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.

Species duration data for living benthic foraminifera derived from an extensive literature search have been compiled and analyzed to investigate rates and patterns of species origination. The same data subjected to taxonomic standardization through examination of many specimens lodged in museum collections indicate strikingly different, and more realistic, rates and patterns.

Evolutionary generalizations based on data generated from the literature only are often unreliable and may be directly in opposition to reality. Extensive attempts at taxonomic standardization should be the norm in paleobiological investigations.

Biologic asymmetry is present in all bilaterally symmetric organisms as a result of normal developmental instability. However, fossilized organisms, which have undergone distortion due to burial, may have additional asymmetry as a result of taphonomic processes. To investigate this issue, we evaluated the magnitude of shape variation resulting from taphonomy on vertebrate bone using a novel application of fluctuating asymmetry. We quantified the amount of total variance attributed to asymmetry in a taphonomically distorted fossil taxon and compared it with that of three extant taxa. The fossil taxon had an average of 27% higher asymmetry than the extant taxa. In spite of the high amount of taphonomic input, the major axes of shape variation were not greatly altered by removal of the asymmetric component of shape variation. This presents the possibility that either underlying biologic trends drive the principal directions of shape change irrespective of asymmetric taphonomic distortion or that the symmetric taphonomic component is large enough that removing only the asymmetric component is inadequate to restore fossil shape. Our study is the first to present quantitative data on the relative magnitude of taphonomic shape change and presents a new method to further explore how taphonomic processes impact our interpretation of the fossil record.

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.

All else being equal, species with short life spans are expected to be overrepresented in time-averaged death assemblages relative to their standing abundance in the living community, but the magnitude of the distortion of proportional abundance and assemblage evenness has received little attention. Here, information from 30 data sets on the living and dead abundances of marine bivalves in local habitats is combined with a global compilation of bivalve life spans to determine whether bias from mortality rate can explain observed differences in species proportional abundances. Although bivalve maximum life spans range from one to 75 years in these data sets, indicating annual mortality rates of 0.97 to 0.09, the “life span bias” (LB) of a species–the difference between its proportional abundance expected dead and that observed alive–is consistently small in magnitude (average change <2%, maximum about 20%) and random in sign relative to observed discordance (OD = difference between that species' proportional abundance observed dead and that observed alive). The aggregate result for 413 living species occurrences is a significantly positive but weak correlation of OD to LB, with only 10% of variation in OD explained. The model performs better among longer-lived species than among shorter-lived species, probably because longer-lived species conform better to the model assumption that species maintain a constant proportional abundance in the living assemblage over time. Among individual data sets, only seven exhibit significant positive correlations between OD and LB. The model also under-predicts the cases where a death assemblage is dominated by a species that is shorter lived than the dominant species in the living assemblage, indicating that some factor(s) other than or in addition to mortality rate is responsible for OD. We can find no evidence of preservational bias linked to life span, for example through body size. This negative outcome reflects a weak biological relationship between life span and living abundance among bivalves in local habitats, contrary to the terrestrial paradigm, and points toward a simpler model of time-averaged death assemblage formation where higher abundances reflect (under-sampled) past populations. Contrary to long-held expectations, variation in population turnover among species is not a major source of taphonomic bias in time-averaged death assemblages among bivalves and perhaps among other marine groups: bias must arise largely from other factors.

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.

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.

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.

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 evolution of powered flight has traditionally been associated with the origin of birds, the most successful clade of modern tetrapods, as exemplified by the nearly 10,000 species alive today. Flight requires a suite of morphological changes to skeletal anatomy to create a light yet resistant framework for an airfoil and advanced nervous motor control. Given the level of morphological integration necessary to create a suitable aerofoil, the origin of flight may be intuitively assumed to be coupled with high evolutionary rates of wing-related morphologies. Here we show that the origin of birds is associated with little or no evolutionary change to the skeletal anatomy of the forelimb, and thus Archaeopteryx is unlikely to be the “Rosetta Stone” for the origin of flight it was once believed to be. Using comparative statistics and time-series analyses on a data set constructed from all known forelimb skeletal anatomy of non-avian theropod dinosaurs and a diverse assemblage of early birds, we demonstrate three focused peaks of rapid forelimb evolution at Tetanurae, Eumaniraptora, and Ornithothoraces. The peaks are not associated with missing data and remain stable under multiple perturbations to the phylogenetic arrangements. Different regions of the forelimbs are demonstrated to have undergone asynchronous periods of evolutionary peaks and stasis. Our results evince a more complicated stepwise mode of forelimb evolution before and after the origin of Aves than previously supposed.

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.

Earliest Paleocene megafloras from North America are hypothesized to be low diversity and dominated by long-lived cosmopolitan species following the Cretaceous/Paleogene (K/Pg) mass extinction. However, megafloras used to develop this hypothesis are from the Northern Great Plains (NGP) of North America, and relatively little is known about floras from southern basins. Here, we present a quantitative analysis of an earliest Paleocene megaflora (<350 kyr after K/Pg boundary) from the Ojo Alamo Sandstone in the San Juan Basin (SJB), New Mexico. The megaflora, comprising 53 morphotypes, was dominated by angiosperms, with accessory taxa composed of pteridophytes, lycophytes, and conifers. Diversity analyses indicate a species-rich, highly uneven, and laterally heterogeneous flora. Paleoclimate estimates using multivariate and univariate methods indicate warm temperatures and relatively high precipitation consistent with a modern tropical seasonal forest.

