In a rare lapse of judgment, T. H. Huxley defined science as “organized common sense.” Huxley's motivation cannot be faulted, for he wished, by this definition, to stress the accessibility of science to any well-educated person. In the first paragraph of his most famous work on popular science (The Crayfish: An Introduction to the Study of Zoology, 1880), Huxley stated the fallacy that he wished to dispel with this definition:

Homalozoans include four classes of non-pentamerous Paleozoic echinoderms: Homostelea (cinctans), Ctenocystoidea (ctenoid-bearing homalozoans), Homoiostelea (solutes), and Stylophora (cornutes and mitrates). Their atypical morphologies have historically made it difficult to relate them to other classes. Therefore, their systematic positions have been represented by two hypotheses (H): as stem taxa to echinoderms (H1) or as stem taxa to chordates (H2). These conclusions rest on previous inability to recognize synapomorphies with more crownward echinoderms, resulting in a forcing of the homalozoans down the phylogenetic tree that is more artifactual than evolutionary. The Extraxial-Axial Theory (EAT) identifies body-wall homologies, common ontogenetic patterns, and major events in bodyplan evolution. Therefore, the EAT can identify synapomorphies among even the most disparate of echinoderms. Application of the EAT undermines both H1 and H2 and strongly suggests that the bizarre asymmetry of homalozoans is a derived characteristic, and not indicative of plesiomorphic morphology for either chordates or echinoderms. Each of the four homalozoan clades and their major features are reexamined using the EAT. New findings are presented concerning homologies of thecal body wall, but we focus on stems, arms, and brachioles, which are recognized as very distinct products of independent evolutionary events. The results support a new interpretation (H3) of homalozoans as a polyphyletic assemblage that can be parsed out into other, clearly echinoderm clades. The Homoiostelea and Homostelea share the blastozoan synapomorphy of a brachiole. The enigmatic Ctenocystoidea also seem to have brachioles. The Stylophora have an arm as in crinoids. H3 is also more congruent with the known fossil record. Although they are stratigraphically early echinoderms, homalozoans are not indicative of the plesiomorphic morphology of the phylum.

Temporal diversity patterns have traditionally been analyzed by counting the number of families or genera present over a series of time periods. This approach has been criticized on the grounds that paraphyletic taxa might introduce artifacts. Sepkoski and Kendrick (1993) simulated phylogenetic trees and different classifications of those trees and concluded that paraphyletic taxa need not be rejected. We have reimplemented their model, extended it, and carried out statistical analyses under a variety of experimental conditions. Our results show that the focus on monophyly vs. paraphyly is misplaced. Instead, it appears that the number of groups in the classification and the distribution of the sizes of those groups have dramatic effects on the recovery of diversity information. Furthermore, the influence of these factors depends on whether the fossil record represents a low- or high-frequency sampling of lineages. When sampling is good, the best results are achieved by classifications with large numbers of small taxa. When sampling is poor, however, the best results are achieved by classifications that include some large and medium-sized groups as well as many smaller groups. This suggests that the best estimates of underlying diversity will be achieved by counting (in the same study) taxa assigned to different ranks, so as to best match the inferred quality of the paleontological sample. In practice this will mean abandoning the commitment to counting taxa at a single rank.

The study of evolutionary trends is one of the oldest and most intriguing topics in evolutionary biology and paleobiology (McNamara 1990). Workers since Cuvier, Lyell, and Owen have wanted to know if the fossil record demonstrates “progression” within temporal sequences of related organisms. Regardless of whether changes in the average values of morphological characters are progressive in any meaningful sense, these changes are still of great interest. In practice, questions about trends are most commonly framed by paleontologists in terms of “complexity” (however defined) or body size (McShea 1998a).

Encrusting bryozoans provide one of the few systems in the fossil record in which ecological competition can be observed directly at local scales. The macroevolutionary history of diversity of cyclostome and cheilostome bryozoans is consistent with a coupled-logistic model of clade displacement predicated on species within clades interacting competitively. The model matches observed diversity history if the model is perturbed by a mass extinction with a position and magnitude analogous to the Cretaceous / Tertiary boundary event. Although it is difficult to measure all parameters in the model from fossil data, critical factors are intrinsic rates of extinction, which can be measured. Cyclostomes maintained a rather low rate of extinction the model solutions predict that they would lose diversity only slowly as competitively superior species of cheilostomes diversified into their environment. Thus, the microecological record of preserved competitive interactions between cyclostome and cheilostome bryozoans and the macroevolutionary record of global diversity are consistent in regard to competition as a significant influence on diversity histories of post-Paleozoic bryozoans.

