Evolutionary patterns in the articulate brachiopod hinge are investigated and this empirical example is used as a model to examine evolutionary trends and evolutionary constraints. Several features of the hinge-system geometry exhibit persistent directional change from the Cambrian to the Recent and result in increased mechanical advantage in opening the valves. These geometric changes are reflected also in general features of shell morphology and growth and in patterns of familial diversity through time. Specific, identifiable constraints on brachiopod morphology and function related to the position of the pedicle and muscles and nature of the hinge line and hinge structures may be said to direct the observed trends. The pattern of evolutionary change among all articulate brachiopods is most satisfactorily accommodated by a diffusion model of morphological change. Examined independently, the deltidiodonts (with noninterlocking hinge structures) appear to reflect a process of directional selection through time, whereas change in the cyrtomatodont hinge mechanism (with interlocking teeth and sockets) may reflect the incidental effects of selection for increasing mantle-cavity volume rather than increasing diductor (opening) muscle moment. The “transition” from deltidiodont to cyrtomatodont hinge structures over the course of brachiopod evolutionary history may be interpreted from at least two distinctly different perspectives. Paleoecologically, this transition appears to reflect a habitat shift from soft sediment to hard, rocky substrates. Phylogenetically, the transition reflects the process of evolution: deltidiodonts persist as their modified descendants, the cyrtomatodonts. Investigating the acquisition of evolutionary novelties over time, or the transition from primitive to derived morphological features, may be the most informative approach to follow in studying evolutionary trends.

The per-stage extinction rate is the product of the per-taxon extinction rate and stage length, and the per-stage origination rate is defined similarly. These rates decline from ancient to recent times because of the pull of the Recent, because there is more young than old fossiliferous rock, and because average stage length increases from the recent to the past. More specifically, the present model assumes that the graphs of ln(per-stage extinction rate) and ln(per-stage origination rate) versus geologic time have slope zero in the absence of sampling biases, and shows how sampling biases cause both these graphs to appear to have slope min(h,q) + s in the distant past, where h and q are the fossil loss and actual per-taxon extinction rates, and the stratigraphic constant, s, quantifies how stage length changes through time.

Although the per-stage rates of bivalve families and marine invertebrate genera decline toward the recent, the magnitudes of these declines are entirely consistent with what the present model predicts sampling biases will produce. Hence there is no need to postulate a biological explanation for these patterns.

The mass, wingspan, and wing area of pterosaurs were reconstructed. Mass was estimated by determining volume and multiplying by avian density. This method was considered appropriate only for smaller pterosaur species because there is evidence for lower density in larger species. These reconstructions were used to compare the wing shapes of Triassic and Jurassic pterosaurs with those of birds. Pterosaurs had wings of below-average loading and above-average aspect compared to the avian mean. This wing design was compatible with relatively slow and highly efficient flight, with high maneuverability. Wing area depends on the reconstruction model adopted; wings attached to the hindlimb principally reduced aspect, and secondarily reduced loading, which would improve take-off performance at the expense of efficiency. The wing shape and cranial feeding adaptations of pterosaurs were most compatible with a marine or aerial predatory adaptive zone. The reconstructed pterosaurs show a limited range of wing shape compared to birds. This may partly reflect preservational bias favoring species living in marine or lagoonal environments, but this is not a complete explanation because there is a lack of pterosaurs with wings of high loading like the marine ducks and auks. Structural, physiological, or adaptive factors may have limited pterosaur wing shape.

This study assesses swimming potential in a variety of ammonoid shell shapes on the basis of coefficients of drag (Cd) and the power needed to maintain a constant velocity. Reynolds numbers (Re) relevant to swimming ammonoids, and lower than those previously studied, were examined. Power consumption was scaled to a range of sizes and swimming velocities. Estimates of power available derived from studies of oxygen consumption in modern cephalopods and fish were used to calculate maximum sustainable swimming velocities (MSV).

Laterally compressed, small thickness ratio (t. r.) ammonoids, previously assumed to be the most efficient swimmers, do not experience the lowest drag or power consumption at all sizes and velocities. At low values of size and velocity associated with Reynolds numbers below 104, less compressed forms have smaller drag coefficients and reduced power requirements. At hatching a roughly spherical shell shape would have minimized drag in ammonoids; with increasing size, hydrodynamic optima shift toward compressed morphologies.

