Using computer simulations and analytic calculations, we have evaluated whether conspicuous expansions and contractions of natural clades may have represented chance fluctuations that occurred while probabilities of speciation and extinction remained equal and constant. Our results differ from those of previous workers, who have not scaled generating parameters empirically at the species level. We have found that the waxing and waning of many real clades have almost certainly resulted from changes in probabilities of speciation and extinction. For some of these changes, likely explanations are evident. The emplacement of adaptive innovations, for example, has at times elevated probability of speciation. We conclude that chance factors have not played a dominant role in producing dramatic changes in standing diversity within speciose higher taxa.

We studied mammal and bird track formation at the northern edge of Lake Manyara, Tanzania, to develop models for interpreting fossil tracks and trackways. Lake Manyara is a closed-basin, alkaline lake in the East African Rift System. The area has a high vertebrate diversity, allowing us to investigate tracks in an environment similar to that of many ancient track-bearing sequences. Three study sites, two on mud flats adjacent to the lake margin and a third on a delta floodplain, provided contrasting environments in which to assess the types of biological data that can potentially be extracted from fossil trackways.

Our censuses of mammals and their tracks revealed that most species that occur within the study area leave a track record, and that common species leave abundant tracks, although numbers of trackways are not proportional to numbers of individuals. Logarithmic increases in track sampling area yield a linear increase in the proportion of both the medium and large-sized local mammals represented in a track record. Transect vs. area mapping methods produced different censusing results, probably because of differences in monitoring periods and areal coverage.

We developed a model of expected track production rates that incorporates activity budget and stride length data in addition to abundance data. By using these additional variables in a study of diurnal birds, we obtained a much better estimator relating track abundance to trackmaker abundance than that provided by census data alone. Proportions of different types of tracks predicted by the model differ significantly from the observed proportions, almost certainly because of microenvironmental differences between the censusing and track counting localities. Censuses of fossil tracks will be biased toward greater numbers of depositional-environment generalists and away from habitat-specific species.

Trackways of migratory animals were dominantly shoreline-parallel, whereas trackways of sedentary species were more variable. A strong shoreline-parallel environmental zonation at the Alkaline Flats site exerted an influence on trackmaker distribution patterns, initial track formation, and track preservation. Variations in habitat usage by different species, as well as species abundance and directionality of movement, were all important in determining the number of preservable tracks a species produced within a given environmental zone.

Fossil trackways are time-averaged, although over entirely different temporal scales than are bones. Unlike bones, tracks are not space-averaged. Therefore, wherever possible, fossil track and bone studies should be used to complement each other, as they provide fundamentally different pictures of paleocommunities. Tracks provide “snapshot” views of localized assemblages of organisms useful in reconstructing autecological relationships, whereas bones yield a broader image of a local fauna in which seasonal and microenvironmental variation are more commonly smoothed out.

C4 grasses form the foundation of warm-climate grasslands and savannas and provide important food crops such as corn, but their Neogene rise to dominance is still not fully understood. Carbon isotope ratios of tooth enamel, soil carbonate, carbonate cements, and plant lipids indicate a late Miocene-Pliocene (8–2 Ma) transition from C3 vegetation to dominantly C4 grasses at many sites around the world. However, these isotopic proxies cannot identify whether the C4 grasses replaced woody vegetation (trees and shrubs) or C3 grasses. Here we propose a method for reconstructing the carbon isotope ratio of Neogene grasses using the carbon isotope ratio of organic matter trapped in plant silica bodies (phytoliths). Although a wide range of plants produce phytoliths, we hypothesize that in grass-dominated ecosystems the majority of phytoliths will be derived from grasses, and will yield a grass carbon isotope signature. Phytolith extracts can be contaminated by non-phytolith silica (e.g., volcanic ash). To test the feasibility of the method given these potential problems, we examined sample purity (phytolith versus non-phytolith silica), abundance of grass versus non-grass phytoliths, and carbon isotope ratios of phytolith extracts from late Miocene-Pliocene paleosols of the central Great Plains. Isotope results from the purest samples are compared with phytolith assemblage analysis of these same extracts. The dual record spans the interval of focus (ca. 12–2 Ma), allowing us, for the first time, to investigate how isotopic shifts correlate with floral change.

