The living members of 113 families of bivalves and gastropods of the Californian Province include 698 species living at shelf depths, of which 538 or 77% are known as Pleistocene fossils from the same region; another 113 fossil species are extralimital, and 98 are extinct. Living species not found as fossils are chiefly rare today, and/or minute, fragile, and/or from deeper shelf habitats. Sampling of the Pleistocene record has been biased towards shallow-water assemblages. Fragile and minute forms are probably underrepresented in the record. Rare forms, however, are still appearing as new studies are conducted, and many rare species are yet to be discovered. At least 85% of durably skeletonized living species may have been captured in the record. It is probable that most durably skeletonized invertebrate species were represented in lithostratigraphic units throughout the Phanerozoic, but that this record is lost owing to erosion, burial, and destruction of skeletons in situ. The bulk of the marine invertebrate fossil record does not represent a series of unusual skeletal accumulations, but rather the preserved remnants of an excellent original record formed through ordinary though episodic processes.

In studies of heterochrony, timing of development is generally measured relative to some intrinsic dimension, usually size, which serves as a proxy for time. A complementary approach is to measure timing relative to chronological time, an extrinsic dimension. Here, colony growth of a marine hydroid, Hydractinia echinata, is used to compare size- and age-based approaches to rate heterochronies. Colonies consist of feeding polyps; a basal, encrusting mat; and, in some cases, stolons which extend from the mat. Size is measured by the area covered by the mat and, if present, the stolons. Rate of polyp production was compared between colonies using clonal replicates grown in a common environment. Age-based comparisons employed polyp specific growth rates calculated over the same ontogenetic time interval for each replicate. Specific growth rates for colony area were also calculated; rates-ratios (polyp/area) provided size-based measures of polyp growth, analogous to allometric coefficients commonly used in studies of rate heterochronies. These comparisons show that size can be a misleading referent for between-colony comparisons because the rate of areal growth is not constant among colonies. Size-based measures of changes in developmental rates may provide insight into patterns, but not necessarily processes, of heterochrony. The implications of the pattern-oriented framework of size-based heterochrony should be considered in paleontological studies.

Geographical restriction to refuges implies the regional extinction of taxa in areas of the previous range falling outside the refuge. A comparison of the circumstances in the refuge with those in areas from which the taxa were eliminated is potentially informative for pinpointing the causes of extinction. A synthesis of data on the geographical and stratigraphical distributions of cool-water molluscs of the North Pacific and North Atlantic Oceans during the late Neogene reveals four patterns of geographical restriction, at least two of which imply that climatic cooling was not the only cause of extinction during the last several million years. These four patterns are (1) the northwestern Pacific restriction, involving 15 taxa whose amphi-Pacific distributions during the late Neogene became subsequently restricted to the Asian side of the Pacific; (2) the northwestern Atlantic restriction, involving six taxa whose early Pleistocene distribution is inferred to have been amphi-Atlantic, but whose present-day and late Pleistocene ranges are confined to the northwestern Atlantic; (3) a vicariant Pacific pattern, in which many ancestral amphi-Pacific taxa gave rise to separate eastern and western descendants; and (4) the circumboreal restriction, involving six taxa whose early Pleistocene distribution, encompassing both the Atlantic and Pacific Oceans, became subsequently limited to the North Pacific. Like the Pliocene extinctions in the Atlantic, previously studied by Stanley and others, the vicariant Pacific pattern is most reasonably interpreted as having resulted from regional extinction of northern populations in response to cooling. The northwestern Pacific and Atlantic restrictions, however, cannot be accounted for in this way. In contrast to the northeastern margins of the Pacific and Atlantic, the northwestern margins are today characterized by wide temperature fluctuations and by extensive development of shore ice in winter. Northeastern, rather than northwestern, restriction would be expected if cooling were the overriding cause of regional extinction. Among the other possible causes of extinction, only a decrease in primary productivity can account for the observed northwestern and circumboreal patterns of restriction. Geographical patterns of body size and the distribution of siliceous deposits provide supporting evidence that primary productivity declined after the Miocene in the northeastern Pacific, but remained high in the northwestern Pacific, and that productivity in the Pacific is generally higher than it is in the Atlantic. The patterns of geographical restriction in the northern oceans thus provide additional support to previous inferences that reductions in primary productivity have played a significant role in marine extinctions.