It has been said that most scientists would rather use another scientist's toothbrush than his terminology. In this issue's Matters of the Record, John Alroy suggests certain conceptual and methodological changes for the study of mechanisms, or dynamics, of large-scale trends, and also a revision of current trend-mechanism terminology. On the conceptual and methodological matters, Alroy's suggestions are good ones (with one important exception), and here I applaud them and explain further why they ought to be taken seriously. Predictably, however, I do not endorse (some of) his terminological revisions.

Issues of directionality in the history of life can be framed in terms of six major evolutionary steps, or megatrajectories (cf. Maynard Smith and Szathmáry 1995): (1) evolution from the origin of life to the last common ancestor of extant organisms, (2) the metabolic diversification of bacteria and archaea, (3) evolution of eukaryotic cells, (4) multicellularity, (5) the invasion of the land and (6) technological intelligence. Within each megatrajectory, overall diversification conforms to a pattern of increasing variance bounded by a right wall as well as one on the left. However, the expanding envelope of forms and physiologies also reflects—at least in part—directional evolution within clades. Each megatrajectory has introduced fundamentally new evolutionary entities that garner resources in new ways, resulting in an unambiguously directional pattern of increasing ecological complexity marked by expanding ecospace utilization. The sequential addition of megatrajectories adheres to logical rules of ecosystem function, providing a blueprint for evolution that may have been followed to varying degrees wherever life has arisen.

A major new inventory of living marine Bivalvia (Mollusca) is based on 29 regional faunas. These again pick out strong latitudinal and longitudinal gradients in taxonomic diversity, but there are indications that the patterns are not so regular as previously thought. There are signs of asymmetry between the Northern and Southern Hemisphere latitudinal gradients, with the former tending to be more regular than the latter. Northern gradients are also characterized by a marked inflection at approximately 30°N, and the three Australian provinces seem to form a distinct “hot-spot” in the Southern Hemisphere. The larger of the two tropical high-diversity foci (the southern China-Indonesia-NE Australia one) appears to be much more nearly arcuate in plan view than oval and is closely associated with the world's richest development of coral reefs.

A taxonomic and stratigraphic analysis reveals that the steepest latitudinal gradients are associated with the youngest bivalve clades. The most striking pattern is that shown by the heteroconchs, an essentially infaunal taxon that radiated extensively throughout the Cenozoic era. Steep gradients are also characteristic of the relatively young anomalodesmatan and arcoid clades and, somewhat surprisingly, the predominantly epifaunal pteriomorphs. Although the latter taxon falls within an older (i.e., “late Paleozoic-Jurassic”) group of clades, it is apparent that certain elements within it (and in particular the Pectinidae) radiated extensively in the latest Mesozoic-Cenozoic. A small but significant component of the later stages of the adaptive radiation of the Bivalvia comprised epifaunal taxa.

The presence of the steepest latitudinal gradients in the youngest clades provides further evidence that the Tropics have served as a major center of evolutionary innovation. Even though some sort of retraction mechanism cannot be completely ruled out, these gradients are most likely the product of primary radiations. Clade history can be an important determinant of contemporary large-scale biodiversity patterns. The markedly lower diversity of some bivalve clades, such as the heteroconchs, in the high-latitude and polar regions may simply reflect the fact that they are not yet fully established there. In a way that is reminiscent of the onshore-offshore radiation of certain benthic marine invertebrate taxa, it may take periods of tens or even hundreds of millions of years for bivalve clades to disseminate fully across the earth's surface.

The persistent spread of taxa from low- to high-latitude regions should perhaps come as no great surprise, as the tropical ocean is very much older than either of the polar ones. The late Cretaceous-Cenozoic evolutionary radiation of the Bivalvia was accompanied by a marked deterioration in global climates, and many new groups have yet to be fully assimilated into cool- and cold-water benthic ecosystems.

Exploration of the theoretical morphospace of erect helical colony form in Bryozoa, created by McKinney and Raup (1982), reveals that only a small volume of the three-dimensional space of hypothetical form is occupied by actual colonies of the Paleozoic fenestrates (Class Stenolaemata) Archimedes and Helicopora, helical species of the cheilostome (Class Gymnolaemata) Bugula, and the cyclostome (Class Stenolaemata) Crisidmonea archimediformis. Actual helical-colony bryozoans are not found in regions of the morphospace characterized by colony geometries that possess the largest surface areas of filtration sheet. Examination of computer-simulated colonies in the theoretical morphospace reveals that, although possessing high surface areas, colonies in the empty region of high-surface-area morphospace possess other aspects of geometry that are unrealistic as filter-feeding geometries: the filtration-sheet whorls are held at small acute angles to the central colony axis and are deeply nested within one another, both of which are disadvantageous conditions for the system of filter feeding used by the extant cheilostome Bugula, and presumably by extinct helical-colony bryozoans as well.

