Taxon discovery underlies many studies in evolutionary biology, including biodiversity and conservation biology. Synonymy has been recognized as an issue, and as many as 30–60% of named species later turn out to be invalid as a result of synonymy or other errors in taxonomic practice. This error level cannot be ignored, because users of taxon lists do not know whether their data sets are clean or riddled with erroneous taxa. A year-by-year study of a large clade, Dinosauria, comprising over 1000 taxa, reveals how systematists have worked. The group has been subject to heavy review and revision over the decades, and the error rate is about 40% at generic level and 50% at species level. The naming of new species and genera of dinosaurs is proportional to the number of people at work in the field. But the number of valid new dinosaurian taxa depends mainly on the discovery of new territory, particularly new sedimentary basins, as well as the number of paleontologists. Error rates are highest (>50%) for dinosaurs from Europe; less well studied continents show lower totals of taxa, exponential discovery curves, and lower synonymy rates. The most prolific author of new dinosaur names was Othniel Marsh, who named 80 species, closely followed by Friedrich von Huene (71) and Edward Cope (64), but the “success rate” (proportion of dinosaurs named that are still regarded as valid) was low (0.14–0.29) for these earlier authors, and it appears to improve through time, partly a reflection of reduction in revision time, but mainly because modern workers base their new taxa on more complete specimens. If only 50% of species are valid, evolutionary biologists and conservationists must exercise care in their use of unrevised taxon lists.

Concavo-convex brachiopods are generally assumed to have lived with their convex valves against the sediment. This orientation is based primarily on the a priori assumption that the upturned commissure would prevent fouling of the brachiopods' mantle cavity. Here, I present epibiontic and taphonomic evidence from orthids and nonproductid strophomenids that concavo-convex brachiopods lived in the reverse life orientation, with the convex valve on top.

Ten testable epibiontic and taphonomic criteria are proposed to establish the uppermost valve during encrustation and whether epibionts encrusted primarily live or dead hosts. The criteria are evaluated for 11 collections of Ordovician and Devonian nonproductid concavo-convex brachiopods that contain a total of over 500 brachiopods and 4000 epibionts. In all cases, the results support a convex-up orientation and the encrustation of live hosts. Five criteria concern epibiont growth patterns and show that (1) epibionts predominate on convex valves, (2) epibionts are not restricted to shell margins, (3) distinct exposed and cryptic faunas, as predicted theoretically and described in previous studies, exist on convex and concave valves respectively, (4) epibiont colonies were often truncated by the commissures and hinges of live hosts, and (5) growth patterns of some epibionts indicate live hosts. Five taphonomic criteria show that (1) concavo-convex shells have greater epibiont cover than other morphologies, (2) internal surfaces of disarticulated specimens are rarely encrusted, (3) interiors of articulated specimens are rarely mud-filled, (4) standard taphofacies indicators suggest little postmortem exposure, and (5) sedimentological reconstructions suggest rapid burial.

Convex-up brachiopods suggest that the Paleozoic mud bottoms they lived on were probably firmer than usually assumed and that a more complex functional interpretation of concavo-convexity is required. Hydrodynamic stability was important to many concavo-convex, plano-convex, and unequally bi-convex brachiopods. Brachiopods with these morphologies probably lived in the more stable orientation on their concave or flat valve. Productid brachiopods, although also concavo-convex, lived convex valve down and are ecologically distinct from earlier concavo-convex taxa. Productids were anchored, semi-infaunally, by spines and were more similar to Mesozoic oysters in orientation and shell function.

Capture-mark-recapture of Nautilus belauensis in Palau shows that (a) immature animals added an average of 0.1 mm of shell per day at the ventral circumference (projected mean annual increment of 37 mm), but this rate decreases rapidly as maturity is approached; (b) rate of growth line secretion is irregular, ranging from 4 to 8 days per growth line in immature animals; (c) overall weight is an imprecise measure of size and growth in Nautilus due to fluctuations in crop content and chamber liquid; (d) this species is shown to live for at least 4 yr beyond maturity. Age estimates based on the highest growth rate obtained (0.12 mm/day = 43.8 mm/yr) range from 14.5 to 17 yr for seven animals recaptured 45–355 days after release. Combined, these data indicate that the life span of Nautilus may exceed 20 yr.

