Occurrence data from the Paleobiology Database are used to analyze the waxing and waning of genera over time. Irrespective of whether we tabulate species richness, frequency of occurrence, geographic range, or other measures, the average rise and fall of genera is remarkably symmetrical. Genera tend already to be in a state of decline when they become extinct. Genera that last appear in the major mass extinction stages, however, are more frequently truncated while they are holding steady or even increasing. This need not imply that mass extinctions are qualitatively different from other events; it is consistent with the expected effects of simply increasing the magnitude of extinction. For reasons that are not completely clear, post-Paleozoic genera show less of a rise and fall on average and tend to be less symmetrical than do Paleozoic genera.

Radiocarbon-calibrated amino acid racemization ages of 428 individually dated shells representing four molluscan taxa are used to quantify time-averaging and shell half-lives with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contains a distinct, essentially modern assemblage. Shells recovered at depths from 25 to 125 cm are age-homogeneous and significantly older than the surface layer. Taxon age distributions within sedimentary layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a sub-century scale. The age distributions and shell half-lives of four taxa (Ethalia, Natica, Tellina, and Turbo) are found to be largely distinct. Shell half-lives do not coincide with any single morphological characteristic thought to infer greater durability, but they are strongly related to a combined durability score based on shell density, thickness, and shape. These results illustrate the importance of bioturbation in tropical sedimentary environments, indicate that age estimates in this depositional setting are sensitive to taxon choice, and quantify a taxon-dependent bias in shell longevity and death assemblage formation.

Cohort analysis is used to investigate survivorship of trilobites originating during the Cambrian and Ordovician. Using a time-homogeneous branching model, it is estimated that trilobite genera and species originating during the Ordovician survived three times longer than Cambrian genera and species. Monte Carlo simulation of survivorship is used to show that (1) Cambrian and Ordovician survivorship are significantly different, (2) Ordovician cohorts conform more closely to the time-homogeneous model than do Cambrian cohorts, and (3) deviations from temporal homogeneity are more often produced by extraordinary extinction than by unusually slow turnover.

When Early Ordovician cohorts are decomposed into genera within families that originated in the Cambrian versus the Ordovician, no evidence that Cambrian and Ordovician survivorship differences are clade-specific can be found. Ordovician genera of Cambrian affinity and of Ordovician affinity become extinct at similar rates.

Some of the ultimate causes of these differences in survivorship include (1) taxonomic inconsistency, (2) greater environmental stability in the Ordovician, and (3) more highly structured ecosystems in the Ordovician that may have led to the weeding out of extinction-prone taxa.

Evolutionary paleontologists seek explanations of form and pattern in a diversity of contexts. A review of this diversity is presented to underscore the difficulty of producing a general model of the factors controlling the morphology of real organisms. A model specific to the marine prosobranch gastropod radula identifies seven factors contributing to form and pattern: (1) phylogenetic, (2) mechanical, (3) ecological, (4) programmatic, (5) maturational, (6) degenerative, and (7) constructional. Aspects of radular morphology attributable to each factor are illustrated in scanning electron micrographs of radulae of marine prosobranch gastropods. Applications of and extrapolations from the radular model are made to promote recognition of a broader range of interacting factors in other systems. Some aspects of form and pattern convey important information about non-evolutionary processes and phenomena and are best examined outside of the evolutionary framework.

Taxonomic survivorship curves may reflect stratigraphic as well as biologic factors. The approximately lognormal distribution of lengths of Phanerozoic time intervals produces an error in the estimation of taxonomic durations that is also lognormally distributed. As demonstrated by several simulated examples, this error may cause concave taxonomic survivorship curves to appear linear, especially if the maximum durations involved are relatively short. The error of estimation also makes highly concave taxonomic survivorship curves virtually unrecognizable. Incomplete sampling of the fossil record, on the other hand, may not be a serious problem in survivorship analysis. Simulated paleontological sampling employing a simple model suggests that survivorship curves tend to retain their original shapes even when as few as 20% of the taxa have been discovered. However, concave taxonomic survivorship curves tend to lose their concavity as efficiency of sampling declines.