When compared with contemporaneous floras from the Denver Basin (DB) of Colorado and the Williston Basin (WB) of North Dakota, the SJB flora had significantly higher species richness but lower evenness. Paleoclimate estimates from the SJB were 7–14°C warmer than the estimates for the DB and WB, indicating a shift from a temperate forest in the NGP to a tropical forest in the SJB. These results demonstrate the presence of a latitudinal floral diversity and paleoclimatic gradient during the earliest Paleocene in western North America. We hypothesize that the warm, wet conditions in the earliest Paleocene SJB drove rapid rates of speciation following the K/Pg boundary, resulting in a diverse and heterogeneous flora.

Cosmopolitanism occurred recurrently during the geologic past, especially after mass extinctions, but the underlying mechanisms remain poorly known. Three theoretical models, not mutually exclusive, can lead to cosmopolitanism: (1) selective extinction in endemic taxa, (2) endemic taxa becoming cosmopolitan after the extinction and (3) an increase in the number of newly originated cosmopolitan taxa after extinction. We analyzed an updated occurrence dataset including 831 middle Permian to Middle Triassic ammonoid genera and used two network methods to distinguish major episodes of ammonoid cosmopolitanism during this time interval. Then, we tested the three proposed models in these case studies. Our results confirm that at least two remarkable cosmopolitanism events occurred after the Permian–Triassic and late Smithian (Early Triassic) extinctions, respectively. Partitioned analyses of survivors and newcomers revealed that the immediate cosmopolitanism event (Griesbachian) after the Permian–Triassic event can be attributed to endemic genera becoming cosmopolitan (model 2) and an increase in the number of newly originated cosmopolitan genera after the extinction (model 3). Late Smithian cosmopolitanism is caused by selective extinction in endemic taxa (model 1) and an increase in the number of newly originated cosmopolitan genera (model 3). We found that the survivors of the Permian–Triassic mass extinction did not show a wider geographic range, suggesting that this mass extinction is nonselective among the biogeographic ranges, while late Smithian survivors exhibit a wide geographic range, indicating selective survivorship among cosmopolitan genera. These successive cosmopolitanism events during severe extinctions are associated with marked environmental upheavals such as rapid climate changes and oceanic anoxic events, suggesting that environmental fluctuations play a significant role in cosmopolitanism.

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.

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.

Pelagic (open-ocean) species have enormous population sizes and broad, even global, distributions. These characteristics should damp rates of speciation in allopatric and vicariant evolutionary models since dispersal should swamp diverging populations and prevent divergence. Yet the fossil record suggests that rates of evolutionary turnover in pelagic organisms are often quite rapid, comparable to rates observed in much more highly fragmented terrestrial and shallow-marine environments. Furthermore, genetic and ecological studies increasingly suggest that species diversity is considerably higher in the pelagic realm than inferred from many morphological taxonomies.

Zoogeographic evidence suggests that ranges of many pelagic groups are much more limited by their ability to maintain viable populations than by any inability to disperse past tectonic and hydrographic barriers to population exchange. Freely dispersing pelagic taxa resemble airborne spores or wind-dispersed seeds that can drift almost anywhere but complete the entire life cycle only in favorable habitats. It seems likely that vicariant and allopatric models for speciation are far less important in pelagic evolution than sympatric or parapatric speciation in which dispersal is not limiting. Nevertheless, speciation can be quite rapid and involve cladogenesis even in cases where morphological data suggest gradual species transitions. Indeed, recent paleoecological and molecular studies increasingly suggest that classic examples of “phyletic gradualism” involve multiple, cryptic speciation events.

Paleoceanographic and climatic change seem to influence rates of turnover by modifying surface water masses and environmental gradients between them to create new habitats rather than by preventing dispersal. Changes in the vertical structure and seasonality of water masses may be particularly important since these can lead to changes in the depth and timing of reproduction. Long-distance dispersal may actually promote evolution by regularly carrying variants of a species across major oceanic fronts and exposing them to very different selection pressures than occur in their home range. High dispersal in pelagic taxa also implies that extinction should be difficult to achieve except though global perturbations that prevent populations from reestablishing themselves following local extinction. High rates of extinction in some pelagic groups suggests either that global perturbations are common, or that the species are much more narrowly adapted than we would infer from current taxonomies.

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.

The fossil record of Phanerozoic brachiopod genera and Late Cenozoic New World mammal genera is examined for evidence of evolutionary equilibria. One necessary (but insufficient) condition is met: within temporal intervals, numbers of originations correlate with numbers of extinctions. Eliminating temporally short-ranging brachiopods, however, reduces the correlation so that it explains only 16% of the variation. More decisive tests of the equilibrium hypothesis appear impossible with available data. Difficulties of temporal and geographic scale, taxonomic level, and ecological consistency must be resolved before equilibrium models can be applied in paleontology for other than inspiration.