We found that many samples contained high abundances of non-biogenic silica; therefore, only a small subset of “pure” samples (>50% of phytoliths by volume) with good preservation were considered to provide reliable carbon isotope ratios. All phytolith assemblages contained high proportions (on average 85%) of grass phytoliths, supporting our hypothesis for grass-dominated communities. Therefore, the carbon isotope ratio of pure, well-preserved samples that are dominated by grass biosilica is considered a reliable measure of the proportion of C3 and C4 grasses in the Neogene.

The carbon isotope ratios of the pure fossil phytolith samples indicate a transition from predominantly C3 grasses to mixed C3-C4 grasses by 5.5 Ma and then a shift to more than 80% C4 grasses by 3–2 Ma. With the exception of the Pliocene sample, these isotopic data are broadly concordant with phytolith assemblages that show a general increase in C4 grasses in the late Miocene. However, phytolith assemblage analysis indicates lower relative abundance of C4 grasses in overall vegetation than do the carbon isotopes from the same phytolith assemblages. The discrepancy may relate to either (1) incomplete identification of (C4) PACMAD phytoliths, (2) higher production of non-diagnostic phytoliths in C4 grasses compared to C3 grasses, or (3) biases in the isotope record toward grasses rather than overall vegetation. The impact of potential incomplete characterization of (C4) PACMAD phytoliths on assemblage estimates of proportion of C4, though important, cannot reconcile discrepancies between the methods. We explore hypothesis (2) by analyzing a previously published data set of silica content in grasses and a small data set of modern grass leaf assemblage composition using analysis of variance, independent contrasts, and sign tests. These tests suggest that C4 grasses do not have more silica than C3 grasses; there is also no difference with regard to production of non-diagnostic phytoliths. Thus, it is most likely that the discrepancy between phytolith assemblages and isotope ratios is a consequence of hypothesis (3), that the isotope signature is influenced by the contribution of non-diagnostic grass phytoliths, whereas the assemblage composition is not. Assemblage-based estimates of % C4 within grasses, rather than overall vegetation, are in considerably better agreement with the isotope-based estimates. These results support the idea that, in grass-dominated assemblages, the phytolith carbon isotope method predominantly records shifts in dominant photosynthetic pathways among grasses, whereas phytolith assemblage analysis detects changes in overall vegetation. Carbon isotope ratios of fossil phytoliths in conjunction with phytolith assemblage analysis suggest that the late Neogene expansion of C4 grasses was largely at the expense of C3 grasses rather than C3 shrubs/trees. Stable isotopic analysis of phytoliths can therefore provide unique information about grass community changes during the Neogene, as well as help test how grass phytolith morphology relates to photosynthetic pathway.

The Late Neogene vertebrate fossil record from Yushe Basin presents multiple, superposed assemblages from a single area, spanning roughly the interval of 6–2 Ma. Both large and small mammals show peak species richness in the middle Pliocene but indicate relative faunal stability throughout the Pliocene. Large mammals show turnover, especially extinction, around 5 and 2.5 Ma. Small mammals indicate change (over half of the species and several genera), as well as turnover at the species level, between 4 and 3.4 Ma. The loosely controlled dating of these events does not disprove hypothetical correlation with events in North America and with global climatic shifts. Elements that lack Yushe antecedents, some being long-distance dispersers, appear throughout the section, but with little effect on the resident assemblage. First records of well-documented immigrants (from North America, Europe, Africa, southern Asia, or high latitudes) generally do not coincide with ecomorph extinctions. Early Pliocene exchange between Asia and North America appears to have been balanced in both directions and involved a small proportion of the fauna. Immigration probably was opportunistic and contributed to faunal enrichment. We interpret the Yushe Pliocene mammalian assemblages as representing a fauna that was stable from ca. 5 to 2.5 Ma and changed mainly by additions and congeneric species substitutions.

The morphologic radiation of Early Jurassic ammonites following the near extinction at the end of the Triassic is analyzed from 436 species of 156 genera that form a representative sample of morphs occurring worldwide in the first three stages of the Jurassic (Hettangian, Sinemurian, Pliensbachian: 36 subzones, 24 m.y.). Morphologic diversity is analyzed independently of taxonomy by processing 18 shape parameters using multivariate analysis and clustering techniques. The morphospace thus defined indicates that morphs fall readily into two groups made up of four and five adjacent morpho-subsets. The temporal pattern of morphospace occupation in the 36 Lower Jurassic subzones displays diversification, depletion (sometimes total), and displacement of successive parts of the morphospace, reflecting a complex history in which morphologic radiation appears to be more than a process of diffusion. The history of the morphologic evolution is tentatively related to sea-level changes and there is a suggestion that morphologic diversity increases during second-order transgressive periods.