Even though actual bryozoans are found only in the low to intermediate surface-area regions of the theoretical morphospace, surface area of filtration sheet is a major determinant of form in these helical colonies, as is evidenced by a negative correlation in values of the parameters BWANG and ELEV exhibited by the colony data. Minimum values of BWANG are even further constrained by the apparent need of the Archimedes colonies to maintain filtration-sheet branching densities within the range of 20 to 50.

Members of the neogastropod muricid subfamily Rapaninae are abundant, shallow-water predators whose phylogeny was previously investigated by Kool (1993b), who used mainly anatomical characters. In order to deepen understanding of the evolution of this important clade and to incorporate functional, ecological fossil evidence, we performed a phylogenetic analysis based on 34 shell characters in 45 genus-level taxa, including five muricid outgroups. Cladograms based on shell characters alone differed from those founded on anatomical features these analyses differed from the phylogenetic reconstruction combining all available morphological evidence. The preferred cladogram incorporates all evidence and reveals a “Thais group” and an “Ergalatax clade” that both emerge from the derived portion of a more primitive, paraphyletic group of other rapanines. The Ocenebrinae, the other four outgroup taxa three ergalataxine taxa all lie outside the rapanine clade that includes the remaining ergalataxines as a derived subclade.

We used the phylogenetic results to probe aspects of the ecological history of the Rapaninae. Our data imply that antipredatory shell defenses (elongated aperture, denticles on the inner side of the outer lip robust external spines and tubercles) evolved multiple times, mainly in post–early Miocene clades in the Indo–West Pacific region. These results support earlier nonphylogenetic inferences.

We compared known prey types and methods of predation of living rapanines with their distribution on our phylogenetic tree. The plesiomorphic mode of feeding in the Rapaninae is drilling of hard-shelled prey. Feeding by other means and on such soft-bodied prey as sipunculan and polychaete worms evolved several times independently among post–early Miocene rapanines in the Indo–West Pacific. Methods of predation on hard-shelled prey that involve edge-drilling or attack by way of the aperture also evolved independently several times, but did so throughout the geographical range of the subfamily.

Specialization for life on the upper shore occurred in at least eight lineages, all but two of which are confined to the Indo–West Pacific. Ecological diversification of the Rapaninae was therefore most common in the tropical Indo–West Pacific during and after early Miocene time. This diversification occurred in a setting of already high biological diversity and intense competition and predation.

Reefs containing abundant calcified metazoans occur at several stratigraphic levels within carbonate platforms of the terminal Proterozoic Nama Group, central and southern Namibia. The reef-bearing strata span an interval ranging from approximately 550 Ma to 543 Ma. The reefs are composed of thrombolites (clotted internal texture) and stromatolites (laminated internal texture) that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs ranging in scale from a meter to several kilometers in width. Stromatolite-dominated reefs occur in depositionally updip positions within carbonate ramps, whereas thrombolite-dominated reefs occur broadly across the ramp profile and are well developed as pinnacle reefs in downdip positions.

The three-dimensional morphology of reef-associated fossils was reconstructed by computer, based on digitized images of sections taken at 25-micron intervals through 15 fossil specimens and additionally supported by observations of over 90 sets of serial sections. Most variation observed in outcrop can be accounted for by a single species of cm-scale, lightly calcified goblet-shaped fossils herein described as Namacalathus hermanastes gen. et sp. nov. These fossils are characterized by a hollow stem open at both ends attached to a broadly spheroidal cup marked by a circular opening with a downturned lip and six (or seven) side holes interpreted as diagenetic features of underlying biological structure. The goblets lived atop the rough topography created by ecologically complex microbial-algal carpets; they appear to have been sessile benthos attached either to the biohermal substrate or to soft-bodied macrobenthos such as seaweeds that grew on the reef surface. The phylogenetic affinities of Namacalathus are uncertain, although preserved morphology is consistent with a cnidarian-like bodyplan. In general aspect, these fossils resemble some of the unmineralized, radially symmetric taxa found in contemporaneous sandstones and shales, but do not appear to be closely related to the well-skeletonized bilaterian animals that radiated in younger oceans. Nama reefs demonstrate that biohermal associations of invertebrates and thrombolite-forming microorganisms antedate the Cambrian Period.

