Interpretations of morphologic radiations and macroevolutionary patterns are dependent on a priori choices of taxonomic and geographic scales of study. The results of disparity analysis at varying taxonomic (species and genus) and geographic (regional, biofacies, and community) scales are examined in a study of Ordovician though Early Silurian crinoids. Using discrete morphologic characters, we examined the disparity of 421 crinoids from 65 Laurentian biofacies. Crinoid disparity differs when analyzed at the regional and biofacies levels. Regardless of fluctuations in regional crinoid disparity, average within-biofacies disparity was static throughout the Ordovician, deviating only during the Silurian because of the proliferation of the morphologically aberrant myelodactylid crinoids. The choice of taxonomic level does not have an effect at the biofacies level. However, at the regional level, the two taxonomic scales (genus and species) can produce different results because of variation in the number of species per genus through time and the amount of morphologic variation within individual genera. Weighting disparity by abundance provides a metric combining morphology and community structure. Average weighted disparity at the community level showed patterns similar to that of the biofacies-level disparity curve, but this metric has a greater degree of variation between biofacies. Biofacies with a low ratio of weighted to unweighted disparity display the distinctive community structure (based on aerosol filtration theory) that is often reported in crinoid assemblages.

The regeneration abilities of crinoids not only are important to understanding crinoid ecology, but also can serve as the basis for assessing the pressure exerted on crinoids by predators both in the Recent and in the geologic past. This is especially true of regenerating arms, because arm loss, and subsequent regeneration, is thought to result from interactions with predators, primarily fish. However, the commonly used regeneration-based proxy for predation pressure (proportion of individuals with regenerating arms) does not provide a measure of the rate at which those events occurred. Here we present a method for reconstructing the arm-loss rate per individual, a more direct proxy of predation pressure. This metric accounts for differences in arm length, arm number, and branching pattern, features highly variable among taxa, among environments, and through geologic time. Normalizing for those characters permits the transformation of observed proportions of regenerating arms to rates that can be compared across samples of morphologically distinct crinoids. Applying this method to a Recent crinoid (Cenometra bella) reveals that this shallow-water comatulid loses arms at a rate of about once every ten days. The same approach reveals that Mississippian shallow-water crinoids (Rhodocrinites kirbyi) experienced arm loss much less frequently, approximately once every 36 days, suggesting that predation pressure on crinoids today is greater than it was in the Mississippian.

A number of facts indicate that the diversity of chthamaloid barnacles is much lower now than it has been in the past. Among these are the relict, often disjunct geographic distributions of distinctive chthamaloid higher taxa and their representation by only a few living species. The living chthamaloid barnacles also have a disjunct bathymetric distribution: as we will document here in detail, they are concentrated in the intertidal zone and in deep water. In his rejoinder to our paper, Paine makes no cogent argument against our interpretations of this powerful evidence.

Our case that the balanoids have competitively displaced the more primitive chthamaloids is based on several facts: (1) the Chthamaloidea have dwindled during the Cenozoic Era, so that most have relict or refugial distributions, whereas the Balanoidea have undergone a remarkable adaptive radiation; (2) balanoid species are known to defeat chthamaloid species in competition for space; (3) the competitive success of the balanoids can be attributed to an inherent biological feature—a tubiferous skeleton, which fosters rapid growth; (4) the peak diversity of living balanoids coincides precisely with the bathymetric gap in the chthamaloids’ distribution.

We argue that the radiation of balanoids during the past 40 Myr or so has caused a decline in chthamaloid diversity by reducing and destabilizing low intertidal and shallow subtidal populations, thus elevating average rate of extinction and depressing average rate of speciation. This model allows for exceptions to the rule of balanoid competitive dominance, but contrary to Paine's claim, the exceptions that exist are either trivial or support our view.

Paine offers no alternative to our competition hypothesis. He suggests that an increase in predation has caused a decline in chthamaloid body size, but neither a Cenozoic trend in body size nor one in predation pressure has empirical support; furthermore, the body-size hypothesis has no direct bearing on the case we have presented.

The rarefaction graphs and trophic nuclei of dead molluscan shell cumulates and their associated live molluscan fauna from seven ‘ecological’ habitats in the Essex Chenier Plain facies are examined and compared. The results show that while the trophic nuclei of the live and dead faunas tend to be dissimilar, the rarefaction graphs indicate that the dead fauna will be more diverse than the live, though changes in the diversity of the live fauna tend to be mirrored in the dead fauna.

