Studies of taxonomic diversity over time commonly count and compare first- and last-appearance data (FADs and LADs) over a succession of temporal intervals, and interpret them with respect to taxon origination and extinction. Singleton taxa, which first appear and last appear in the same temporal interval, are often removed from analyses because they might result from preservational biases rather than evolutionary processes, or they might represent non-independent FADs and LADs. Should singleton taxa always be excluded? We argue that in the case of Paleozoic terebratulide brachiopods, although they may be sensitive to biases in sampling intensity, singleton genera should be included in diversity studies because they do not appear to result from more typical biases, such as Lagerstätten and temporal interval length, that arguably can result in artificially high numbers of singleton genera.

Singleton genera can be critical and effective when used to test hypotheses regarding the existence and generation of latitudinal diversity gradients. Contrary to the anti-tropical diversity pattern of modern articulated brachiopods, Paleozoic terebratulides show a latitudinal diversity gradient that peaks in the Tropics. The hypothesis that the Tropics are either a diversity source or sink can be tested by comparing FAD and LAD latitudes. For singleton genera, FAD and LAD latitudes are taken from the same data points and must be removed for statistical comparisons to be valid. We suggest that taxon age distributions can accommodate singleton data, as the taxon age metric considers origination and extinction simultaneously. We generated taxon age distributions to test the hypothesis that the observed Paleozoic diversity gradient results from a latitudinal bias in generic turnover rate. We discovered that singletons are not randomly distributed over latitude, with proportionally more singleton genera occurring in the Tropics. In this case, singleton genera may reflect rapid evolutionary turnover of taxa, rather than simply preservational bias. Methods that can accommodate singleton taxa should be used to study the diversity of Paleozoic terebratulides and possibly other well-skeletonized marine metazoans.

Predatory marine snails and their prey provide a unique look at many aspects of predation events, allowing behavioral inference and studies of coevolution. This study examines differential predation patterns, rates, and success of two co-occurring gastropod predator families which drill two co-occurring species of Turritella (Turritellidae: Gastropoda) in the Gulf of California. Both naticid and muricid predators, identified by the shapes of their respective boreholes, attacked the thinner-shelled Turritella leucostoma more frequently than the thicker-shelled Turritella gonostoma. Both species were drilled more frequently and more successfully by naticid, as compared to muricid, predators. Naticids drilled prey in the 40- to 70-mm size class most frequently. Prey over 100 mm in length were relatively safe from all drilling predators. Predator size (estimated by borehole diameter) in naticids was correlated with prey size in both species, but for a given-sized prey, predators on T. gonostoma were proportionally larger. There was no size correlation for muricid predators. Unsuccessful attempts (incomplete drilling) were started on the suture more often than were completed holes, for both predator families on both prey species. Naticids began drilling T. leucostoma on the suture significantly less than expected by chance. We looked for possible changes over evolutionary time by analyzing prey shells from Pleistocene and Recent storm deposits. We found no evidence of change in any aspect of implied predatory behavior over the past 100 k.y.

Previous observations about the stable nature of coral-rich assemblages from the Middle Devonian Hamilton Group have led some researchers to invoke the primacy of ecological controls in maintaining biofacies structure through time. However, few analyses have examined the degree to which recurring biofacies vary quantitatively, and none have assessed lateral variability as a benchmark for testing the significance of temporal variability. Thus, the extent to which Hamilton biofacies persist and the mechanism(s) responsible for their hypothesized stability remain contentious. In this study, recurring coral-rich biofacies were evaluated from two stratigraphic horizons within the Middle Devonian Appalachian Basin to examine (1) the extent to which species assemblages persisted within the basin through space and time, and (2) whether ecological interactions may be a plausible mechanism for generating the degree of stasis observed in this case.