More than 1600 valves of Late Cretaceous and early Paleocene Northern Atlantic Coastal Plain gryphaeid oysters (Exogyrinae and Pycnodonteinae) were examined for breakage-induced shell repair and morphologic variability to evaluate the hypothesis of escalation. The Exogyrinae show disproportionately higher average repair frequency (0.41) relative to the ecologically and functionally similar unornamented pycnodonts (0.19). An increase in repair frequency (independent evidence of the action of a selective agent, e.g., predation) through the stratigraphic interval supports escalation. Variation in repair frequencies may reflect differences in oyster morphology and in the strength and diversity of shell crushers across an onshore-offshore gradient. Escalation of antipredatory adaptation characterized the evolutionary response of gryphaeid oysters to their durophagous predators. Adaptation generally occurred by the enhancement of existing traits in both oyster lineages. Characters that confer a selective advantage against predators are not all expressed or improved concurrently in both oyster lineages. Morphologic adaptations to minimize shell breakage include the development of expansive, broad commissural shelves, thickened valves, and surface ornamentation (Exogyrinae). Surface ornament in the Exogyrinae gradually increased with time. For some characters, such as thickness, conflicting functional demands (e.g., valve stabilization) may have limited adaptation to predators.

The study of evolution has increasingly incorporated considerations of history, scale, and hierarchy, in terms of both the origin of variation and the sorting of that variation. Although the macroevolutionary exploration of developmental genetics has just begun, considerable progress has been made in understanding the origin of evolutionary novelty in terms of the potential for coordinated morphological change and the potential for imposing uneven probabilities on different evolutionary directions. Global or whole-organism heterochrony, local heterochrony (affecting single structures, regions, or organ systems) and heterotopies (changes in the location of developmental events), and epigenetic mechanisms (which help to integrate the developing parts of an organism into a functional whole) together contribute to profound nonlinearities between genetic and morphologic change, by permitting the generation and accommodation of evolutionary novelties without pervasive, coordinated genetic changes; the limits of these developmental processes are poorly understood, however. The discordance across hierarchical levels in the production of evolutionary novelties through time, and among latitudes and environments, is an intriguing paleontological pattern whose explanation is controversial, in part because separating effects of genetics and ecology has proven difficult. At finer scales, species in the fossil record tend to be static over geologic time, although this stasis—to which there are gradualistic exceptions—generally appears to be underlain by extensive, nondirectional change rather than absolute invariance. Only a few studies have met the necessary protocols for the analysis of evolutionary tempo and mode at the species level, and so the distribution of evolutionary patterns among clades, environments, and modes of life remains poorly understood. Sorting among taxa is widely accepted in principle as an evolutionary mechanism, but detailed analyses are scarce; if geographic range or population density can be treated as traits above the organismic level, then the paleontological and mac̀roecological literature abounds in potential raw material for such analyses. Even if taxon sorting operates on traits that are not emergent at the species level, the differential speciation and extinction rates can shape large-scale evolutionary patterns in ways that are not simple extrapolations from short-term evolution at the organismal level. Changes in origination and extinction rates can evidently be mediated by interactions with other clades, although such interactions need to be studied in a geographically explicit fashion before the relative roles of biotic and physical factors can be assessed. Incumbency effects are important at many scales, with the most dramatic manifestation being the postextinction diversifications that follow the removal of incumbents. However, mass extinctions are evolutionarily important not only for the removal of dominant taxa, which can occur according to rules that differ from those operating during times of lower extinction intensity, but also for the dramatic diversifications that follow upon the removal or depletion of incumbents. Mass extinctions do not entirely reset the evolutionary clock, so survivors can exhibit unbroken evolutionary continuity, trends that suffer setbacks but then resume, or failure to participate in the recovery.

Changes in genus diversity within higher taxa of marine animals on the temporal scale of a few million years are more strongly correlated with changes in extinction rate than with changes in origination rate during the Paleozoic. After the Paleozoic the relative roles of origination and extinction in diversity dynamics are reversed. Metazoa as well as individual higher taxa shift from one mode of diversity dynamics to the other. The magnitude of taxonomic rates, the relative variance of origination and extinction rates, and the presence or absence of a long-term secular increase in diversity all fail to account for the shift in importance of origination and extinction in diversity changes. Origination and extinction rates both tend to be diversity-dependent, but different modes of diversity-dependence may contribute to the change in diversity dynamics from the Paleozoic to the post-Paleozoic. During the Paleozoic, there is a weak tendency for extinction rates to be more diversity-dependent than origination rates, whereas after the Paleozoic the two rates are about equally diversity-dependent on average.