Cutan, a resistant non-hydrolyzable aliphatic biopolymer, was first reported in the cuticle of Agave americana and has generally been considered ubiquitous in leaf cuticles along with the structural biopolyester cutin. Because leaves and cuticles in the fossil record almost always have an aliphatic composition, it was argued that selective preservation of cutan played an important role in leaf preservation. However, the analysis of leaves using chemical degradation techniques involving hydrolysis to test for the presence of cutan reveals that it is absent in 16 of 19 taxa (angiosperm and gymnosperm), including many previously reported to contain cutan on the basis of pyrolysis data. Cutan is clearly much less widespread in leaves than previously thought, and its presence or absence does not exert any major bias on the preservation of leaves in the fossil record. In the absence of cutan, other constituents—cutin, plant waxes, and internal plant lipids—are incorporated into the geomacromolecule and contribute to the formation of a resistant aliphatic polymer by in situ polymerization during diagenesis.

The variation in time-averaging between different types of marine skeletal accumulations within a depositional system is not well understood. Here we provide quantitative data on the magnitude of time-averaging and the age structure of the sub-fossil record of two species with divergent physical and ecological characteristics, the brachiopod Bouchardia rosea and the bivalve Semele casali. Material was collected from two sites on a mixed carbonate-siliciclastic shelf off the coast of Brazil where both species are dominant components of the local fauna.

Individual shells (n = 178) were dated using amino acid racemization (aspartic acid) calibrated with 24 AMS radiocarbon dates. Shell ages range from modern to 8118 years b.p. for brachiopods, and modern to 4437 years for bivalves. Significant differences in the shape and central tendency of age-frequency distributions are apparent between each sample. Such differences in time-averaging magnitude confirm the assumption that taphonomic processes are subject to stochastic variation at all spatial and temporal scales. Despite these differences, each sample is temporally incomplete at centennial resolution and three of the four samples have similar right-skewed age-frequency distributions. Simulations of temporal completeness indicate that samples of both species from the shallow site are consistent with a more strongly right-skewed and less-complete age-frequency distribution than those from the deep site.

We conclude that intrinsic characteristics of each species exert less control on the time-averaging signature of these samples than do extrinsic factors such as variation in rates of sedimentation and taphonomic destruction. This suggests that brachiopod-dominated and bivalve-dominated shell accumulations may be more similar in temporal resolution than previously thought, and that the temporal resolution of multi-taxic shell accumulations may depend more on site-to-site differences than on the intrinsic properties of the constituent organisms.

Plesiadapiforms were among the most diverse and abundant of North American mammalian groups during the Paleocene epoch. Their decline and extinction has been attributed to a variety of causes, including competitive exclusion by rodents and/or by primates of modern aspect (euprimates), and the effects of late Paleocene–early Eocene climatic warming. However, analyses of generic and species richness and relative abundance patterns indicate that the decline of plesiadapiforms was more complex than usually supposed. The two major superfamilies, Plesiadapoidea and Microsyopoidea, exhibited very different patterns of decline. Microsyopoids and paromomyid plesiadapoids do not exhibit a pronounced decline in taxonomic richness and relative abundance during the Early Tertiary. Non-paromomyid plesiadapoids do decline markedly, but, when their taxonomic richness and relative abundance patterns are compared with those of potentially competing taxa (rodents and euprimates) only non-paromomyid plesiadapoids and rodents show the inverse relationship consistent with taxonomic displacement. Euprimates appear in North America after non-paromomyid plesiadapoids were virtually extinct. Analyses of body size, diet, diel activity patterns, and locomotion support the hypothesis that rodents and non-paromomyid plesiadapoids may have competed for the same resources; their paleobiogeographic histories also are consistent with the competition argument.

Currently available paleoclimatological data for the Early Tertiary largely pertain to global temperature trends and are not detailed enough to demonstrate unequivocally whether the decline of non-paromomyid plesiadapoids coincided with climatic change. A more complete record of paleoclimatic indicators, especially from the Western Interior of North America, is needed to test the climate hypothesis adequately. The several independent lines of evidence considered here and elsewhere in reference to other mammalian taxa strongly support the hypothesis that competition with rodents was an important factor in a major restructuring of Early Tertiary mammalian communities in North America.

