of disparity. The initial radiation of stylophorans is characterized by a low diversity and a rapid increase in disparity (Middle Cambrian–Tremadocian). The subsequent diversification involved filling and little expansion of morphospace (Arenig–Middle Ordovician). Finally, both stylophoran diversity and disparity decreased relatively steadily from the Late Ordovician to the Middle Devonian, with the exception of a second (lower) peak in the Early Devonian. Such a pattern is comparable to that of other Paleozoic marine invertebrates such as blastozoans and orthid brachiopods. During the Lower to Middle Ordovician, the most dramatic diversification of stylophorans took place with a paleogeographic dispersion essentially limited to the periphery of Gondwana. In the Late Ordovician, stylophorans steadily extended toward lower paleolatitudes, and new environmental conditions, where some of them radiated, and finally survived the end-Ordovician mass extinction (e.g., anomalocystitids). This pattern of paleobiogeographic dispersion is comparable to that of other examples of Paleozoic groups of marine invertebrates, such as bivalve mollusks.

Documenting sexual dimorphism for structures that exhibit indeterminate growth can be more difficult than for structures exhibiting determinate growth. Most proboscidean tusks are ever-growing structures that change size and shape throughout life. Sexual dimorphism is pronounced in tusks of mature individuals, but the external form of tusks offers no clear evidence of maturation, and it is difficult to distinguish a young male's tusk from that of an older female. Thus, with previous approaches, knowledge of age was often required to assess sex from tusk measurements. This study examines sexual dimorphism of American mastodon (Mammut americanum) tusks through principal components analysis to determine which aspects of tusk form contribute most strongly to the variance among measurements and to explore the relationship between tusk form and individual age and sex. Twenty-one mastodon tusks from the Great Lakes region were evaluated in two analyses, the first focusing on geometrically distinct aspects of tusk form and the second adding measurements that reflect ontogenetic changes in a single aspect of morphology (circumference). Both analyses separated mastodons by sex (PC-I) and sorted them by age (PC-II). The distribution of tusks on the PC-II versus PC-I plane provides better discrimination of sex than univariate or bivariate methods because tusks of similar size and opposite sex appear near opposite ends of an age spectrum. The second analysis enhances sorting by age, thereby clarifying assessment of sex. This work contributes to studies of mastodon paleobiology by presenting a reliable method for assessing the sex of an individual from tusk measurements without requiring independent knowledge of age.

The environmental transformations that occurred during the Neogene had profound effects on spatiotemporal biodiversity patterns, yet the modulating role of traits (i.e., physiological, ecological, and life-history traits) remains little understood. We tested this idea using the Neogene fossil record of chondrichthyans along the temperate Pacific coast of South America (TPSA). Information for georeferenced occurrences and ecological and life-history information of 38 chondrichthyan fossil genera in 42 Neogene sites was collected. Global georeferenced records were used to estimate present-day biogeographic distributions of the genera and to characterize the range of oceanographic conditions in which each genus lives as a proxy of their realized niche. Biogeographic range shifts (Neogene–present) were evaluated at regional and local scales. The role of traits as drivers of different range dynamics was evaluated using random forest models. The magnitude and direction of biogeographic range shifts were different at both spatial scales. At a regional scale, 34% of genera contracted their ranges, disappearing from the TPSA. At a local scale, a similar proportion of genera expanded and contracted their southern endpoints of distribution. The models showed a high precision at both spatial scales of analyses, but the relative importance of predictor variables differed. At a regional scale, disappearing genera tended to have a higher tolerance to salinity, lower sea surface temperature (SST) range, and smaller body sizes. At a local scale, genera contracting their ranges tended to live at greater depths, tolerate lower levels of primary productivity, and show a reduced tolerance to higher and lower SST ranges. The magnitude and direction of the changes in the range distribution were scale dependent and variable across the genera. Hence, multiple environmental exogenous factors interacted with taxon traits during the Neogene, creating a mosaic of biogeographic dynamics.

The accuracy of numerical summaries of data from the fossil record has been hotly contested in recent years. In this paper I present a computer simulation which mimics the process of preservation and the resultant loss of data in survivorship records. The simulation accepts as its input data a hypothetical “original” record and a time dependent model of preservational loss. These parameters are used to generate a “preserved” record, and the “original” and “preserved” data sets are then compared. Eleven hypothetical original records having different patterns of diversity and turnover were “preserved” in this manner. Indices of diversity, origination, extinction, turnover, and longevity were evaluated for each of five different models of preservational bias. All of the indices behaved in a single characteristic fashion. Taxonomic data that contained large fluctuations (of the order 100%) were preserved accurately. Similarly the large scale changes in preserved records matched original distributions. Records with small fluctuations (30%) were variably preserved and when such fluctuations were present in the preserved record they did not always correlate with events in the original record. Longevities were more accurately preserved than were other forms of taxonomic data. The results are believed to reflect the levels of accuracy obtainable from generic and familial data and suggest ways in which data sets warranting more detailed study may be singled out.