Posterior eccentricity has evolved independently in three lineages of northern Pacific sand dollars; it is best known in the genus Dendraster. In these forms, the anterior areas of the aboral surface are much more highly developed than the posterior areas; consequently, the apical system is located posteriorly, rather than centrally. This morphology is linked to an unusual and highly successful mode of suspension-feeding. The evolution of eccentricity appears to be related to regulatory mechanisms found in many non-eccentric sand dollars, such as Echinarachnius parma. During the ontogeny of this form, the growth rates of the ambulacra and interambulacra are correlated with their position along the longitudinal axis. Early in ontogeny, the anterior areas develop at a faster rate than the posterior areas; later in ontogeny, this relationship is completely reversed. Normally, these two phases of unequal growth counterbalance each other, and the mature test appears symmetrical. However, if the balance between the two phases were upset, eccentricity would naturally ensue. In fact, aberrant Recent and fossil Echinarachnius with the predicted anteriorly-and posteriorly-eccentric morphologies actually do exist. Posterior eccentricity is apparently produced by retention of the earlier unequal growth pattern, which favors anterior development. This represents trait neoteny; however, since the retained trait is a regulatory mechanism that controls growth rates over the entire aboral surface, the morphological effects are particularly profound. Thus, the seemingly bizarre morphology of Dendraster can be derived by a change in the timing of an existing regulatory mechanism. This may help to explain the sudden appearance of Dendraster in the fossil record and the absence of transitional forms. The unusual suspension-feeding behavior of Dendraster may have been derived from a righting response that is common among other sand dollars.

Several very large, taxonomically standardized data sets have been compiled and utilized to investigate biogeographic and evolutionary patterns of continental margin benthic foraminifera. Mean partial species durations for 87 frequently occurring and 180 rarely occurring species on the Atlantic continental margin of North America are the same, namely 21 m.y. The global fossil record of these species indicates no center or centers of origin and indicates very rapid dispersal. The Miocene had the greatest number of first occurrences with 46%, followed by the Pleistocene, Pliocene and Oligocene with approximately 13% each. The remaining 14% first occur in the Eocene, Paleocene, and Cretaceous. Species with a wide geographic distribution often exhibit longer species durations than those with narrow geographic ranges. The vast majority of endemic species (150 of 175) occur rarely and have no fossil record.

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

Pteroscaphites are small, rare species of scaphitid ammonites from the Upper Cretaceous (Turonian-Santonian) of the Western Interior of North America. Their evolution appears to parallel that of the larger species of Scaphites or Clioscaphites, with which they co-occur. To investigate this evolutionary pattern, I constructed a phylogeny of all these species based on their distribution of shared derived characters including ornamentation, ammonitella size, whorl shape, umbilical diameter, sutures, number of whorls, adult size, shape of the adult body chamber, and apertural modifications. This analysis revealed numerous instances of congruence in the preadult morphology of the pteroscaphites and that of the co-occurring scaphites or clioscaphites. However, there is a marked divergence at maturity. Adult pteroscaphites are uniformly small (2.5 postembryonic whorls in their phragmocone) and develop apertural projections. The scaphites and clioscaphites exhibit a number of morphological changes at approximately the same whorl number, but secrete as many as two more whorls in their phragmocone before forming a mature body chamber with a relatively unmodified aperture. Both groups display sexual dimorphism. The process of progenesis may explain this conflicting pattern of congruence prior to maturity and divergence at maturity. Adult pteroscaphites are not mature replicas of the juvenile shells of the larger scaphites or clioscaphites, because maturation produces its own set of morphological modifications. Nevertheless, such features as the apertural projections on adult pteroscaphites may be interpreted as extrapolations of juvenile patterns of growth into maturity. The repeated associations of pteroscaphites with scaphites or clioscaphites in the Upper Cretaceous suggest that progenesis was iterative and may have represented an optional developmental pathway common to all of these species.