Differences in the larval ecology of Ordovician trilobites directly influenced the outcome of the Ashgill extinction (latest Ordovician) and indirectly governed the pattern of evolution in post-Ordovician trilobites. Larval ecology also affected survivorship patterns within the Ordovician, particularly between the Llandeilo and the Caradoc stages. All taxa with pelagic adults became extinct by the end of the Ordovician. Similarly, trilobites with entirely planktonic larvae had all but disappeared by the end of the Ordovician. Although suffering a significant loss in diversity, taxa with benthic larvae provided the ancestral stock for the majority of post-Ordovician lineages. Trilobites with a two-stage protaspid period (single planktonic followed by benthic larvae) suffered least during the Ashgill extinction, giving rise to approximately 23% of the new genera appearing in the early Silurian.

Patterns of extinction/survivorship among trilobite taxa with different developmental strategies indicate that the so-called Ashgill extinction was most likely the result of a composite phenomenon, including environmental perturbations, ecosystem breakdown and biogeographic restriction, and was not the consequence of a single catastrophic event (e.g., bolide collision). Indeed, our results are consistent with extinction models which invoke periodic global cooling and sea level regression associated with glaciation. Taxa with planktonic larvae first suffered a marked decline in diversity between the Llandeilo and the Caradoc (end of Middle Ordovician), coincident with the onset of the Ordovician-Silurian glaciation. During this time interval, the paleogeographic ranges of most surviving genera with solely planktonic larvae were severely constricted to lower latitude paleobiogeographic provinces. During the Ashgill (Rawtheyan to Hirnantian), climatic and sea level fluctuations were most extreme because of continued and more extensive glaciation. At this time, when extinctions were particularly severe, there was also a significant effect upon the survivorship of taxa with benthic larvae.

Our results indicate that the proportion of the life cycle spent in the water column, dependent upon a planktonic food source, was a critical factor in the survivorship of trilobite taxa during Middle to Late Ordovician time. Consequently, differences in life history patterns predisposed individual taxa to survival or extinction.

Principal components analysis of Upper Carboniferous (Pennsylvanian) ammonoids (all 117 genera), using 21 variables to measure shell geometry, sculpture and suture complexity, shows that following a sharp decline (∼30%) in generic diversity after the mid-Carboniferous boundary, seven morphotypes persisted throughout the Pennsylvanian (ca. 30 m.y.). Six of these were polyphyletically adopted at different times, while the seventh was monopolized by the prolecanitids, a group whose evolution accelerated during the Pennsylvanian and later gave rise to Mesozoic ammonoids. Innovations in suture geometry distinguished at least 17 of 39 (44%) Pennsylvanian ammonoid families. Average suture complexity increased almost threefold; this was achieved by various methods (lobe serration, insertion of umbilical elements, prong subdivision, lobe trifurcation, and secondary bifurcation), which were recurrent and crossed morphotype boundaries. The Pennsylvanian record supports suggestions that Paleozoic ammonoids were confined to a certain suite of basic shell geometries, showing preference for a limited number of sites in the spectrum of available morphospace. However, these morphic constraints did not, with one possible exception (the prolecanitids), control the emergence of increasing sutural complexity during the Pennsylvanian, which occurred among different lineages in all seven morphotypes.

The post-Paleozoic radiation of echinoids entailed a rapid diversification not only of adult morphology, but also of larval morphology. The timing, order, and phylogenetic distribution of evolutionary transformations in echinopluteus larvae are reconstructed here under maximum parsimony assumptions from a large neontological data base. Many echinoid larval apomorphies apparently evolved within the Paleozoic stem lineage and were subsequently retained during much of the crown-group radiation. This suite of apomorphies includes most (and perhaps all) of the skeletal elements and some features of soft anatomy such as vibratile lobes. Other features apparently arose or were modified during the post-Paleozoic radiation. These include skeletal features such as arm-rod structure and length, and soft structures such as epaulettes and skeletal muscles. Transformational hypotheses of this kind can be supported or rejected with further neontological data, and many can potentially be tested from fossil evidence. Many post-Paleozoic transformations in echinopluteus structure may be adaptive. For example, increases in arm length and flattening of arm ectoderm may increase feeding rate and efficiency, and both types of transformation have occurred several times within the crown group. Relational hypotheses of this nature can be tested through comparative functional studies in extant echinoplutei. Parallel evolutionary losses of feeding in echinoplutei are accompanied by loss or modification of characteristic structures. This suggests that developmental constraints do not fully explain the conservation of these structures in planktotrophic echinoplutei. Comparisons of larval and adult morphology over congruent time intervals demonstrate that the origin of many orders was accompanied by a suite of synapomorphies in larval morphology that was subsequently conserved. Many details of echinopluteus morphology are therefore of taxonomic significance. Intraordinal patterns of larval diversity, however, vary considerably. That larval morphology has diversified independently of adult morphology indicates that mosaic evolution has occurred within the life cycle and suggests that echinoid larvae and adults can and do respond to selection independently. Taken together, these findings underscore the complex ways in which complex life cycles can evolve.