Variations in species composition and abundance were examined across multiple spatial scales within both sampled coral-rich horizons. This permitted the establishment of a baseline against which temporal differences in biofacies composition and structure could be evaluated. Although successive coral-rich horizons remained taxonomically stable, their dominance structures changed significantly through the 1.5 Myr study interval. Moreover, additional comparisons among older Hamilton coral-rich horizons corroborate our primary results. These findings support a model in which species respond individually to fluctuations in the physical environment, as indicated by the fluidity of their relative abundances geographically and temporally.

A method is provided to calculate the expected distribution of species' ranges for use as a basis of comparison for paleontological interpretation of species' range charts. If the method is used for such studies, the possibility of suggesting an environmental or biological cause for observed patterns, when in fact no such cause exists, will be diminished. The method is applied to a large, well-studied fossil data set to illustrate some possible species' range patterns which result only from sample size inequities. As examples, stepwise extinction and species richness patterns are predicted for situations where, in reality, none exist.

The Brontotheriidae (Perissodactyla, Mammalia) are often used as an illustration of vertebrate macroevolutionary trends because their morphological evolution includes significant size increases accompanied by the disproportionate lengthening of bony frontonasal horns. The positive phylogenetic allometry for horn length vs. skull length is among the strongest known of such relationships in vertebrate phylogeny. Hypotheses explaining the change from small, incipient horns in Eocene ancestors to longer horns in Oligocene descendants have included two heterochronic mechanisms, hypermorphosis (extrapolation) and predisplacement (earlier onset time of horn growth). These proposed peramorphic mechanisms derive from interpretation of adult intergeneric allometries in logarithmic data spaces. Analysis of the raw (unlogged) data shows that the simple allometric model previously used is not an appropriate model for this specific problem. The heterochronic interpretations derived from them are therefore unsupported (but not disproven) by the allometries. A more appropriate allometric model for the data (full model) does not support any heterochronic interpretation. Previously unaccounted for in the heterochronic hypotheses is a complication due to body-size scaling effects on life history stage lengths. Neontological scaling patterns suggest that brontothere size increases were probably accompanied by increasing life spans and longer developmental stages. This effect broadens the types of heterochronies that may be postulated. Semiquantitative analyses comparing brontotheres with similarly sized extant ungulates show the hypothesized effect of larger size on brontothere life history stages. A scaled descendant ontogeny introduces the problem of relative vs. absolute time frames within which to view ontogenetic onset times. Thus, predisplacements, postdisplacements, or nondisplacements may be viewed as relative or absolute with respect to ancestral ontogenies. This raises a fundamental question about how development scales, which in turn affects how heterochronies are interpreted. A scaling effect suggests that brontothere horns are more likely postdisplaced in the traditional absolute time sense. Paradoxically then, while the descendant adult horn is peramorphic, its onset time may have shifted in a paedomorphic direction. Data for two Oligocene juvenile brontotheres suggest that most horn growth occurred late in their longer (i.e., descendant) ontogenies (hypermorphosis), and that the horns probably grew at faster rates (acceleration) than in Eocene taxa.

Using computer simulations and a quantitative method for describing bifurcating structures, the morphology of branching patterns seen in early land plants is analyzed. Four types or models of random branching (regular, geometric, binomial, and poisson) are shown to adequately describe the range of observed branching in most early land plants. Approximately 57% of all randomly generated computer patterns show reiterative branching events (=three successive identical modes of branching). Artificial canalization of reiterative events results in branching patterns structurally analogous with that of ancient fossil plants. Simulated phylogenetic changes among early land plant lineages, based on parsimonious transitions in branching patterns, indicate that most observed trends can be related directly to those seen in randomly generating branching patterns in which “size” is increased. The trimerophyte to progymnosperm trend in changing branching patterns is an exception, since the binomial model describing the progymnosperms has not been simulated by random processes.

While the apparent phylogenetic changes among early land plant groups do not require deterministic explanations, the transition from regular to geometric branching and the “canalization” of reiterative branching patterns may represent a grade level response to selective pressures related to mechanical design and vegetative reproduction.