Nonparametric randomization tests were used to determine whether fossil marine families classified in different higher taxa are significantly different in their durations, their rates of origination and extinction, and their proportion of originations. For each of these properties, significant differences among orders and classes were found that could not be explained by differences at other taxonomic ranks, but no such differences were found among phyla. Thus, orders and classes appear to possess an evolutionary coherence that phyla lack.

Growth of the colony is a basic element of morphological evolution and life history in cheilostome bryozoans. Here we consider the occurrence of different modes of growth in encrusting cheilostomes through geologic time and in well-studied living associations. We assess patterns of zooid formation by direct examination of skeletal characters in species from nearly all diverse fossil assemblages reported from North America and quantify within-assemblage diversities and abundance rankings for fossil encrusting species with different modes of growth. These data document macroevolutionary trends showing a transition from dominance of an apparently primitive mode of budding in the Early Cretaceous to derived modes through the Tertiary. The trends are characterized by their long duration and apparent convergence among systematic subgroups within the Cheilostomata. We then consider the validity of our observations as adaptive trends. Patterns of ecological dominance among living and fossil species indicate that different patterns of zooid formation are important determinants of success of colonies as reflected by their abundance, competitive ability, survivorship, and recovery from injury or predation. The consistency of the long-term trends and evidence for the existence of ecological mechanisms in fossil assemblages suggest a major evolutionary role for biotic interactions.

Functional consequences of the variation in geometry and morphology of the articulate brachiopod hinge mechanism are poorly understood, despite the fact that hinge structures have considerable importance in brachiopod taxonomy. Jaanusson (1971) proposed that the ability to resorb shell material during growth, particularly in the hinge structures, can be used to distinguish two groups within the articulates, the deltidiodonts and the cyrtomatodonts. He considered the two groups to be morphologically distinct, “natural” phylogenetic groups, separated by a “functional discontinuity.” In order to test the morphological, functional, and phylogenetic implications of shell resorption, comparisons of the hinge-system geometry and diductor muscle moment are made here between deltidiodont and cyrtomatodont brachiopods. A truss network composed of landmarks relevant to the valve opening mechanism is constructed to characterize hinge-system geometry. The function of the hinge mechanism is analyzed in the context of valve opening, and diductor muscle force, effort lever arm, and moment are compared between deltidiodonts and cyrtomatodonts. The distribution of resorption among brachiopods is investigated with respect to a phylogenetic hypothesis proposed by Williams and Rowell (1965a).

Deltidiodont brachiopods are morphologically more variable than cyrtomatodonts, and a greater proportion of the variability is correlated with size. Deltidiodonts and cyrtomatodonts employ different strategies to open the valves; deltidiodont lever arms are relatively longer, whereas cyrtomatodont diductor muscles have relatively larger cross-sectional areas. The greatest muscle moment in deltidiodont hinge systems is realized in the maintenance of a gape angle; in the cyrtomatodont system, it is achieved at the initiation of a gape. Although they are morphologically and functionally distinct, it is doubtful that the two groups are separated by a “functional discontinuity.” Because the phylogenetic relationships among brachiopod orders are not yet resolved, the status of shell resorption as a homologue is still unclear. Resorption is manifest in at least some members of each major group of articulates (except the pentameraceans); it is likely that resorption has evolved independently several times in brachiopod evolution, in part because of the increased morphological flexibility it confers.