Oxygen and carbon isotopic analyses have been conducted on Miocene to Pliocene (6.0 to 2.9 Ma) members of the gradually evolving, deep-dwelling planktonic foraminiferal clade, Globoconella, in temperate waters of the southwest Pacific, Deep Sea Drilling Project (DSDP) Site 593. In the late Miocene the clade began with Globoconella conoidea, and continued through G. conomiozea, G. sphericomiozea, and G. puncticulata to the extant form G. inflata. Isotopic analyses were performed on ancestor-descendant species within the clade to determine if isotopic differences exist between these species which would, in turn, suggest depth and/or seasonal habitat differences and perhaps segregation, as well as ecological changes in the clade. Isotopic analyses were also conducted on the relatively shallow-dwelling planktonic foraminifer Orbulina universa and the benthic form Cibicidoides wuellerstorfi to determine if any relationships exist between the evolution of Globoconella and paleoceanographic/paleoclimatic change.

Small (usually 0.1–0.15 ‰ but up to 0.3 ‰) oxygen isotopic differences exist between ancestor and descendant forms that we believe represent small (∼1°) temperature differences. These temperature differences are inferred to indicate depth and/or seasonal habitat differences and possible segregation between the species during the gradual evolution. The largest differences in oxygen isotopic values occur between ancestor and descendant forms during the most conspicuous morphological transition within the clade near the Miocene/Pliocene boundary. During this interval, the clade underwent a transformation from conical to spherical forms and there was a loss of the keel. No consistent differences were observed between ancestor and descendant carbon isotopic values.

Both morphological and ecological evolution appear to have been associated with paleoceanographic/paleoclimatic changes. Intervals marked by warming of surface-to-upper intermediate waters are associated with evolution of forms with a spherical test and inferred adaptation to cooler waters relative to ancestral forms. We propose two alternative models for the evolution of the Globoconella clade. In the first model, we assume that depth and/or seasonal segregation between ancestor and descendant forms provided a partial barrier to gene flow and that evolution resulted from genetic drift and/or differential selective pressures acting on each morphotype. In the second model, we assume that coeval members of Globoconella formed a vertical and/or seasonal morphocline in the water column and that segregation did not provide an effective barrier to gene flow. Evolution proceeded as directional selection acted on morphotypes of Globoconella inhabiting selectively advantageous positions in the water column.

Fitting the generalized inverse Gaussian-Poisson distribution (GIGP) to observed frequency distributions of taxa from the Late Triassic Molteno Formation of South Africa has yielded estimates of the corresponding preserved biodiversities. Three extrapolations have been made on the basis of the uniquely rich megaflora/insect coassemblages from 100 taphocoenoses: insect species—335 observed, 7740 preserved; vegetative species—206 observed, 667 preserved; gymnosperm ovulate orders—16 observed, 84 preserved. The reliability of the results varies according to the abundance and observed diversity of the taxa. These results, with further estimations in a companion paper of existed diversity (regional, continental and global), hint at Late Triassic floral and faunal richness akin to today. This conflicts with the traditionally held model of an increasing cone of biodiversity through time and suggests a phase of explosive evolution in the Triassic hitherto unsuspected. Application of the GIGP to other well-documented collections from other periods might reveal a pattern of diversity trends offering fundamentally new insights into the evolving terrestrial biosphere.

Extensive investigation of the close association between biological diversity and environmental temperature has not yet yielded a generally accepted, empirically validated mechanism to explain latitudinal gradients of species diversity, which occur in most taxa. Using the highly resolved late Cenozoic fossil records of four major taxa of marine plankton, we show that their gradients arise as a consequence of asymmetric geographic range expansion rather than latitudinal variation in diversification rate, as commonly believed. Neither per capita speciation nor extinction rates trend significantly with temperature or latitude for these marine plankton. Species of planktonic foraminifera and calcareous nannoplankton that originate in the temperate zone preferentially spread toward and arrive earlier in the tropics to produce a normal gradient with tropical diversity peaks; by contrast, temperate-zone originating species of diatoms and radiolarians preferentially spread toward and arrive earlier in polar regions to produce reversed gradients with high-latitude diversity peaks. Our results suggest that temperature affects latitudinal diversity gradients chiefly by its effect on species’ range limits rather than on probabilities of speciation and extinction. We show that this mechanism also appears to operate in various multicellular taxa, thus providing a widely applicable explanation for the origin of latitudinal diversity gradients.