We have investigated whether changes in marine faunal diversity during the Pleistocene would have been affected by changes in sea level. Our estimate of changes in area of shallow marine seas between low stand and high stand of sea level (approximately 200 m) leads to a prediction of approximately a 25% change in species diversity, 10% change in generic diversity and less than a 5% change in familial diversity. This agrees with the observation that large scale generic or familial changes have not been widely noted for the Pleistocene. As in other intervals of geologic time, changes in the sizes of faunal provinces owing to changes in sea level are less likely to be the significant factor in changing marine diversity than are changes in the number of faunal provinces.

Even though Neogene outcrops along the temperate Pacific coast of South America harbor a rich marine vertebrate fossil record, no studies have examined the diversification patterns of these taxa. Here, we analyze diversification trends based on the stratigraphic ranges of 86 genera of marine vertebrates, including sharks, rays, chimaeras, marine mammals, and seabirds. The richness of genera shows a hump-shaped trend, with maximum values around the late Miocene, driven by a large pulse of origination during mid-Miocene and higher extinction rates during the Pliocene. Trends varied markedly among taxa and departed largely from expectations based on global diversification patterns. Moreover, these trends cannot be explained solely as a sampling artifact derived from sampling intensity (i.e., number of occurrences) or sedimentary rock availably (i.e., number of geologic maps). A large fraction of genera (42%) went globally extinct by the late Pliocene–Pleistocene, and the extinction was highly selective according to different ecological and life-history traits. An analysis using “randomForest” showed that taxonomic structure and the geographic midpoint of distribution could explain up to 83% of extinction of genera. The extinction was taxonomically clumped (i.e., disproportionally high in Cetacea and very low in Carcharhiniformes) and concentrated in the northern area of the temperate Pacific coast of South America. Our results suggest that the particular paleogeographic, paleoclimatic, and paleoceanographic events that took place during the Neogene along the temperate Pacific coast of South America had a significant effect on the structure of marine biodiversity.

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

One of the major evolutionary transitions of the mammaliaform lineage was the origin of a typically mammalian pattern of growth. This is characterized by rapid juvenile growth followed by abrupt cessation of growth at adult size and may be linked with other important mammaliaform apomorphies of dental replacement and morphology. Investigation of growth patterns in the tritylodontid cynodont Oligokyphus and the basal mammaliaform Morganucodon provides insight into this crucial transition. We collected mandibular depth measurements from large samples of Morganucodon and Oligokyphus and constructed distributions of mandibular depth versus frequency for each species. These were compared with distributions from species from three different growth classes of extant amniote: testudines + crocodilians, mammals + birds, and lepidosaurs. Discriminant function analysis was used to differentiate between known growth classes by using different combinations of three measures of mandibular depth distribution shape (skew, kurtosis, and coefficient of variation) as proxies for different juvenile and adult growth patterns. Classification of the fossil species showed that Morganucodon closely resembled extant placental mammals in having rapid juvenile growth followed by truncated, determinate adult growth. Oligokyphus showed intermediate growth patterns, with more extended adult growth patterns than Morganucodon and slightly slower juvenile growth. This suggests a gradual evolution of mammalian growth patterns across the cynodont to mammaliaform transition, possibly with the origin of rapid juvenile growth preceding that of truncated, determinate adult growth. In turn, acquisition of both these aspects of mammalian growth was likely necessary for the evolution of diphyodont tooth replacement in the mammaliaform lineage.

Preservation in the fossil record is never perfect in the sense that we cannot sample all individuals of a given population in time and space. Incomplete detection (i.e., preservation and modern-day sampling of fossils) often affects estimates of other paleobiological parameters of interest, such as occupancy and turnover. Here, I simultaneously model the occupancy and detection probability of taxa, teasing apart the zeros in data that reflect true absences and those that imply non-detection of taxa that were actually present in the space and time of interest. Occupancy modeling, an approach first developed in population ecology, can easily incorporate covariates of interest, such as sampling effort and habitat variables. I use a data set of brachiopod taxa from the Paleozoic to illustrate the utility of this approach for paleontological questions. I demonstrate a range of models, including those that allow colonization between time intervals and those that incorporate facies as site covariates. I also suggest how future data collection can be improved so that process- and sampling-oriented approaches such as occupancy modeling can be applied with ease to paleobiological settings to answer important paleoecological and evolutionary questions.