A diffusion model of the distribution of a phenotypic character in a group of species is developed and analyzed. The model incorporates the combined effects of phyletic evolution, speciation and extinction. Directed speciation is modeled by assuming there is some bias to phenotypic changes during speciation. Species selection is modeled by assuming there is some dependence of either speciation or extinction rates on the phenotypic character. Three examples are analyzed to illustrate the use of the model. A model of completely random changes due to both phyletic evolution and speciation shows how between-species differences are established. A model of directed speciation due to multiplicative changes during speciation shows how a simple assumption about the speciation process can produce macroevolutionary trends. A model of species selection due to differences in extinction rates shows how the efficacy of species selection depends on the between-species variance produced both by speciation and by phyletic evolution.

In recent articles in this journal Hansen (1980) and Jablonski (1980) discussed the planktonic larval stages of fossil benthic invertebrates and the relation of type of larval development to geographic ranges, nearshore-offshore position, speciation, and extinction. The fossil record provides a view of long term consequences of types of larval development, and the emphasis of these, and other, paleobiological studies was on selection among species which are characterized by different developmental adaptations.

The pedicle differentiates brachiopods from all other phyla. With its associated musculature, it is more variable than any other brachiopod system at or above the generic level and is considered to contribute to the versatility of behavior possessed by some species. Differences in pedicle type are reflected clearly in the beak.

Studies of ecological structure and diversity over time in extinct groups have always been challenged by the inability to observe the behavior of fossil taxa directly. The only available evidence for function, behavior, and interactions between taxa is the morphological characteristics of the preserved fossils. Recent studies on modern groups have shown that morphological analyses may give misleading results in terms of ecological pattern and diversity. An alternative approach is to focus on functionally relevant aspects of morphology through a paleobiomechanical paradigm. The purpose of this research is to examine variation in the lower jaw morphology in Late Devonian arthrodire placoderms and develop biomechanical metrics that can be used to quantify functional diversity among this fossil group. Nine functionally relevant morphological characters were collected for 94 isolated arthrodire inferognathals from the Gogo Formation in Western Australia and the Cleveland Shale in Ohio. These data were used to address aspects of functional morphology, biomechanical disparity, and ecological structure in arthrodire placoderms from the Late Devonian. Results were compared with results from previous morphometric work on the same set of jaws.

Statistical tests show a significant difference in functional characters between the two faunas. The differences may be related to phylogenetic differences between faunas, as the two major clades of arthrodire taxa included in this study are almost completely segregated between faunas. Average pairwise disparity analyses of the mechanical characters indicate that there is no significant difference in overall functional diversity between the Cleveland Shale and Gogo Reef arthrodire faunas. This result is at odds with previous results that show overall morphological disparity to be much higher in the Cleveland Shale. Clustering patterns within a multivariate function-space show tightly constrained functional groups of taxa independent of phylogenetic or shape-based morphological similarity. These functional groups illustrate a level of ecological diversity in Late Devonian arthrodires that is comparable to that in certain modern faunas.

Further statistical analysis of the morphological and functional disparity of these Late Devonian taxa shows a disjoint between the two measures. Model I regression analysis of and Spearman rank-correlation analysis of average pairwise morphological and functional disparity measures indicate no significant relationship between morphological and functional disparity among the jaws used in this study. Although function is obviously derived from morphology, these results show that morphological shape analysis is not necessarily a good proxy for eco-functional diversity.

Dental morphology changes dramatically across the artiodactyl-cetacean transition, and it is generally assumed that this reflects the evolutionary change from herbivory and omnivory to carnivory. To test hypotheses regarding tooth function and diet, we studied size and position of wear facets on the lower molars and the stable isotopes of enamel samples. We found that nearly all investigated Eocene cetaceans had dental wear different from typical wear in ungulates and isotope values indicating that they hunted similar prey and processed it similarly. The only exception is the protocetid Babiacetus, which probably ate larger prey with harder skeletons. The closest relative of cetaceans, the raoellid artiodactyl Indohyus, had wear facets that resemble those of Eocene cetaceans more than they do facets of basal artiodactyls. This is in spite of Indohyus's tooth crown morphology, which is unlike that of cetaceans, and its herbivorous diet, as indicated by stable isotopes. This implies that the evolution of masticatory function preceded that of crown morphology and diet at the origin of cetaceans.

Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging, because early twentieth-century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early twentieth century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late twentieth century at the Po prodelta. The twentieth century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late twentieth century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of assemblages from ~25–50 years to ~10–20 years in both regions. This shift reflects a decline in the depth of the fully mixed layer from more than 20 cm to a few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ca. a.d. 1950. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion, even when hypoxic conditions were limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in cores was enhanced by higher frequency of hypoxia that reduced bioturbation in the late twentieth century.

Application of computed tomography to the most recently recognized specimen of Archaeopteryx demonstrates the potential of this technique for paleontological issues. Contrary to earlier interpretations of these specimens, the results of computed tomography appear to show the presence of an avian-like doubled-headed quadrate bone in this earliest true bird.