The bulk of Neoproterozoic trace fossils can be interpreted as horizontal creeping trails produced by minute vermiform organisms moving on or just beneath the seafloor or under algal mats. We have investigated the formation of trails by living cnidarians and platyhelminths that creep by cilia on mucus ribbons. These relatively simple metazoans produce trails that are similar in size and morphology to some Neoproterozoic traces, owing to the entrainment of sediment within their mucus trails. Thus a mucociliary locomotory system provides sufficient means to form some types of Neoproterozoic traces. It follows that the body architectures of the Neoproterozoic trace-makers may have been quite simple, though complex bodyplans are, of course, not ruled out. Thus, the use of Neoproterozoic trace fossils to constrain the time of origin of bilaterians or of any crown-group bilaterian taxon remains questionable.

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

Geometric properties of the shells of 123 species of extant Bivalvia were analyzed from the viewpoint of theoretical morphology. The effects of shell form and the structure of ligament on the interumbonal space and the maximum shell opening received particular attention. The results of computer simulation and morphospace analysis indicate that possessing both prosogyrous shell form and an extended hinge without the parivincular ligament tends to cause space conflict between umbones or dorsal shell margins of right and left valves. To a large degree, a prosogyrous shell form with a long parivincular ligament helps shell opening without umbonal conflict, if the shell is flat enough to avoid the mutual interference between dorsal shell margins of closed valves. Extension of the ligament and plunging of the anterior part of the coiling axis into the ventral side provide enough space along the dorsal shell margins in which a parivincular ligament and its substrata are developed.

The general allometric equations for the logarithmic helicospiral can fit many extraconical shapes, but the isometric conditions traditionally used limit study only to conical growth. We present evidence to show that in real gastropod shells, the logarithmic helicospiral equations fit the suture. Poor location of the coiling axis and / or an inappropriate pole for the logarithmic helicospiral has often led to the rejection of this model. The differences between the errors associated with measurement or previously available models are discussed. Two methods, based on suture trace measurements, are proposed to locate the coiling axis both in apical and lateral views. The first is a graphical method based on an elementary property of the logarithmic spiral. The second is a computational method based on iterative reprojections of the suture. It is shown that the protoconch and the teleoconch must be treated separately. The precision of the new methods (especially the computing method) enables deviations from logarithmic helicospiral trajectory to be identified and differentiated from irregularities of the shell and sequential growth phases. Application of these methods may be useful not only for other gastropod morphological features, but also for other taxa such as brachiopods and other mollusks.

Vase-shaped microfossils (VSMs) occur globally in Neoproterozoic rocks, but until now their biological relationships have remained problematic. Exceptionally preserved new populations from the uppermost Chuar Group, Grand Canyon, Arizona, display details of morphology and taphonomy that collectively point to affinities with the testate amoebae. The fossils are tear-shaped tests, ∼20–300 μm long and ∼10–200 μm wide, that are circular in transverse section, expand aborally toward a rounded or slightly pointed pole, and taper orally toward a “neck” that ends in a single aperture. Apertures may be circular, hexagonal, triangular, or crenulate, and may be rimmed by a distinct collar. Approximately 25% of the Chuar VSMs are curved, such that the oral and aboral poles do not lie opposite each other. Tests are preserved as mineralized casts and molds, commonly coated with organic debris or iron minerals, but they were originally composed of nonresistant organic matter. Approximately 1% have a “honeycomb-patterned” wall attributable to the original presence of mineralized scales whose bases were arranged regularly in the test wall. Scale-bearing testate amoebae, such as members of the Euglyphidae, are essentially identical to the honeycomb VSMs, and a close relationship between other Grand Canyon VSMs and additional testate amoebae, both lobose and filose, is likely. The VSM population therefore most likely represents a multispecies assemblage whose spatial association reflects a common habitat and/or taphonomic circumstances that favor test preservation. The assignment of these fossils to the testate amoebae strengthens the case for a major diversification of eukaryotic organisms by mid-Neoproterozoic times and, more significantly, provides the earliest morphological evidence for heterotrophic eukaryotes in marine ecosystems.