The evolutionary history of the Order Carnivora is marked by episodes of iterative evolution. Although this pattern is widely reported in different carnivoran families, the mechanisms driving the evolution of carnivoran skull morphology remain largely unexplored. In this study we use coordinate-point extended eigenshape analysis (CP-EES) to summarize aspects of skull shape in large fissiped carnivores. Results of these comparisons enable the evaluation of the role of different factors constraining the evolution of carnivoran skull design. Empirical morphospaces derived from mandible anatomy show that all hypercarnivores (i.e., those species with a diet that consists almost entirely of vertebrate flesh) share a set of traits involved in a functional compromise between bite force and gape angle, which is reflected in a strong pattern of morphological convergence. Although the paths followed by different taxa to reach this “hypercarnivore shape-space” differ because of phylogenetic constraints, the morphological signature of hypercarnivory in the mandible is remarkably narrow and well constrained. In contrast, CP-EES of cranial morphology does not reveal a similar pattern of shape convergence among hypercarnivores. This suggests a lesser degree of morphological plasticity in the cranium compared to the mandible, which probably results from a compromise between different functional demands in the cranium (e.g., feeding, vision, olfactory sense, and brain processing) whereas the mandible is only involved in food acquisition and processing. Combined analysis of theoretical and empirical morphospaces for these skull data also show the lower anatomical disparity of felids and hyaenids compared to canids and ursids. This indicates that increasing specialization within the hypercarnivorous niche may constrain subsequent morphological and ecological flexibility. During the Cenozoic, similar skull traits appeared in different carnivoran lineages, generated by similar selection pressures (e.g., toward hypercarnivory) and shared developmental pathways. These pathways were likely the proximate source of constraints on the degree of variation associated with carnivoran skull evolution and on its direction.

The claw equipped forelimbs have been shown to be an important hunting weapon for modern felids. In light of its functional importance, the claw retractile mechanism for modern felids was compared with that of the saber-toothed felids. In this regard, the functional anatomy of claw retraction for saber-toothed felids was found to be the same as that of modern forms. Body proportions of modern felids were also compared with saber-toothed felids and the relationship of their morphology to habitat structure and habitat utilization were studied. It was found that the relative body proportions for Hoplophoneus and Smilodon were similar to modern forest felids (dwellers of high structured dense forest), while Dinictis and Machairodus could probably compete in more open terrain (open woodland, meadow). It is postulated that saber-toothed felids used their claw equipped forelimbs to grasp and hold their prey as do modern felids. In this fashion, the enlarged upper canines could then be used to kill the victim, and this was probably done by a stab to the nape of the neck. It is also thought that Smilodon, like the modern lion, adapted to open habitats by forming prides.

Paleotectonic reconstructions for the late Proterozoic have differed over the timing of the Cordilleran rifting between Laurentia and the East Gondwana cratons. Parsimony Analysis of Endemism (PAE) and phenetic clustering of the “Ediacara biota” were carried out, for comparison with competing paleotectonic hypotheses. All analyses show a common pattern of similarities among biotas. The biotas of Charnwood Forest and Newfoundland consistently group together, while the south Australian biota is closest to those of Baltica, northern Laurentia, and Siberia. The biota of southwest North America, though still poorly known, strikingly resembles that of Namibia—not that of northwestern Canada. This pattern is not obviously due to facies-related or preservational bias and is very different from Cambrian biogeographic patterns. The overall pattern is most consistent with the hypothesis that the western margin of Laurentia was in close contact with East Gondwana, with rifting taking place either during or just before the Vendian. This arrangement has been previously proposed as a paleotectonic hypothesis; however, most recent paleomagnetic and structural studies support the alternate hypothesis that this rifting took place more than 100 million years before the Vendian. Resolving this contradiction will require much more data on both organismal distribution and cratonal position.

While the importance of allopatric speciation in the fossil record has long been underestimated, phyletic change within single unbranching lineages also occurs. The 50% increase in thoracic width observed in the radiolarian species Pseudocubus vema from an Antarctic deep-sea core is a clear example of a long-term phyletic trend in a continuous fossil sequence. Phyletic change in P. vema occurred at varying rates, but changes in the morphologic rate of evolution do not correspond to any obvious breaks in the fossil record such as would be indicated by missing segments of the core's magnetic stratigraphy. Variation in thoracic width, as measured by the coefficient of variation, does not depend on the morphologic rate of evolution, proportional rate of evolution, nor the amount of time required for the width to change by one standard deviation, so much as it depends on whether change was accomplished by addition or removal of extreme phenotypes to or from the population.