The Mesozoic marine revolution theory postulates a causal connection between the Cretaceous radiations of durophagous predators and the decline of suspension-feeding echinoderms in shallow-water habitats. In order to test the temporal distribution of dense ophiuroid populations for such a decline, I present a method of calculating the expected distribution of populations or communities in different geologic timespans. This statistical null hypothesis may then be compared with data from the fossil record to draw paleoecological inferences. The model takes into account the relative lengths of time blocks, the decay of sedimentary rock, and changes in shallow sea area through time. Although mass extinctions did not cause the immediate disappearance of shallow-water “brittlestar beds,” brittlestar beds show a significant decline in the Early Cretaceous. Results of several studies suggest that predators were at least partially responsible for reducing the distribution of dense ophiuroid populations.

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.

The borophagine canids were bone-cracking scavengers in the Miocene-Pleistocene of North America. In this they parallel the Recent hyenas. This paper analyzes the borophagine adaptation in relation to that of hyaenids, using Osteoborus cyonoides as an example. The emphasis during canid evolution on the posterior molars, which is a derived condition, created a constraint on the adaptation of borophagines. This constraint meant that the borophagines used P4/4 as bone-cracking teeth, whereas hyaenids use P3/3. The latter adaptation has the advantage of separating the bone-cracking teeth from the meat-cutting portion of the dentition, thereby allowing a dual purpose dentition in hyaenids. In borophagines, no such dual purpose was possible, and it is suggested that they were closer to obligate bone-cracking scavengers than Recent hyaenids. Other than the evolution of a specialized bone-cracking tooth, the borophagines adapted to bone cracking by evolving a vaulted and strengthened skull for the dissipation of the strong forces generated during bone cracking. In this they again parallel the hyaenids. Evolution within borophagines involved an elaboration of patterns already set at the group's inception, creating an evolutionary trend which was mediated by the constraint on the bone-cracking morphology. This trend may be due to selection or sorting, or may, under certain assumptions, be stochastic. Other evolutionary trends may also be epiphenomena of constraints that lock morphological evolution.

A general model of species dynamics must incorporate the effects of species number on the processes of speciation and extinction. Previous models make specific assumptions about these effects, but do not consider the effects of dynamics of lower level entities on speciation and extinction rates. A hierarchical model is developed which explicitly describes the effects of energy use by species on speciation and extinction rates. The effects of energy use are represented by parameters that characterize the average effects of energy use by each species in the biota on speciation and extinction rates. The dynamics of the model describe a sigmoidal increase in species number over time, as does the logistic model of species dynamics. However, the mechanisms of those dynamics are assumed to be different in the two models. Empirical analysis of a data set on the diversification of fossil Miocene horses suggests that the logistic and hierarchical models have similar descriptive power. The hierarchical model incorporates insights from recent considerations of the nature of a hierarchical theory of biology. Further progress in developing such a theory will depend on the success with which relationships among levels in the biological hierarchy are able to be defined.

An investigation of shell morphometrics of the prosobranch genus Paramelania from Lake Tanganyika shows striking contrasts between some live and dead populations from identical localities. Living populations and surficial dead shells were collected from 15 sublittoral-profundal localities along the east side of the lake. Interpopulation variability in this endemic gastropod is clinal (N-S) for several shell characters. Intrapopulation variability of dead shell populations frequently exceeds that of live populations. Distinctive morphs may be present in a local dead shell population which are absent in the live population from the same locality. However, the phenodeviant dead shell morphology may occur among live snails elsewhere in the lake. Phenodeviant shells may be encrusted, but are unabraded and show no preferred orientation.

Radiocarbon ages of >2000 yr b.p. on dead phenodeviants suggest that surficial shell accumulations in Lake Tanganyika are strongly time-averaged. Although wave activity and biogenic concentration are important processes in Lake Tanganyika, neither is sufficient under modern lake conditions to account for these admixed assemblages. Winnowing and in situ stratigraphic condensation during periods of lowered lake levels is the most likely explanation for the occurrence of phenodeviant surficial shells. This hypothesis is consistent with the observation that extensive shell lags are exposed on the lake floor in areas currently inhabited by the same snails.