The commonly observed fact that the principal biologic groups appear early in an adaptive radiation is shown to be an artifact of the geometry of the evolutionary tree. For marine invertebrates of the Phanerozoic, total diversity and species turnover rates combine to make early origins of major groups inevitable. This is demonstrated through an analysis of the theoretical probability distribution of divergence time (time to most recent common ancestor) in comparison with observed distributions. Whereas the fact of early origins does not require ecological explanations, the interpretation of the morphologic distinctness of major groups when they first appear remains an ecological problem.

Eocene lake beds of Horsefly, British Columbia, are preserved in varves, or discrete yearly layers representing seasonal changes in the lake. These varves allow study of temporal variation and rates of change in morphological and ecological characters on a very short time scale. One of the most sensitive indicators of the paleoenvironmental conditions on the floor of the lake may be the taphonomic condition of the fishes, which vary between perfectly articulated and completely disarticulated skeletons. Patterns of disarticulation correspond to those produced by scavengers. The taphonomy supports the hypothesis that the lake was warm monomictic, circulating in the winter, at which time scavengers could gain access to the bottom of the lake. Larger-scale environmental events (on the order of hundreds of years) are suggested by the fact that the proportion of well-preserved specimens reached two peaks within the seven centuries of deposition, one peak during the second century and another during the fifth and sixth centuries. These results clearly demonstrate two principles: that taphonomy can be a sensitive indicator of paleoenvironmental conditions, and that temporal averaging can affect the taphonomic properties of this fossil site, and presumably of others with equal or lower time resolution.

Phanerozoic trends in shell and life habit traits linked to postmortem durability were evaluated for the most common fossil brachiopod, gastropod, and bivalve genera in order to test for changes in taphonomic bias. Using the Paleobiology Database, we tabulated occurrence frequencies of genera for 48 intervals of ∼11 Myr duration. The most frequently occurring genera, cumulatively representing 40% of occurrences in each time bin, were scored for intrinsic durability on the basis of shell size, reinforcement (ribs, folds, and spines), life habit, and mineralogy.

Shell durability is positively correlated with the number of genera in a time bin, but durability traits exhibit different temporal patterns across higher taxa, with notable offsets in the timing of changes in these traits. We find no evidence for temporal decreases in durability that would indicate taphonomic bias at the Phanerozoic scale among commonly occurring genera. Also, all three groups show a remarkable stability in mean shell size through the Phanerozoic, an unlikely pattern if strong size-filtering taphonomic megabiases were affecting the fossil record of shelly faunas. Moreover, small shell sizes are attained in the early Paleozoic in brachiopods and in the latest Paleozoic in gastropods but are steady in bivalves; unreinforced shells are common to all groups across the entire Phanerozoic; organophosphatic and aragonitic shells dominate only the oldest and youngest time bins; and microstructures having high organic content are most common in the oldest time bins.

In most cases, the timing of changes in durability-related traits is inconsistent with a late Mesozoic Marine Revolution. The post-Paleozoic increase in mean gastropod reinforcement occurs in the early Triassic, suggesting either an earlier appearance and expansion of durophagous predators or other drivers. Increases in shell durability hypothesized to be the result of increased predation in the late Mesozoic are not evident in the common genera examined here. Infaunal life habit does increase in the late Mesozoic, but it does not become more common than levels already attained during the Paleozoic, and only among bivalves does the elevated late Mesozoic level persist through the Holocene.

These temporal patterns suggest control on the occurrence of durability-related traits by individual evolutionary histories rather than taphonomic megabiases. Our findings do not mean taphonomic biases are absent from the fossil record, but rather that their effects apparently have had little net effect on the relative occurrence of shell traits generally thought to confer higher preservation potential over long time scales.