Organisms emit, detect, and respond to a huge array of environmental signals. The distribution of a given signal is dependent, first of all, upon the original spatial distribution of signal sources, the source landscape. The signal sources can be fixed or moving and their output can be stable or ephemeral. Different sources can also occupy the same general spatial location, such as insects living on a host plant. The emitted signals are modified by relevant transport processes, which are often strongly scale and environment dependent. Chemical signals, for example, are propagated by diffusion and turbulence. The resulting complex, three-dimensional, and dynamic distribution of signals in the environment is the signal landscape; it is the environment of potentially available information in which sensory systems function and have evolved. Organisms also differ widely in what signals they can actually detect; the distribution of signals that an organism can potentially respond to is its information landscape. Although increasing the kinds and specificity of signals that can be detected and processed can lead to improved decision making, it almost always comes at an increased cost. The greater the spatial and temporal complexity of the environment, the greater are the costs of incomplete information and the more advantageous is the development of improved information-gathering capabilities. Studies with simulation models suggest how variability in the spatial structure of source and signal landscapes may control patterns of animal movement that could be represented in the trace fossil record. Information landscapes and the corresponding sensory systems should have evolved in concert with major transitions in the history of life. The Ediacaran to Cambrian interval is one of the most intensively studied periods in the history of life, characterized by the profound environmental and biological changes associated with the bilaterian radiation. These include the advent of macroscopic predation, an increase in the size and energy content of organisms, and the transition in seafloors from laminated matgrounds to mixgrounds produced by the development of macroscopic infaunal bioturbation. The overall effect of these transitions was to markedly increase the spatial complexity of the marine environment. We suggest that this increased spatial complexity, in turn, drove the evolution of macroscopic sense organs in mobile bilaterians, leading to their first appearance during the Cambrian. The morphology and distribution of these sense organs should reflect the life habits of the animals that possessed them. Our overall hypothesis was that there was a “Cambrian Information Revolution,” a coevolutionary increase in the information content of the marine environment and in the ability of and necessity for organisms to obtain and process this information. A preliminary analysis of the Maotianshan Shale (Chengjiang) biota indicates that the distribution of eyes and antennae in these animals is consistent with predictions based on their life habit.

New evidence from fossil plants at the Middle Miocene fossil quarry near Fort Ternan, Kenya, together with that from paleosols, allow reconstruction of a mosaic of early successional woodland (on Dhero paleosols), grassy woodland (on Chogo clay eroded phase and ferruginized nodule variant paleosols) and wooded grassland (on type Chogo and Onuria clay paleosols). This grassy open vegetation was on a high plateau of phonolite at the foot of a carbonatite-nephelinite stratovolcano, which probably supported dry Afromontane forest, alpine meadows, and marsh. This earliest savanna-mosaic vegetation yet documented from Africa, was probably recruited from dry lateritic soils elsewhere in Africa during climatic drying and cooling some 15 Ma. These early grassland ecosystems were very different from Early Miocene forest ecosystems of East Africa, but not altogether like modern grasslands either. Already present were grasses with dense growth and rich in silica bodies, and abundant antelope with moderately high crowned teeth and cursorial limb structure. These mammalian adaptations to grasslands, however, were not nearly so pronounced as they are in modern African grassland faunas, which include zebra and other Asiatic immigrants, as well as antelope. Grasses of the subfamily Chloridoideae and supertribe Panicanae were common in tropical Africa by Middle Miocene time, if not much earlier, but there is not yet evidence so far back in time for the grass supertribe Andropogonae which is now dominant in seasonally arid, overgrazed, and burned African grasslands.