Differences in zooid skeletal ontogeny are strongly linked to the distribution of bryozoan species, even along such large-scale environmental gradients as depth and latitude. Three growth patterns broadly characterize zooid formation for encrusting cheilostome bryozoans: zooidal and intrazooidal budding, which facilitate growth in two dimensions at colony margins; and frontal budding, which also permits upward growth in the third dimension. Analyses of skeletal growth patterns of encrusting species present in 230 Recent assemblages from the North Atlantic, Mediterranean, North Sea, and Gulf of Mexico show that species with zooidal budding dominate overwhelmingly at lower latitudes and in shallower water, as assessed by percentages of species within assemblages. In contrast, patterns within moderately diverse fossil assemblages from North America indicate that within the geographic and environmental limits sampled, species with intrazooidal budding once dominated and probably originated in shallow, warm water environments. Intrazooidal budding now occurs with greater relative frequency among species from deeper and higher latitude assemblages. Patterns for frontal budding suggest a slight decrease with increasing depth, but no consistent relationship with latitude. These paleoenvironmental trends occur withn a putatively monophyletic clade and are markedly similar to onshore/offshore trends recognized in other groups of marine benthos, based on patterns of occurrence of supra-specific taxa and novel morphotypes. Results presented here are based on relative dominance of species within assemblages and thus provide a novel comparative test of these previously reported trends.

The Guadalupian (middle Permian) extinction may have triggered substantial ecological restructuring in level-bottom communities, such as turnover in dominant brachiopod genera or a shift from abundant brachiopods to mollusks, despite comparatively minor taxonomic losses. However, ecological changes in relative abundance have been inferred from limited data; as a result, constraints on important shifts like the brachiopod-mollusk transition are imprecise. Here, I reevaluate the magnitude of ecological shifts during the Guadalupian–Lopingian (G-L) interval by supplementing previous census counts of silicified assemblages with counts from non-silicified assemblages and global occurrence data, both sourced from the Paleobiology Database. Brachiopod occurrences are consistent with more pronounced faunal composition changes from the Guadalupian to Lopingian than among stages within those intervals, but only in Iran and South China, and not in Pakistan or a Tethys-wide data set. In Iran and South China, Bray-Curtis dissimilarity values comparing occurrence frequencies between adjacent stages were elevated across the G-L transition, although other intervals exhibited similarly large shifts. However, genus occurrence frequencies were less strongly correlated or were anti-correlated across the G-L transition, suggesting moderate faunal turnover among dominant brachiopod genera. In contrast to previous inferences from silicified faunas, abundances of brachiopods, bivalves, and gastropods remained consistent from the Guadalupian to Lopingian in non-silicified local counts and global occurrences, implying that the brachiopod-mollusk shift did not occur until the end-Permian extinction. Ecological and taxonomic consequences were both minor in level-bottom settings, suggesting that severe environmental perturbations may not be necessary to explain biotic changes during the Guadalupian-Lopingian transition.

We outline a plausible evolutionary sequence that led from prokaryotes to the origin of the first nucleated cell. The nucleus is postulated to evolve after the archaebacterium and eubacterium merged to form the symbiotic ancestor of amitochondriate protists. Descendants of these amitochondriate cells (archaeprotists) today thrive in organic-rich anoxic habitats where they are amenable to study. Eukaryosis, the origin of nucleated cells, occurred by the middle Proterozoic Eon prior to the deposition in sediments of well-preserved microfossils such as Vandalosphaeridium and the spiny spheres in the Doushantou cherts of China.

Significant warming of Earth's climate in the near term seems increasingly likely. If significant enough, this climatic regime could, in the long term, come to resemble previous greenhouse intervals in earth history. Consequently, analysis of the fossil record during periods of extreme warmth may provide important lessons for species biology, including biogeography, in a much warmer world. To explore this issue, we analyzed the biogeographic response of 63 molluscan species to the long-term global warmth in the Late Cretaceous Western Interior Seaway (WIS) of North America, using Geographic Information Systems (GIS) to quantitatively measure changes in range size and distribution throughout this interval. We specifically considered the role that geographic range size played in mediating extinction resistance and invasion potential of these WIS species. We found no relationship between geographic range size and survivorship. However, endemic species with small range sizes were more likely to become invasive. Finally, mollusks did not experience a poleward shift in range out of the tropics during this warm regime. To the extent that these patterns are representative, and the WIS and taxa considered constitute a reasonable ancient analogue to a warmer future world, these results suggest that some biogeographic “rules” may not prevail under greenhouse conditions of long-term, equable warmth. They also suggest that other factors beyond geographic range size, including distinctive niche characteristics, may play quite important roles in species survival and invasion potential. This potentially complicates predictions regarding the future responses of extant species to long-term warming.

An unusual isolated specimen representing an immature rhabdosome of Dicellograptus sp. from the Middle Ordovician Athens Shale of Alabama has a flattened sphere attached to the distal end of the nema. The flattened sphere can be interpreted as a float that probably conferred buoyancy on the rhabdosome. Such floats, which became detached in mature rhabdosomes, may be common in the fossil record. The presence of nematophorous siculae in all planktonic graptolites suggests that for immature rhabdosomes a passive, buoyant mode of life, attained by means of a float, might have been widespread taxonomically. This mode of life is compatible with the theory of passive response for the mode of life of mature rhabdosomes, but it is difficult to reconcile with the theory of automobility.