The high energetic cost of ammonoid locomotion may have limited dispersal and excluded ammonoids from high current velocity environments.

The temporal dimension of fossil sequences provides a critical component to the study of intraspecific dynamics and species formation. Here I report on the branching and subsequent morphological evolution of two gastropods (Melanopsis fossilis and M. vindobonensis) from the Late Miocene of the Pannonian basin in eastern and central Europe. Although morphological divergence between species is rapid, intermediates between the two species co-occur with typical individuals for approximately 1 m.y. and then disappear. The long-term persistence of intermediates followed by their ultimate disappearance is a pattern that, to my knowledge, has not been previously observed.

Distinguishing genetic from ecophenotypic influences on shell form in freshwater prosobranchs is difficult. Nevertheless, consideration of the temporal, geographic, lithologic, and paleoecologic patterns of this sequence suggests that the morphologic differences between M. fossilis and M. vindobonensis had some genetic basis. Whether these forms were initially morphs of a single species or two species with some hybridization between them is impossible to determine. In either case, the morphological changes that resulted in M. vindobonensis were rapid, but the attainment of complete isolation between M. fossilis and M. vindobonensis apparently did not occur until approximately 1 m.y. later.

Polymorphism is being actively investigated by ostracode specialists because the phenomenon is widespread and may be a key to understanding speciation patterns and evolutionary processes. However, the methods for recognizing the phenomenon and the cause or causes of polymorphism are still under debate. This study presents an example of genetically controlled ostracode dimorphism in which the relationship between the two forms can be explained in terms of heterochrony. The living males as well as the females of Loxoconcha uranouchiensis in Aburatsubo Cove, central Japan, are found to be dimorphic in size, shape, and surface ornamentation of the carapace. The dimorphic forms appear concurrently in the same microhabitat, suggesting that the dimorphism is genetically rather than environmentally controlled. Although the large and the small forms are indistinguishable in their early stages, morphological differences gradually appear and become complete after the final molting, which is the seventh molting for both forms. The characters determined by the number of moltings, such as the numbers of reticules, bristles, and pores, are identical between adults of the two forms. In contrast, for characters affected by the growth condition of the epidermal-cell system, such as carapace shape and fine surface ornamentation, the small-form adult is similar to the penultimate instar of the large form. Taking the large form as a standard, the small form is paedomorphic. These two forms are reproductively isolated and should be considered different species.

A biomechanical study of archaeocyathan (phylum Archaeocyatha) skeletal construction was undertaken in order to compare its function with that of poriferans. Flume experiments were conducted on three cylindrical, brass models of regular archaeocyathans. Two of these, the porous-septate and aporous-septate models (i.e., possessing septa either with or without pores), represent an ontogenetic series; regular archaeocyathans (class Regulares) typically exhibit a reduction in septal porosity as they grow and many have aporous septa as adults. The third model is aseptate and represents a morphology that is not found in the fossil record. All models exhibit passive entrainment of flow during flume testing, a phenomenon on which modern sponges depend for suspension feeding. Flow direction through the models is consistent with predictions of the spongiomorph-affinity hypothesis. The three models behave quite differently, however. The aseptate model is least effective at passive entrainment. Although some fluid exits the top of the central cavity (or osculum), a great deal of fluid is entrained out the top of the intervallum and also leaks out the outer wall. Flow induction from the oscula of the septate models is augmented when compared to the aseptate model. The porous-septate model exhibits slight leakage from the outer wall, and a dye-rich plume exits the top of the intervallum. Alternatively, the aporous-septate model exhibits no outer-wall leakage and no entrainment from the intervallum. These differences in flow pattern between the porous- and aporous-septate models suggest a hitherto unknown function for septa. Imperforate septa prohibit the migration of fluid through the intervallum to the low-pressure, downstream side where leakage occurs. The ontogenetic shift in septal porosity, common to many archaeocyathan species, may be a mechanism by which outer-wall leakage is avoided later in life. Archaeocyathans would have encountered progressively higher ambient current velocities as their height increased through growth. Outer-wall leakage is not a problem at low velocities or small sizes, but leakage becomes serious at higher velocities when tall, adult morphologies are attained.