Gastropod opercula are classified here on a new morphogenetic basis, which distinguishes three main types: (1) flexiclaudent spiral (mostly multispiral) operculum, the shape of which does not coincide with that of the aperture, (2) rigiclaudent spiral (usually paucispiral) operculum, the shape of which fits that of the aperture, and (3) rigiclaudent concentric operculum, also aperture-fitting. The first type fits by flexing into the aperture and is secreted when the soft parts are partly or wholly extended (i.e., when the operculum is not in a closed position). The other two types do not flex upon retraction (except at the very margin) and grow when the operculum closes over the aperture, with or without rotation. A study of opercular types at the family level confirms the systematic and evolutionary significance of opercula. Types 1 and 2 are the only ones present in archaeogastropods, Type 1 being predominant. Opercula (if present) in Neritopsina are always rigiclaudent. Within Caenogastropoda, Type 2 predominates; the only flexiclaudent spiral opercula are found in certain basal cerithioidean families. Concentric opercula are predominant in higher neotaenioglossans and exclusive in neogastropods. Except for one family, opercula in Heterostropha are always rigiclaudent spiral. Morphological, systematic, and histological criteria point to the flexiclaudent spiral operculum as the ancestral form. This leads us to propose the “periostracum shaving” model in prosobranchs to account for the origin of this kind of operculum. According to this model, in the earliest trochospiral gastropods the periostracum ceased to serve a shell-formation function at the band of overlap between whorls (the parietal band). The periostracal band was then extruded from the shell to constitute an incipient operculum, taking on the appearance of a spiral strip coiling opposite to the shell. The parietal segment of the periostracal groove migrated toward the epipodium and became independent from the rest of the mantle. The concomitant development of an opercular disc allowed the successive turns of periostracal strip to seal together. In this way, a spiral operculum emerged, coiling counterclockwise without matching the aperture shape. During the course of prosobranch evolution, rigiclaudent spiral opercula emerged several times from the ancestral flexiclaudent type, although they were always restricted to apertures with a spiral-shaped outer (labral) edge. Such opercula enlarged the range of shell morphologies for which the operculum constituted an efficient protective barrier to include those of neritoidean or naticoidean type. The onset of calcification in opercula took place with the rigiclaudent type. Concentric opercula also evolved independently from rigiclaudent spiral opercula in several gastropod groups, thus further broadening the spectrum of apertures and, hence, of shell morphologies using opercula for protection. From the standpoint of adaptation, the concentric type was probably the only one available to neogastropods having long and wide siphonal canals.

Problems of stratigraphic completeness and poor temporal resolution make analysis of faunal change in terrestrial sequences difficult. The fluvial Neogene Siwalik formations of India and Pakistan are an exception. They contain a long vertebrate record and have good chronostratigraphic control, making it possible to assess the influence of biotic interchange on Siwalik fossil communities. In Pakistan, the interval between 18 and 7 Ma has been most intensively studied and changes in diversity and relative abundance of ruminant artiodactyls and muroid rodents are documented with temporal resolution of 200,000 years. Within this interval, diversity varies considerably, including an abrupt rise in species number between 15 and 13 Ma, followed by a decline in ruminant diversity after 12 Ma and a decline in muroid diversity in two steps at 13 and 10 Ma. Significant changes in relative abundance of taxa include an increase in bovids between 16.5 and 15 Ma, a decrease in tragulids after 9 Ma, and a very abrupt increase in murids at 12 Ma. Megacricetodontine rodents also decrease significantly at 12 Ma, and smaller declines are recorded among myocricetodontine and copemyine rodents after 16 Ma. An increase of dendromurine rodents at 15.5 Ma is also observed. There is also a trend of progressive size increase among giraffoids and bovids throughout the sequence.

We have also investigated relationships between biotic interchange and diversity, body size, and relative abundance, concluding that (1) the rapid increase in ruminant and muroid diversity was largely due to immigration, whereas in situ speciation had only a secondary role; (2) during intervals of increasing diversity, resident lineages did not have higher than average rates of in situ speciation; (3) during intervals with rising diversity, greater extinction did not accompany increased immigration; (4) during intervals with falling diversity, there may have been greater extinction in recently invading lineages; and (5) change in diversity was independent of changes in relative abundance and body size.

Fossil data suggest that the great majority of metazoan classes that existed in the Early Cambrian arose after about 700 my ago. The rectangular model of evolution, which views most evolutionary change as being concentrated in speciation events, can easily accommodate the implied rate of divergence.