Changes in morphological variability within fossil populations of planktonic protists provide insight into the processes responsible for morphological change. Evolution in the radiolarian genera Theocorythium and Lamprocyclas is documented biometrically in Pliocene and Pleistocene core material from the equatorial Pacific and Indian Oceans. The patterns of morphological change in Theocorythium within a single Pacific core could be interpreted as indicating the in situ evolution of T. trachelium trachelium from T. vetulum via intermediate forms during the Pleistocene. However, consideration of biogeographic data shows that this is not the case and supports instead two alternate interpretations: (1) immigration coupled with extensive hybridization and introgression, or (2) multiple polymorphs of a single species changing in frequency along a cline. Analyses of evolutionary change in Theocorythium and the closely related genus Lamprocyclas during the Pliocene suggest that hybridization may be a significant factor in the production of new species. Evolutionary models and phylogenetic analyses should consider that barriers to gene flow may vary significantly with time and permit the exchange of genetic information between populations previously reproductively isolated.

Consideration of the physics of sinking of hollow, rigid bodies leads to equations of motion for sinking cephalopod shells. We have derived equations of motion for three post-mortem sinking situations: sinking with a fixed amount of water in the phragmocone; rapid phragmocone filling (no siphuncular tube); and slow phragmocone filling (siphuncular tube intact). In all three cases sinking speed can be closely approximated by the terminal velocity calculated from the total weight, buoyancy, and drag parameters of the shell.

Experiments on modern Nautilus shells yield sinking velocities in agreement with calculated values. The experiments also show that orientation of a sinking Nautilus shell varies as the phragmocone fills with water. With small negative buoyancy the shell sinks with its plane of symmetry upright, but as it fills, it begins to rock from side to side and leans over and sinks with its plane of symmetry horizontal when the camerae are about 55% full.

The maximum sinking speed of upright adult Nautilus shells is approximately 30 cm/sec, which appears to be too small for embedding in the bottom upon impact. The maximum depth to which Nautilus sinks in the upright position ranges from about 7 m for rapidly filling shells to as much as 600 m for slowly filling shells. In the latter case, the shell will continue to fill after coming to rest on the bottom, and the stability of the vertical orientation will be removed within 1 or 2 days. Thus, primary vertical preservation of cephalopod shells indicates water depths less than about 10 m.

Psychological evidence suggests that the visual outline of an object is the most important character for discriminating differences in external morphology. External morphology is an important taxonomic character in describing living and fossil species, for example, the blastoid Pentremites. Comparison of different views of the same specimen utilizing Fourier series suggests that the skeleton of Pentremites commonly is not rotationally symmetrical and that the asymmetry is not associated with any specific ray. Analysis of a growth series indicates that the amplitudes of the second and third harmonics are significantly correlated with growth, which is demonstrated to be anisometric. Definable changes in the lateral outline can be attributed to changes in harmonic amplitudes and a wide range of morphological forms comparable to known taxa can be generated by systematically varying the amplitudes of the first four harmonics. The population used in this study probably represents Pentremites robustus Lyon and is from Bangor Limestone in the abandoned Moulton Quarry, Lawrence County, Alabama.

Wolpoff's (1984) recent discussion of evolutionary rates in Homo erectus deserves careful study. Whether Homo erectus or other hominid species exhibit gradual, continuous change in key characters or whether instead there is evidence for morphological stasis in the fossil record is an important question. By allocating all Homo erectus specimens to three groups of early, intermediate, and later geological age and by comparing group means for 13 measurements, Wolpoff attempts to show that gradualism is the rule for this mid-Pleistocene taxon. This method is straightforward, but it is crucial that the samples be composed in a manner which is biologically reasonable. I argue here that Wolpoff has not done this. While there may be legitimate doubt concerning sorting of the fossils, especially where specimens are incomplete, several of the individuals said to be representative of Homo erectus are simply inappropriate for use in this analysis. Wolpoff insists that he has employed a “conservative” definition of the species, but instead he has measured everything in sight. This approach to the record does influence his results.

The linear survivorship curve of Van Valen's Law can be generated from stochastic loss of lineages, given a decreasing sample size within an adaptive zone. No special biological explanation is required or called for in order to explain “linearity” in taxonomic survivorship curves.

