Selected shell specimens of extant and fossil streamlined pectinids, which have or presumably had level-swimming ability, were examined experimentally to elucidate their hydrodynamic properties, in particular, airfoil efficiency estimated by lift-drag ratio. Using a stationary water tank for nautical engineering, lift and drag forces were measured at various attack angles. Of the examined species, Amusium japonicum, which is characterized by an unusually shiny surface, upward-cambered commissure and sharpened trailing edge, is the most efficient level swimmer and has the lowest drag coefficient and the highest value of lift-drag ratio at any attack angle. Amussiopecten praesignis from the Plio-Pleistocene may have swum horizontally because its airfoil efficiency is superior to that of a living level swimmer, Placopecten magellanicus. Although its shell shape is analogous to that of Placopecten, Camptonectes (Maclearnia) cinctus from the Lower Cretaceous shows a much inferior efficiency and a significant flow separation from the surface. Bernoulli's effect in convex-upward species may contribute to increase lift, but a certain attack angle is always required for level flight. The strategy of level swimming probably evolved independently in several pectinacean lineages in which swimming rather than shell robustness became the preferred defense against predators. One problem that must be solved is that some feedback mechanism is required to check pitching, rolling, and yawing of the shell to attain stability in level flight.

Many organisms continue to grow their skeletons throughout ontogeny. In the shells of molluscs, protists, and brachiopods and in bovid horns, accretionary spiral growth provides a detailed and continuous growth history. Although a shell may be described as a single static form, the overall morphology is a summation of the ongoing accretionary process. For this reason, an explicitly ontogenetic characterization of form provides insight into the final form achieved. Analysis of landmark transformations offers direct access to major components of morphological variation, both among adult individuals and through an individual's ontogeny. Parameters of preconceived, abstract geometric models can also be used to characterize morphological variation, but there is no guarantee that these parameters will coincide with the major features of shape variation.

In order to locate landmarks at equivalent ontogenetic stages, features that indicate ontogenetic stage of coiled forms must be identified (e.g., growth increments, age, size, whorls). The gastropod Epitonium (Nitidiscala) tinctum exhibits prominent varices that provide landmark locations throughout ontogeny. Recent specimens of this species were obtained from three localities in Baja, Mexico. The morphological variation among individuals, treated as whole shells and within individual ontogenies, was analyzed using shape coordinates of landmark configurations. Deformation of shape is expressed in the uniform and nonuniform shape subspaces. The empirical components of shape variation found are similar to those generated by two parameters of an equiangular spiral: θ, the angle between consecutive varices, and W, the whorl expansion rate. The distribution of individuals is examined within morphospaces constructed from these shape features.

Three scales of analysis are necessary to characterize adequately the shape variation within and among specimens. The smallest scale is equivalent to increment-by-increment changes in θ and W. The middle scale comprises variation equivalent to whorls resulting from systematic changes in θ and W during an individual's ontogeny. Finally, there is the overall ontogenetic trajectory. Mean shape must be a function of initial shape and ontogenetic trajectory in shape. Mean forms that are found to have similar shapes at the same arbitrary growth increment may achieve that shape in different ways.

A kill curve for Phanerozoic species is developed from an analysis of the stratigraphic ranges of 17,621 genera, as compiled by Sepkoski. The kill curve shows that a typical species' risk of extinction varies greatly, with most time intervals being characterized by very low risk. The mean extinction rate of 0.25/m.y. is thus a mixture of long periods of negligible extinction and occasional pulses of much higher rate. Because the kill curve is merely a description of the fossil record, it does not speak directly to the causes of extinction. The kill curve may be useful, however, to limit choices of extinction mechanisms.

Taxon-age distributions were compiled for families of marine animals surviving or becoming extinct in each stage of the Phanerozoic. I demonstrate, through the use of a modified bootstrap analysis, that there is no difference between the longevity of families becoming extinct during times of background extinction and times of mass extinction. In both mass and background extinction intervals the mean age of families that become extinct is 2 standard deviations below the geometric mean taxon age of families available for extinction. Young families are more susceptible to extinction, perhaps as the result of lower species richness or of occupying a smaller geographic range. There is no tendency during mass extinctions toward loss of families with different taxon ages other than those that become extinct during background times. Thus, in terms of family survival, mass extinction appears to be an exaggeration of processes of background extinction.

Onshore-offshore patterns of faunal change occurred at many taxonomic scales during the Paleozoic Era, ranging from replacement of the Cambrian evolutionary fauna by the Paleozoic fauna to the environmental expansion of many orders and classes. A simple mathematical model is constructed to investigate such change. The environmental gradient across the marine shelf-slope is treated as a linear array of discrete habitats, each of which holds a set number of species, as observed in the fossil record. During any interval of time, some portion of the species in each habitat becomes extinct by background processes, with rates of extinction varying among both clades and habitats, as also observed in the record. After extinction, species are replaced from within the habitat and from immediately adjacent habitats, with proportions dependent on surviving species. This model leads to the prediction that extinction-resistant clades will always diversify at the expense of extinction-prone clades. But if extinction intensity is highest in nearshore habitats, extinction-resistant clades will expand preferentially in the onshore direction, build up diversity there, and then diversify outward toward the offshore. Thus, onshore-offshore patterns of diversification may be the expectation for faunal change quite independently of whether or not clades originate onshore. When the model is parameterized for Paleozoic trilobites and brachiopods, numerical solutions exhibit both a pattern of faunal change and a time span for diversification similar to that seen in the fossil record. They also generate structure similar to that seen in global diversification, including logistic patterns of growth, declining origination but constant extinction within clades through time, and declining overall extinction across clades through time.

Taphonomic processes in deep-water environments differ markedly from those in shallow waters. These differences are illustrated by the preservational style of a large cetacean skeleton lying at the bottom of the Santa Catalina Basin in 1,240 m of water. The degree of skeletal articulation contrasts with that documented in the shallow North Sea where gas-filled, buoyant carcasses disarticulated during flotation. Increased hydrostatic pressure at greater depth is presumed to have prevented the whale carcass from floating and promoted increased levels of preservation. We present a model that relates gas evolution during decay to carcass buoyancy with depth. Application of this model may ultimately allow the degree of skeletal articulation to be used as a rough index of paleobathymetry.