The class Echinoidea apparently originated during the Ordovician Period and diversified slowly through the Paleozoic Era. The clade then mushroomed in diversity beginning in Late Triassic time and continued expanding into the present. Although this evolutionary history is generally accepted, the taphonomic overprint affecting it has not been explored. To gain a more accurate perception of the evolutionary history of the group, I have compared the diversity history of the family Cidaridae (Echinodermata: Echinoidea) with the preservational style of fossil type species using literature-derived data. The Cidaridae apparently originated in Middle Triassic time and diversified slowly through the Neocomian (Early Cretaceous). Diversity was maintained through the remainder of the Cretaceous and Tertiary Periods, reflecting the diversity history of the subclass. Characterization of the preservational style of type fossil material for the family revealed the following breakdown of preservational states: 60% of species were described on the basis of disarticulated skeletal material, primarily spines; 20% based on intact coronas denuded of spines, apical system, Aristotle's lantern and peristomial plates; 10% based on large coronal fragments; and 10% based on other skeletal elements. This distribution may represent the effect of a disarticulation threshold on the condition of echinoid carcasses before final burial and suggests that preservation of intact specimens may be very unlikely. For cidaroids, previous work has suggested that this threshold is likely to be reached after 7 days of decay.

Comparison of the diversity history of the Cidaridae with the preservation data reveals that characteristic patterns of taphonomic overprint have affected the group since its origination in Middle Triassic time, and the nature of that overprint has changed over time: the early diversity history of the group is characterized by occurrences of fragmented fossil material, with spines predominant; further radiation of the group in mid-Jurassic time coincided with an increase in modes of preservation, ranging between exceptionally well-preserved material and disarticulated skeletal elements. Finally, type material is more rarely described from younger stratigraphic intervals (Miocene–Pleistocene) and consists predominantly of disarticulated skeletal elements and coronal fragments larger than an interambulacrum in size. Intact, denuded coronas are noticeably lacking.

The number of type species of Cidaridae described in each stratigraphic interval has not been consistent during post-Paleozoic time. Middle Triassic, Malm (Upper Jurassic), Senonian (Upper Cretaceous) and Eocene series yielded significantly (α = .05) higher numbers of type specimens per million years, while the Lias (Lower Jurassic), Dogger (Mid-Jurassic), Lower Cretaceous and Paleocene yielded significantly (α = .05) lower numbers of type specimens per million years. This may be the result of a combination of taxonomic, sampling, and geographical biases.

The shovel-tusked gomphotheres are normally portrayed scooping up water plants with their shovellike mandibular tusks. This portrayal is based on speculation about the possible functions of the lower tusks and misinterpretation of mandibular-tusk wear patterns that goes back to the 1920s and 1930s. In addition, most literature concerning shovel-tusker feeding behavior ignores the possibility that the upper tusks may have had a role in feeding, as they do in modern African elephants. Because wear patterns on tusks provide direct evidence as to how tusks were used during feeding, I examined and interpreted patterns on the lower and upper tusks of shovel-tusked gomphotheres on the basis of theoretical models of tusk wear to determine how the tusks were used in feeding.

This examination led to the following conclusions. (1) The genus Amebelodon was characterized by feeding opportunism, gathering food with both its lower and upper tusks in a variety of ways. (2) There is evidence for similar flexibility within the genus Serbelodon. (3) Wear patterns on mandibular tusks of Platybelodon grangeri and Torynobelodon barnumbrowni indicate that these taxa did not practice mandibular shoveling but used these tusks to cut tough vegetation in a specialized fashion. (4) The flaplike trunks (probosces) generally attributed to shovel-tusked gomphotheres in restorations are shown almost certainly to be erroneous; morphological and functional evidence suggests that these animals had trunks similar to those of modern elephants. (5) Strong tusk sexual dimorphism is unknown in the shovel-tusked gomphotheres except for Platybelodon and should not be found in any taxa that relied on their tusks for procuring food. New restorations of both Amebelodon and Platybelodon are provided that are more consistent with the actual evidence bearing on their feeding behavior.