A model of biogeographic range fluctuations for particular Paramelania morphs during the Holocene, in concert with lake level fluctuations, can account for the shell assemblages seen on the lake floor today. The complex history of these accumulations suggests that taphonomic admixtures may obscure the interpretation of evolutionary sequences in the lacustrine stratigraphic record.

Several groups of siliceous microorganisms possess a skeletal latticework of interconnected rods. Skeletal configurations of one of these groups, the silicoflagellates, can be produced by a simple mathematical model that minimizes the apical surface area for a given basal area and internal volume. A similar model that minimizes the total length of the skeletal elements, and thus the silica utilization and skeletal weight, produces configurations that are generally less common in silicoflagellates. The diversity of silicoflagellate skeletal morphologies suggests that both the minimization of apical surface area and the conservation of skeletal material may be important factors in skeletal design. The two most important morphologies found in modern oceans, the four-sided Dictyocha and the six-sided Distephanus, can co-occur in an environment where both factors have some relative importance. However, these models do not explain the range of silicoflagellate skeletal morphology found in nature.

A moving reference frame is introduced for the analysis of accretionary shell growth. Simple principles of motion and stepwise growth define the model. At each growth step, the aperture migrates from its present position to a new position, according to locally defined rules. The aperture becomes the focus of the analysis, mathematically and conceptually, in conformity with biological reality. Kinematic principles provide the analytical framework for describing the aperture's trajectory (kinematics is the study of motion). The aperture “translates,” “rotates,” and “dilates.” The model offers exceptional flexibility in the analysis of accretionary growth forms and is particularly well-suited to analysis of conical and loosely coiled shell geometries. Computer simulations illustrate the principles of a moving reference model. The inverse problem of finding the aperture motions from actual shell data is rigorously specified, for both planispiralled and helicospiralled shell forms.

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

Physical models of graptolites have been constructed for a range of morphologies, with emphasis on planar, multiramous forms. The models are life size and have the density of a living graptolite, based on the now-established collagenous nature of the periderm and unavoidable assumptions about the amount of extrathecal tissue present in the living colony. These models have been used to test the two main hypotheses of graptolite life habit developed by Bulman, Rickards, Kirk, and others. Testing of graptoloid models in water suggests that many rhabdosome shapes were designed for passive rotation within the water column. This is caused in the models by a variety of modifications, including changes in thecal and stipe orientation, alterations of colony shape and the addition of vanes and hooks. Rotation would only have been useful when the rhabdosome was in directional motion and the frequency of such modifications seems anomalous if no such movement occurred. Thus movement by some means is required, either passively, by changes in buoyancy, or by automobility. Spiralling action would increase the harvesting path of an individual living on a planar, multiramous colony, making this a theoretically advantageous mode of life for the morphology. It would prevent the individual zooids of scandent biserial and uniserial colonies from feeding from the same narrow band of water.

Survival of North American shelf trilobite families during a mass extinction across the upper boundary of the Upper Cambrian Sunwaptan Stage (=“Ptychaspid Biomere”) cannot be predicted from patterns of turnover among their component species: on the shelf, stratigraphic ranges of species belonging to surviving families do not differ significantly from those of eliminated families. Thus, the sorting of families during the extinctions cannot be explained by simple upward causation from the individual level. However, families that ranged from shelf to slope environments before the extinctions (most of which are pandemic) fared significantly better than those confined to shelf settings (which tend to be endemic to North America), indicating that family survival was influenced by geographic and environmental distribution, a property emergent above the individual level. One factor implicated in clade survival during normal background times, high species richness, did not influence the outcome of mass extinction. The results affirm the importance of a hierarchical approach to the interpretation of macroevolutionary patterns and provide some support to the suggestion that sorting processes operating during mass extinctions differ from those of background times.