Stable isotope analysis of mammalian tooth enamel is a valuable method for examining resource partitioning in modern and ancient environments where there is a mixture of C3 and C4 plants. However, before 7 Ma North American ecosystems were composed predominantly of C3 plants, complicating isotopic assessment of resource partitioning. Study of modern African and North American ecosystems has shown that niche partitioning among mammals may be discerned in communities dominated by C3 plants, suggesting that a similar approach may work for ancient C3 ecosystems. Here, such analyses are applied to explore resource use and niche partitioning in two ancient C3-dominated communities, one from California and one from Florida. Each locality, Black Hawk Ranch (California) and the Love Bone Bed (Florida), occurs in Miocene deposits that accumulated prior to the rapid increase in C4 ecosystems 7 Myr ago. δ13C and δ18O values were obtained from the tooth enamel of eight species from Black Hawk Ranch, and 15 species from the Love Bone Bed. Results from the 197 bulk isotope samples showed significant differences in δ13C among taxa at the Love Bone Bed, but no significant differences were observed among taxa at Black Hawk Ranch. At both localities, equids generally have more positive δ13C values than co-occurring taxa, suggesting that equids occupied more open habitats, whereas antilocaprids, camelids, and proboscideans have more negative values, implying utilization of more closed communities. One result of note is the positive δ13C values of Pediomeryx (Yumaceras) hamiltoni from the Love Bone Bed, which suggests that P. (Y.) hamiltoni incorporated abundant fiber, possibly grass, in the diet similar to the horses from this locality. The lack of significant differences among taxa at Black Hawk Ranch may indicate a relatively homogeneous flora, or presence of abundant resources permitting niche overlap, whereas the opposite is implied by the presence of significantly different isotope values among taxa at the Love Bone Bed. The results from this study highlight the utility of isotopic techniques allowing discernment of resource partitioning in C3-dominated landscapes such as those that persisted for the millions of years before the rapid increase in C4 ecosystems that occurred during the late Miocene.

High-resolution stratigraphic analysis of 18 sections spanning the Cenomanian–Turonian Stage boundary in the western interior of the United States has allowed determination of the magnitude and pattern of molluscan extinction and disruption. Composite range data from all sections show that the faunal turnover across the stage boundary occurs in a series of narrow stratigraphic zones, defined by multiple first and last occurrences, separated by intervals displaying little or no taxonomic turnover. Two of the apparent extinction steps (bottom and top of the Neocardioceras juddii Zone) may be intercontinentally developed. The additional steps apparently reflect cyclic changes in water mass and substrate characteristics in the western interior basin produced in response to orbital forcing of climate. An interval (ca. 10-100 k.y. duration) of changing community structure and general biotic deterioration is found below each of the two potentially intercontinentally developed extinction steps. The most affected mollusks were those having intercontinental distributions (ammonites and inoceramid bivalves), suggesting that disruption of planktotrophic larval dispersal may have played a role in increasing extinction and speciation rates near the C–T boundary. The nekto-benthic ammonites were affected earlier and to a greater degree than the pelagic forms, implying progressive upward expansion of the oxygen minimum zone preceding the stage boundary.

Analysis of post-mortem buoyancy loss in Nautilus shells suggests that extensive nekroplanktonic drifting occurs infrequently. Most shells do not reach the surface but settle to the sea floor, after a short period of ascent. This occurs because the rate of water influx into the phragmocone due to ambient hydrostatic pressure is sufficiently rapid in most cases to overcome positive buoyancy before the shell reaches the surface. The resulting geographic distribution of Nautilus shells would therefore mirror the distribution of the live animals. Thus, post-mortem drift in Nautilus cannot be used as a basis for questioning the validity of cephalopod paleobiogeography. Estimate of influx rates in ammonoid siphuncles indicates that many, if not most, ammonoid shells also would not become nekroplanktonic. This is especially true for small (<5 cm diameter) shells. Cephalopod paleobiogeographic investigation appears less subject to criticism stemming from the supposed obfuscating effects of post-mortem drift than previously thought.