If, as some authors believe, the eukaryotic cell arose long before the start of the Cambrian, an explanation is required for the delay of large-scale metazoan divergence. The advent of sexuality may have triggered diversification, not by accelerating phyletic evolution, as traditionally believed, but by making possible speciation and, hence, adaptive radiation. Another important delaying factor may have been the near-saturation of Precambrian algal systems in the absence of cropping by herbivores. Uncropped Precambrian systems should have been self-limiting in terms of diversification. When advanced heterotrophy finally arose, self-propagating feedback systems of diversification should have been set in motion. Even if the eukaryotic cell arose later than commonly envisioned and triggered the radiation of the Metazoa about 700 my ago, the earlier absence of sexuality and cropping may have delayed the transition from the prokaryotic condition to the eukaryotic condition.

A hierarchical branching model is presented that tracks diversity of lineages and paraclades simultaneously. It is based on three parameters: (1) lineage origination within a paraclade, (2) lineage origination that founds new paraclades, and (3) lineage extinction. The model assumes stochastic constancy of evolutionary rates and, therefore, exponential growth of diversity is the null expectation. The probabilistic model can be used to generate confidence intervals for exponential growth through Monte Carlo simulation. These confidence intervals can be compared to empirical curves to test the null hypothesis of exponential growth. The analytic solution for paraclade diversity gives the statistical expectation for the probabilistic model. It can be applied to empirical diversity curves of supraspecific taxa to estimate total speciation rate, the intrinsic rate of increase, and the proportion of speciation events that found new paraclades. Two factors were identified with the model that may cause diversity curves for higher taxa (e.g., genera, families) to deviate from simple exponential growth during initial phases of diversification: (1) stochastic fluctuation at low diversity, and (2) taxonomic structure (i.e., species/genus ratio). These sources of variability should be evaluated before macroevolutionary explanations for specific diversity trends are invoked. Two kinds of evolutionary radiations were investigated with this model: (1) prolonged periods of low diversity (macroevolutionary lag) in cheilostome bryozoans and mammals prior to their main phases of diversification, and (2) rapid bursts in diversity of bivalves following the late Permian and end-Cretaceous mass extinctions. In all cases these patterns were found to deviate significantly from the null expectation of exponential growth lending support to previous macroevolutionary explanations. Finally, rates of speciation during radiations were compared to rates of speciation during background times for articulate brachiopods, cheilostome bryozoans, bivalves, and mammals. Speciation events that found new paraclades tend to make up a larger proportion of total speciation events during radiations compared to background times indicating that the opportunity for new adaptive zones to be filled at higher taxonomic levels is proportionately higher during these periods of increased evolutionary activity, and is not simply a result of an increased frequency of speciation.

One of the best-recognized patterns in the evolution of organismal size is the tendency for mean and maximum size within a clade to decrease following a major extinction event and to increase during the subsequent recovery interval. Because larger organisms are typically thought to be at higher extinction risk than their smaller relatives, it has commonly been assumed that size reduction mostly reflects the selective extinction of larger species. However, to our knowledge the relative importance of within- and among-lineage processes in driving overall trends in body size has never been compared quantitatively. In this study, we use a global, specimen-level database of foraminifera to study size evolution from the Late Permian through Late Triassic. We explicitly decompose size evolution into within- and among-genus components. We find that size reduction following the end-Permian mass extinction was driven more by size reduction within surviving species and genera than by the selective extinction of larger taxa. Similarly, we find that increase in mean size across taxa during Early Triassic biotic recovery was a product primarily of size increase within survivors and the extinction of unusually small taxa, rather than the origination of new, larger taxa. During background intervals we find no strong or consistent tendency for extinction, origination, or within-lineage change to move the overall size distribution toward larger or smaller sizes. Thus, size stasis during background intervals appears to result from small and inconsistent effects of within- and among-lineage processes rather than from large but offsetting effects of within- and among-taxon components. These observations are compatible with existing data for other taxa and extinction events, implying that mass extinctions do not influence size evolution by simply selecting against larger organisms. Instead, they appear to create conditions favorable to smaller organisms.

The origin of ammonite sutures is one of the most interesting open problems in paleontology. Present theories of suture formation deal with function rather than morphogenesis. Here we present a study showing that sutures are scale-invariant. In each of ten different ammonite genera, the ontogenetic sequence of the suture can be characterized by the same fractal dimension. We propose a morphogenetic theory for ammonite sutures that is based on the similarity between sutures and the Saffman-Taylor instability in both fractal characters and physical mechanism.