Phylogeny-based approaches can be used to infer diversification dynamics and the rate and pattern of trait change. Applying these analyses to fossil data often requires time-scaling a cladogram of morphotaxon relationships. Although several time-scaling methods have been developed for this purpose, the incomplete sampling of the fossil record can distort the apparent timing of branching. It is unclear how well different time-scaling methods reconstruct the true temporal relationships or how any such inaccuracy could affect tree-based evolutionary analyses. I developed process-based simulations of the fossil record that allow the comparison of approximated time-scaled trees to true time-scaled trees. I used this simulation framework to test the effect of time-scaling methods on the fidelity of several commonly applied tree-based analyses, across a range of simulation conditions. When the fidelity of time-scaling methods differed, the stochastic “cal3” time-scaling method with ancestral assignment produced preferable results. Estimating rates and models of continuous trait evolution was particularly sensitive to bias from scenarios that forced the insertion of many short branch lengths, a bias that is not solved by any of the considered time-scaling methods in all scenarios. The cal3 method of time-scaling can be recommended as the preferred time-scaling method among those tested, but caution must be exercised because tree-based analyses are prone to easily overlooked biases.

Synapsids dominated the terrestrial realm between the late Pennsylvanian and the Triassic. Their early evolution includes some of the first amniotes to evolve large size, herbivory, and macro-predators. However, little research has focused on the changes in diversity occurring during this early phase in their evolutionary history, with more effort concentrating on later events such the Permo-Triassic extinction. Here we assess synapsid diversity, at both the species and genus levels, between the Carboniferous (Moscovian) and the Middle Permian (Capitanian). A raw, taxic diversity (richness) estimate is generated, and we use two separate methods to correct for sampling biases in this curve. To remove the effect of anthropogenic sampling bias, we apply a recently published modification of the residual diversity method, and then generate a supertree, using matrix representation with parsimony to infer ghost lineages and obtain a phylogenetic diversity estimate. The general diversity pattern reflects the initial diversification of synapsids in the late Pennsylvanian and early Cisuralian, which was followed by an extinction event during the Sakmarian. Diversity recovered during the Artinskian and Kungurian, coinciding with the radiation of Caseidae, although other families begin to decline. A second extinction event occurred across the Kungurian/Roadian boundary, in which Edaphosauridae and Ophiacodontidae died out although Caseidae and Therapsida diversified. The sampling-corrected curves reveal further extinction during the Roadian, although therapsids were again unaffected. Pelycosaurian-grade synapsids survived during the Wordian and Capitanian, but were a minor part of an otherwise therapsid-dominated fauna. Evidence of significant anthropogenic sampling bias calls into question previous diversity studies that have not employed sampling correction.

If the dinosaurs Protoceratops grangeri and Hypselosaurus sp. grew at rates predicted for their body size from extant reptiles, their expected ages at reproductive maturity would be about 20 and 62 yr respectively. This calculation assumes that dinosaurs grew at ectothermic rates which are about an order of magnitude slower than those of endothermic vertebrates of the same body size. Such delayed reproductive maturity may have set a premium on selection for high juvenile survivorship that is reflected in the relatively large eggs of these dinosaurs and in their only moderately large clutch sizes compared to modern reptiles.

Three closely related species of the irregular echinoid Oligopygus succeed one another in the late Eocene deposits of Florida. This apparent lineage, where each species is similar but larger than the preceding species, offers an excellent opportunity to examine relationships between heterochrony and environmental change. Bivariate and multivariate techniques illustrate patterns of simple and complex allometry and indicate that simple size increase related to changes in the timing of maturation account for most of the morphological differences among species. Those differences that are not the by-products of size increase are due to heterochronic changes dissociated from the otherwise global (i.e., whole organism) hypermorphosis and were probably needed to increase the relative food intake of the larger forms. Unlike the vast majority of allometric studies, which show constant growth ratios, the ontogenies analyzed here often exhibit curvilinear trajectories. Thus, heterochronic events are seen as systematic alterations of already changing growth ratios. It is suggested that these heterochronic changes provided quick, efficient responses to selective pressures on a plurality of characters and behaviors. As the Florida carbonate environment became more stable, responses associated with K-type life history strategies were favored, resulting in “size selection.” Thus, the morphological differentia among these species may not have resulted from selection directed at those traits, but were either simple sequelae of size increase or of changes which occurred to permit the size increase.