This paper proposes a simple, unorthodox model for use in the study of vertebrate jaw mechanics. Central to the new “bifulcral model” is the assumption that the bite point may be regarded as a distinct and independent “occlusal fulcrum” equal in status to the jaw articulation or “joint fulcrum” of more traditional biomechanical models. The bifulcral model allows all mechanical forces acting on the feeding system to be evaluated in terms of their purely rotational and purely translational components defined relative to the occlusal and joint fulcra. The major benefit of this analytical approach is that it permits a new and substantially different perspective on the functional consequences of morpho-geometric organization in feeding systems. The bifulcral model clearly establishes the dynamic relationships among muscle alignment, bite point and the resultant patterns of mechanical stress at the craniomandibular joint (CMJ). It also reveals potentially important modes of competitive interaction between otherwise seemingly synergistic jaw muscles. Further, the bifulcral model encourages preliminary investigations into the possible contribution of intramuscular dynamics to the overall operational plasticity of the feeding machinery.

Specific application of the new model to structural and functional problems concerning the origin of the mammalian feeding complex lead to the following tentative conclusions. (1) Contrary to current opinion, the CMJ of most cynodont therapsids probably did not experience positive vertical loads (= compressive) when the cheek teeth were utilized for mastication. Instead, net CMJ loads were either neutral or somewhat negative (= tensile). (2) The development of a pronounced coronoid process in cynodonts was more directly related to promoting the differentiation of the masseter complex than to either improving the mechanical advantage or prehensile capacity of the temporalis muscle. (3) Differential motor activity within the complex temporalis musculature of cynodonts could have resulted in “derived lines of action” markedly different from the reconstructed lines of action employed in previous analyses of feeding mechanics in these reptiles. Such derived lines of action may have been optimal configurations for the integrated activity of the temporalis and masseter musculature. (4) Selection in cynodonts favored the evolution of a superficial masseter rather than the elaboration of the preexisting and geometrically similar pterygoideus musculature. This occurred because the masseter held the greater potential for improving bite force, motor control and facilitating the reduction of the postdentary bones of the mandible while still preserving the basic spatial economy of the cranial region. (5) The rearward growth of the condylar process of the dentary in cynodonts promoted the reduction of the postdentary unit primarily by shifting the point of load application between the dentary and postdentary units, thereby reducing bending stresses in the postdentary unit. (6) The enlarged squamosal sulcus of cynodonts was occupied mainly by a hypertrophied depressor mandibulae muscle; an auditory tube was also present in the sulcus deep to the muscle. (7) The depressor mandibulae played a major role in CMJ stabilization in therapsids; this was possibly its only function in later cynodonts. The peculiar downturned retroarticular process of the mandible in therapsids appears to have been related to this same function.

Physical environmental factors have been seen as paramount in determining many large-scale biodistributional patterns in time and space. Although this is probably correct for many situations, this view has become so pervasive that it has led to the neglect of the role of biotic interactions in setting large-scale diversity patterns. (In this paper diversity denotes taxonomic richness.) New approaches to this perennial debate on the roles of physical and biotic forces in paleoecology and macroevolution are needed, and here we explore an argument for the role of incumbency or priority effects in the dynamics behind the most dramatic spatial pattern in biodiversity, the latitudinal diversity gradient.

Paedomorphosis has played an important evolutionary role in several vertebrate lineages, yet there are few studies in which fossils have been used to document this evolutionary pattern directly. The unusually good fossil record of lungfishes offers an excellent opportunity to do this. An approach to recognizing paedomorphic character complexes in fossil lineages is formalized, and the interdependence of ontogenetic and phylogenetic studies is reemphasized. Concordant evolutionary changes in seven characters representing a variety of organ systems are discussed. For example, in Devonian lungfishes, the tail is heterocercal, endochondrally ossified bones are present, and the fin rays are similar to those of other Osteichthyes. Recent lungfishes have protocercal tails, suppression of endochondral ossification, and greatly reduced fin rays. The data suggest that paedomorphosis played a pervasive role in the evolution of lungfishes and give further evidence for the evolutionary importance of heterochrony.