Marine shelf lagoon deposits of Middle Ordovician age in Tennessee contain the record of repeated ecological successions which began on soft, azoic, terrigenous muds and progressed to a complex bryozoan-brachiopod community. The stages of succession formed a skeletal limestone bed. The process led to an interbedded shale-limestone stratigraphic sequence. The pioneer was the brachiopod Strophomena basilica which paved the soft mud with accumulated shells. Encrusting bryozoans colonized this shell pavement and were followed by ramose bryozoans. The final stages of succession included bryozoans, gastropods, pelecypods, and pelmatozoans but were dominated by the brachiopod Rostricellula rostrata.

Measurements of total length, width, length to growth lines (Strophomena only), and thickness (Rostricellula only) were used to generate growth-rate, and survivorship curves for these two species. Strophomena shows low juvenile mortality, a moderate, linear growth-rate, and increased mortality in later ages. Rostricellula shows relatively higher juvenile mortality, a rapid non-linear growth rate, and nearly constant mortality. These attributes are contrary to expectations derived from theory, but can be understood in the light of autecological factors for each species. Adaptation to soft substrata was the primary factor controlling population dynamics in Strophomena, and crowding was the main factor in Rostricellula.

The extent of morphologic innovation during the Ediacaran–Cambrian diversification of animals was unique in the history of metazoan life. This episode was also associated with extensive changes in the redox state of the oceans, in the structure of benthic and pelagic marine ecosystems, in the nature of marine sediments, and in the complexity of developmental interactions in Eumetazoa. But did the phylogenetic and morphologic breadth of this episode simply reflect the unusual outcome of recurrent evolutionary processes, or was it the unique result of circumstances, whether in the physical environment, in developmental mechanisms, or in ecological interactions? To better characterize the uniqueness of the events, I distinguish among these components on the basis of the extent of sensitivity to initial conditions and unpredictability, which generates a matrix of possibilities from fully contingent to fully deterministic. Discriminating between these differences is important for informing debates over determinism versus the contingency in the history of life, for understanding the nature of evolutionary theory, and for interpreting historically unique events.

Recent data from mollusks suggest that mode of larval development may have important consequences for rates of speciation and extinction of marine organisms. The present study examines two methods that may be used to infer mode of development in the tests of fossil and Recent echinoids: genital pore size and crystallographic patterns of apical plates. Extant species with known modes of development were examined, and the following hypotheses were tested. 1) Species that produce large eggs and have nonfeeding larval development have larger genital pores than species that produce small eggs and have feeding larval development. 2) Orientations of crystallographic axes (c-axes) of genital and ocular plates differ in species with differing modes of development and can be used to infer mode of development. Genital pore size was found to be strongly dependent on body size within a species. For some taxa, pairwise interspecific comparisons of the relationships between genital pore size and body size support the hypothesis of larger genital pores for species with nonfeeding larval development. However, in multiple comparisons of linear regressions, species with nonfeeding larval development always overlapped other species with feeding larval development. An examination of the allometry of genital pore growth showed some species with nonfeeding larval development differed from those with feeding larval development; other species with differing modes of development could not be distinguished on the basis of allometric growth parameters. Orientations of c-axes of genital plates were found to be accurate indicators of mode of development, but orientations of c-axes of ocular plates were not. Among regular echinoids, 71 of 72 species supported the hypothesis that orientation of c-axes of genital plates is indicative of mode of development. Among 19 spatangoid echinoids studied, orientations of c-axes of genital plates generally allowed separation of species with differing modes of development. This method cannot be used to infer modes of development in taxa with reduced numbers of genital plates such as some spatangoids, some cassiduloids, and all clypeasteroids. Taxonomic differences in c-axis orientations require that inferences be made from comparisons between species within families.