The fundamental goal of biochronology is ordering taxonomic first and last appearance events. The most useful biochronologic data are of the form “the first appearance event of one taxon predates the last appearance event of a second taxon” (FAE < LAE). FAE < LAE data sets are unusually reliable because they converge on a unique solution with greater sampling. The fact that the FAE of one taxon i < the LAE of another taxon j always can be inferred either if i is found lower than j in a stratigraphic section, or if i and j co-occur in at least one taxonomic list. Thus, FAE < LAE data accurately synthesize two disparate sources of information: routine biostratigraphic observations and taxonomic lists that may have no stratigraphic context. Appearance event ordination, the new method introduced here, is intended to summarize FAE < LAE data. The algorithm is founded on the following parsimony criterion: arrangements of FAEs and LAEs should always imply FAEi < LAEj when this is known, and otherwise imply LAEj < FAEi whenever possible. The technique differs from others related to correspondence analysis in its use of FAE < LAE data and explicit definition as a parsimony method. The algorithm is even more unique in that it uses different subsets of FAEi < LAEj statements at each iterative step, converging on separate sets of scores for the FAEs and LAEs. After arranging either the FAEs or the LAEs on the basis of their scores, the other set of scores can be discarded and the best arrangement of the remaining events can be inferred directly. An analysis of the Plio-Pleistocene mammalian record in the Lake Turkana region is used to illustrate the method. Biochronologic resolution on the order of 0.2-1.5 m.y. is achieved. The Turkana species lists by themselves demonstrate enough FAEi < LAEj relationships to resolve the basic biochronologic pattern, but stratigraphic information is still of great use.

Since David Raup's seminal 1976 work, paleontologists have been aware of the relationship between outcrop area and diversity. By incorporating lithologic data derived from Alexander Ronov and coworkers into our own data on areas of mapped outcrops from worldwide geologic maps, we are able to establish a quantitative connection between area of sedimentary rock exposure and diversity from the same general depositional environments. Significant power-law relations are observed at both global and continental scales of consideration. The addition of data on areas of habitable area estimated from paleogeographic maps does not substantially affect these correlations between outcrop area and diversity, indicating that the relation between outcrop area and diversity is primarily a function of sampling and not a common cause such as sea level. We observe a significant diversity-area effect, first noted by Jack Sepkoski in the marine realm. Unlike Sepkoski's, however, our diversity-area effect appears to play a substantial role in influencing diversity through time; a true global diversity signal appears to be contained in the rock record despite the impacts of variable sampling.

Greater outcrop area can serve to increase estimated diversity by increasing both the sample size and the range of habitats and biogeographic provinces sampled. After standardizing for pure sampling intensity by rarefying the number of taxon occurrences, outcrop area continues to explain a substantial portion of global marine diversity. This indicates that coverage, or sampling from multiple habitats and biogeographic provinces, is even more important than sampling intensity. If we remove the effect of outcrop area from our estimate of global biodiversity, we do not observe a net increase in diversity toward the present, lending support to other studies that have not supported a substantial, long-term global increase in biodiversity during the Phanerozoic.

Two kinds of adaptation aid infaunal bivalve mollusks in sustaining their life positions against potentially disruptive water movements: (1) ability to retard scour of surrounding sediment and (2) ability to reburrow rapidly if exhumed.

The shell of the venerid Anomalocardia brasiliana is ornamented with concentric ridges that are asymmetrical in cross-section; experiments show that these ridges aid the animal in burrowing by gripping the sediment during backward rotation of the shell. The shell of the less deeply burrowing venerid Chione cancellata also bears concentric ridges, but these are symmetrical and experiments show that they hinder burrowing; other experiments demonstrate that these ridges reduce scour of sand from around a partly exposed animal. The single cardiid species Trachycardium egmontianum possesses adaptations comparable to those of the two venerid species, in the form of spines of two varieties; experiments show that the flared anterior spines facilitate burrowing and the cupped posterior spines reduce scour.

Conspicuous ornamentation of the kinds considered here was rare throughout the Paleozoic. Its evolutionary deployment occurred primarily during the post-Paleozoic adaptive diversification of infaunal bivalves, which was triggered by the evolution of efficient burrowing mechanisms. The general premium on maintaining infaunal life positions was accentuated for bivalves after the Paleozoic by the origins of important modern predatory taxa.