Although available paleobiological data indicate that the geographic ranges of marine species are maintained throughout their entire observable durations, other evidence suggests, by contrast, that the ranges of higher taxa expand as they age, perhaps in association with increased species richness. Here, I utilize a database of Ordovician genus occurrences collected from the literature for several paleocontinents to demonstrate that a significant aging of the global biota during the Ordovician Radiation was accompanied by a geographic and environmental expansion of genus ranges. The proportion of genera occurring in two or more paleocontinents in the database, and two or more environmental zones within a six-zone onshore-offshore framework, increased significantly in the Caradocian and Ashgillian. Moreover, widespread genera tended to be significantly older than their endemic counterparts, suggesting a direct link between their ages and their environmental and geographic extents. Expansion in association with aging was corroborated further by demonstrating this pattern directly among genera that ranged from the Tremadocian through the Ashgillian. Taken together, these results are significant not only for what they reveal about the kinetics of a major, global-scale diversification, but also for what they suggest about the interpretation of relationships between diversity trends at the α (within-community) and β (between-community) levels.

Using Sepkoski's compendium of fossil marine families (1982a, and updates), we have analyzed the changing pace of familial origination and extinction within 55 extinct and 44 extant higher taxa of marine organisms. Eight different metrics were calculated, and least-squares regression analysis was used to identify within-taxon trends in the data. All metrics and analyses gave essentially the same results. Origination metrics decline significantly with time during the histories of higher taxa, while extinction metrics increase significantly. The number of statistically significant declines of origination metric, however, substantially and invariably exceeds the number of statistically significant increases of extinction metric for each pair of corresponding metrics analyzed. It follows, therefore, that temporal trends in the pace of origination and extinction within higher taxa are highly asymmetrical.

Further analysis shows that truncating data from temporal endpoints has little effect upon the intensity of origination trends, implying that declining pace of origination is a sustained property of the long term histories of taxa. Such truncation, however, reduces the intensity of extinction trends to statistical insignificance and confirms Van Valen's (1985a) suggestion that extinction behaves largely as a stationary process. If the histories of higher taxa are characterized by substantial declines in the pace of origination while the pace of extinction remains largely stationary, it follows that declining pace of origination is an important controlling factor in long term taxic evolution.

Macroevolutionary change from the Middle to the Late Paleozoic crinoid fauna was not the result of mass extinction. The presumption that the decline of the middle Paleozoic crinoid fauna was from a single mass extinction event was tested using seriation, multidimensional scaling (MDS), binomial analysis, and bootstrapping simulations on a data set which is a comprehensive revision of old faunal lists. The data for these analyses were based on temporal distributions of 214 species from 69 late Osagean and early Meramecian localities from the midcontinental United States. The time under consideration is subdivided into seven informal intervals using MDS in conjunction with biostratigraphy. Seriation of species ranges into these intervals results in a gradual pattern of faunal turnover, and sampling bias can be eliminated as a cause for this more gradual pattern. MDS analysis of the crinoid range data is similar to MDS simulations using data with continuous, monotonic species turnover and dissimilar to a simulated mass extinction. Binomial analysis and bootstrapping demonstrate that the observed number of extinctions at the putative extinction boundary were not unusually high. All methods agree that extinctions throughout this time were high but spanned several time intervals and that rapid, monotonic faunal turnover describes the data better than mass extinction. Macroevolutionary processes other than mass extinction and microevolutionary processes must have dictated the character and composition of this remarkable faunal transition among the Crinoidea.

In both jaw geometry and molar morphology, eutherian carnivores (order Carnivora) as a whole display greater diversity (plasticity in evolution from the primitive type) than marsupial carnivores (order Dasyurida). This is related to the difference in tooth replacement between the two taxa. In Carnivora, the permanent carnassial is preceded by a deciduous carnassial; the permanent tooth can erupt in its (geometrically) permanent position, and the post-carnassial molars are free to evolve for specialized functions or be reduced. In Dasyurida, there is relative molar progression, each erupting molar in turn functioning as a carnassial, and subsequently being pushed forwards in the jaw by the next erupting molar. Thus, all molars have carnassiform morphology, and none are free to develop for other functions. The greater plasticity of Carnivora has led to their adaptive zone being broader (as a group they are relatively more eurytopic than Dasyurida), which in turn has led to greater taxonomic diversity within Carnivora than Dasyurida. The resulting pattern from a macroevolutionary point of view is that, even in the absence of direct competition, Carnivora have had greater evolutionary “success” than Dasyurida.