Quantitative analysis of growth rings in pre-Quaternary fossil woods is commonly used as a paleoclimatic indicator. In this paper, a global analysis of the relationship between climate and growth ring parameters in modern trees is presented that, in part, invalidates the use of fossil woods in this way. Data reprocessed from the International Tree-Ring Data Bank are used to analyze three parameters, mean ring width, mean sensitivity, and percentage latewood, from 727 sites across a global climatic range. Results allow the complex relationship between climate and growth ring parameters to be quantified at the global scale for the first time. They reveal the enormous variability in tree response to climate-forcing, which is influenced by disparate factors such as taxonomy, ontogeny, ecology, and environment. Quantitative analysis of fossil growth ring data in light of the modern results indicates that even the largest and most detailed fossil studies conducted to date are probably inadequate in distinguishing a paleoclimate signal from the background noise of variability. The validity of using quantitative growth ring parameters as indicators of Pre-Quaternary climates is therefore questionable. Only in well-constrained studies where paleoclimatic, ontogenetic, and taxonomic sources of variability can be controlled, and data sets are very large, may fossil growth ring analysis provide useful paleoecological data. The findings of this paper do not invalidate in any way the use of growth rings in fossil woods as qualitative paleoclimatic indicators.

Model calculations predict that pathways of alpha- and beta-diversity in diversifying ecosystems notably differ depending on the relative role of competition, predation, positive effects of species' interactions, and environmental parameters. Four scenarios are discussed, in which alpha- and beta-diversity are modeled as a function of increasing gamma-diversity. The graphic illustration of this approach is herein called α-β-γ plot, in which the x-axis indicates increasing diversification rather than absolute time. In purely environmentally controlled systems, beta-diversity maintains near-maximum values throughout the diversification interval, whereas mean alpha-diversity increases linearly, with a slope being reciprocal to beta-diversity. A second scenario is based on the assumption that increasing richness will have predominantly positive effects on the addition of further species; here, alpha- and beta-diversity increase simultaneously (though not necessarily at the same rates) and without reaching a predictable upper limit. In ecosystems that are characterized by low competition between species, mean alpha-diversity asymptotically approaches a saturation level, whereas the increase in beta-diversity accelerates until alpha-diversity stagnates, and then continues to rise linearly. If competition is high, addition of species first increases beta-diversity until no further habitat contraction is possible, followed by a period in which alpha-diversity increase through adaptive divergence becomes the principal drive of diversification. Because there is a continuous transition between the late stage of the low-competition model and the early stage of the high-competition scenario, both can be combined in a single model of diversity partitioning under the premise of a diversity-dependent increase of competition. This summary model predicts three phases of diversity accumulation: (1) a niche overlap phase, (2) a habitat contraction phase, and (3) a niche differentiation phase. The models herein discussed provide a potential tool to assess the question which factors primary controlled the diversification of life over geological times.

The same three subtypes of derived multiserial Hunter-Schreger bands are found in the incisor enamel of African phiomorph rodents from the late Eocene-early Oligocene and the oldest South American Caviomorpha from the Deseadan (late Oligocene). The synapomorphies contained therein, especially arrangement and orientation of interprismatic matrix, make an African origin of the Caviomorpha very probable. A North American origin of the Caviomorpha is thus rejected, as only primitive pauciserial Hunter-Schreger bands have been observed in possible ischyromyoid caviomorph ancestors. A multiserial Schmelzmuster apparently never evolved in the North American rodent fauna.

Paleoecologists studying Paleozoic reef-builders have interpreted their growth forms as responses to conditions of depth and turbulence in reef complexes. Comparison of the shapes of Paleozoic stromatoporoids and corals with the growth forms of modern scleractinians has been used to reconstruct Paleozoic conditions. A review of shape zonation on modern reefs indicates that no general pattern is applicable to all reefs and variations in shape are the result of the interaction of many environmental factors with the genetically dictated growth pattern of the coral. In most zones of a reef a wide range of shapes co-exist. The growth forms of corals on modern reefs are not a simple vegetative response to the many environmental parameters that have been shown to influence form, but are constrained by phylogenetic and developmental influences as well as functional ones.

Interpretations of the environments of western Canadian and other mid-Paleozoic reefs have been based on the growth forms of stromatoporoids. The environmental significance of the shapes has been deduced from comparison with the shapes of modern scleractinians, functional morphology, nature of the enclosing sediment, position of growth, position within the reef, and diversity gradients. The validity of these criteria is open to question and considerable doubt remains concerning the significance of the growth forms. The shapes of reef animals are not specific guides to environments of modern reefs and should not be expected to be guides for ancient ones.