The end-Permian mass extinction occurred alongside a large swath of environmental changes that are often invoked as extinction mechanisms, even when a direct link is lacking. One way to elucidate the cause(s) of a mass extinction is to investigate extinction selectivity, as it can reveal critical information on organismic traits as key determinants of extinction and survival. Here we show that machine learning algorithms, specifically gradient boosted decision trees, can be used to identify determinants of extinction as well as to predict extinction risk. To understand which factors led to the end-Permian mass extinction during an extreme global warming event, we quantified the ecological selectivity of marine extinctions in the well-studied South China region. We find that extinction selectivity varies between different groups of organisms and that a synergy of multiple environmental stressors best explains the overall end-Permian extinction selectivity pattern. Extinction risk was greater for genera that had a low species richness, narrow bathymetric ranges limited to deep-water habitats, a stationary mode of life, a siliceous skeleton, or, less critically, calcitic skeletons. These selective losses directly link the extinctions to the environmental effects of rapid injections of carbon dioxide into the ocean–atmosphere system, specifically the combined effects of expanded oxygen minimum zones, rapid warming, and potentially ocean acidification.

The emergence of Phanerozoic global diversity as a central theme of investigation has resulted from a confluence of factors, including the assembly by several researchers of global taxonomic databases; the advent of computers, which permitted construction and analysis of global Phanerozoic diversity trajectories; and the recognition that Phanerozoic diversity trends are important bellwethers of the evolutionary processes that cause biotic transitions. Despite the enormous progress in the measurement and interpretation of Phanerozoic diversity over the past quarter century, much of which has been reported in Paleobiology, these studies have collectively generated at least as many new questions as they have answered—arguably the mark of an area of inquiry that continues to be vital. In this essay, I discuss several outstanding issues in the investigation of Phanerozoic diversity, ranging from the viability of literature-derived databases for investigating global diversity trends, to the biological significance of the myriad biotic transitions that have taken place throughout the Phanerozoic.

Although much natural historical evidence has been adduced in support of the occurrence of several mass extinctions during the Phanerozoic, unambiguous statistical confirmation of the mass extinction phenomenon has remained elusive. Using bootstrapping techniques that have not previously been applied to the study of mass extinction, we have amassed strong or very strong statistical evidence for mass extinctions (see text for definitions) during the Late Ordovician, Late Permian, and Late Cretaceous. Bootstrapping therefore verifies three of the mass extinction events that were proposed by Raup and Sepkoski (1982). A small amount of bootstrapping evidence is also presented for mass extinctions in the Induan (Triassic) and Coniacean (Cretaceous) Stages, but high overall turnover rates (including high origination) in the Induan and uncertain estimates of the temporal duration of the Coniacean force us to conclude that the evidence is not compelling.

We also present the results of more liberal X2 tests of the differences between expected and observed numbers of familial extinctions for stratigraphic stages. In addition to verifying the mass extinctions identified using bootstrapping, these analyses suggest that several stages that could not be verified as mass extinction stages using bootstrapping (including the last three in the Devonian, and the Norian Stage of the Triassic) should still be regarded as candidates for mass extinction. Further analysis will be required to test these stages in more detail.

Dorsal sails supported by hyperelongate neural spines of dorsal vertebrae were evolved by various tetrapods, but most work on their function has centered on the pelycosaur Dimetrodon, in which the sail has generally been interpreted as a thermoregulatory structure that would permit rapid warming in the morning and cooling during the hot midday. The pelycosaur Edaphosaurus differed from other sailed tetrapods in that the neural spines supporting the sail had laterally directed tubercles or cross-bars. Past interpretations of Edaphosaurus suggested that the cross-bars were embedded in a thick fat-storage structure or extended from a thin sail to enhance its utility for intraspecific display. However, wind tunnel modeling of air flow over a thin sail with laterally projecting cross-bars supports a thermoregulatory interpretation of the sail of Edaphosaurus. The cross-bars would produce a turbulent flow, which would increase the effectiveness of convective cooling. Measurements of heat flow in an instrumented model show that cross-bars increase heat loss from the sail. The cross-bars may have enabled Edaphosaurus to thermoregulate effectively with a smaller and lower dorsal sail than would have been required without them.

Both molecular clocks and the first appearances of major groups in the fossil record suggest that most of the range of diatom morphologies observed today had evolved by the end of the Cretaceous Period. Despite this, a canonical reading of the Cenozoic fossil record suggests a dramatic rise in taxonomic diversity that can be interpreted as an explosion of morphological variety. We investigated this apparent discrepancy by using a discrete-character-based, empirical diatom morphospace, resolved by molecular phylogeny and by fossil occurrences through time. The morphospace shows little correspondence to phylogeny and little Cenozoic change in disparity as measured by mean pairwise distance. There is, however, an increase in the total volume of morphospace occupied. Although the increase in occupied volume through time ostensibly supports a conclusion of increasing morphological variety, sampling biases and other data suggest an underlying stationary pattern more consistent with molecular clock data.