Substantial geographic coverage in paleontological study is essential in testing evolutionary models of phyletic gradualism and punctuated equilibrium. We present a multivariate morphometric study of the late Neogene planktonic foraminiferal clade Globoconella using specimens from four Deep Sea Drilling Project sites (DSDP 284, 207A, 208, and 588) along a latitudinal traverse in the southwest Pacific.

During the Late Miocene (7 Ma to 5 Ma), populations of the ancestral species Globorotalia (Globoconella) conomiozea formed a geographic cline showing continuous morphological variation from the temperate sites (DSDP 284 and 207A) to the warm subtropical sites (DSDP 208 and 588). Populations living to the south had higher conical angle and fewer chambers in the final whorl compared to the northern populations. Nevertheless, populations across the entire cline exhibited a coherent, directional trend towards having larger conical angle and fewer chambers through time. At the Miocene/Pliocene boundary, the intensification of the Tasman Front (Subtropical Divergence) possibly isolated the peripheral populations in the warm subtropics from the central stocks of the temperate water masses. The evolutionary trends became decoupled: the central populations gradually lost their keel and transformed into G. (G.) sphericomiozea, while the peripheral populations in the warm subtropical areas retained their keel and evolved into a flattened species, G. (G.) pliozea.

The gradual transformation of G. (G.) conomiozea terminalis (a form retaining a keel) into G. (G.) sphericomiozea (a form lacking a keel) occurred during an interval of about 0.2 m.y., with all measured morphologic variables showing continuous and steady changes. The evolution of the central populations follows the model of phyletic gradualism. In peripheral populations, the origin of the descendant species G. (G.) pliozea from the ancestor G. (G.) conomiozea terminalis occurred very rapidly within an interval of less than 0.01 m.y. The population size of G. (G.) pliozea was small at the incipient stage at about 5.05 Ma, but increased rapidly to become dominant during the next 0.2 m.y. when the ancestral species G. (G.) conomiozea terminalis became locally extinct. Following speciation, G. (G.) pliozea exhibited morphological stasis for about 0.6 m.y., until the central stock form G. (G.) puncticulata migrated back to the warm subtropics; during the next 0.5 m.y. of their sympatry, there is no sign of hybridization between these two sister species. The evolution of G. (G.) pliozea follows the model of punctuated equilibrium.

The evolution of the Globoconella clade shows both phyletic gradualism and punctuated equilibrium. These two “alternative” evolutionary models complement each other rather than being mutually exclusive. Both models are indispensable towards providing a complete picture of the evolution of Globoconella.

Faunal stasis among Indo-Pacific reef-building corals is explained as the result of chronic environmental disruptions preventing evolutionary processes from approaching completion since the Late Pliocene. The model assumes shallow reefal habitats (<20 m) on the continental shelves are major sites of scleractinian evolution and explores ecological and evolutionary consequences of high-frequency sea-level fluctuations and their associated transgression-regression cycles. Because single generations, dominated by a few large clonal genotypes, may persist indefinitely, local populations may not have experienced enough generations to approach evolutionary equilibrium with their environments during the estimated average duration (≈3200 yr) of existence of shallow habitats. Persistent consequences of chronic evolutionary disturbance may be the extensive intraspecific variation so characteristic of the dominant genera of shallow Indo-Pacific corals and the apparent paucity of recently evolved endemic species. The same disturbances may have accelerated speciation rates among reefal organisms with much shorter generation times.

A computer model is presented which is capable of calculating both the photosynthetic efficiency (I) of any specified plant shape and the stress related to the total moment arm (M) imposed on vertical branching patterns. Computer simulations indicate that a flattened plant thallus and an erect branching growth habit are two plant shapes capable of optimizing photosynthetic efficiency during indeterminate growth. These two morphologies have geometric analogues in the dorsiventral thalli of some bryophytes and in the vertical axes of mosses and tracheophytes, respectively.

Extension of the model to complex, three-dimensional branching patterns indicates that I and I/M are maximized when branching is overtopped (treelike, with lateral branches on a main axis) and when lateral branching systems are planated (frondlike). Geometric alterations of branching patterns that result in optimization of I and I/M can be simulated by computer and are shown to be similar to morphologic alterations attending the early evolution of vascular land plants. It is suggested that a number of major evolutionary trends seen in Upper Silurian to Upper Devonian times can be expressed in terms of optimizing the display of photosynthetic tissues (I) or the balance between photosynthetic efficiency and incurred moment arms (I/M).

The intensity of drilling predation was studied on samples of fossil and Recent species of Turritella, a soft-bottom mesogastropod mollusc. Our data and records in the literature show that the frequency of drilling has remained about the same from the Eocene to the present. There may have been less predation by drilling during the Late Cretaceous. Among living Turritella, there is a sharp increase in intensity of drilling predation from the temperate zones to the tropics. This latitudinal trend is paralleled by an equatorward increase in number of species of drilling gastropods. Strong spiral ribs of some Eocene, Miocene, and Recent species of Turritella confer protection against drilling, but the mechanism of this immunity remains unclear.

Ecomorphological diversity of Mesozoic mammals was presumably constrained by selective pressures imposed by contemporary vertebrates. In accordance, Mesozoic mammals for a long time had been viewed as generalized, terrestrial, small-bodied forms with limited locomotor specializations. Recent discoveries of Mesozoic mammal skeletons with distinctive postcranial morphologies have challenged this hypothesis. However, ecomorphological analyses of these new postcrania have focused on a single taxon, a limited region of the skeleton, or have been largely qualitative.

For more comprehensive locomotor inference in Mesozoic mammals, we applied multivariate analyses to a morphometric data set of extant small-bodied mammals. We used 30 osteological indices derived from linear measurements of appendicular skeletons of 107 extant taxa that sample 15 orders and eight locomotor modes. Canonical variate analyses show that extant small-bodied mammals of different locomotor modes have detectable and predictable morphologies. The resulting morphospace occupation reveals a morphofunctional continuum that extends from terrestrial to scansorial, arboreal, and gliding modes, reflecting an increasingly slender postcranial skeleton with longer limb output levers adapted for speed and agility, and extends from terrestrial to semiaquatic/semifossorial and fossorial modes, reflecting an increasingly robust postcranial skeleton with shorter limb output levers adapted for powerful, propulsive strokes. We used this morphometric data set to predict locomotor mode in ten Mesozoic mammals within the Docodonta, Multituberculata, Eutriconodonta, “Symmetrodonta,” and Eutheria. Our results indicate that these fossil taxa represent five of eight locomotor modes used to classify extant taxa in this study, in some cases confirming and in other cases differing from prior ecomorphological assessments. Together with previous locomotor inferences of 19 additional taxa, these results show that by the Late Jurassic mammals had diversified into all but the saltatorial and active flight locomotor modes, and that this diversification was greatest in the Eutriconodonta and Multituberculata, although sampling of postcranial skeletons remains uneven across taxa and through time.

Speciation processes are only rarely studied with fossil materials, even though in principle hypotheses of speciation patterns are most directly testable in the fossil record. We quantitatively document in two widely separated South Pacific DSDP holes the mid-Pliocene speciation of the planktonic foraminifer Globorotalia truncatulinoides. Speciation, with continuous geographic co-occurrence of ancestor and descendant forms, occurred simultaneously at both localities over a period of ~500,000 years. This suggests a sympatric speciation process that involved a large, geographically extensive population. Globorotalia truncatulinoides underwent its most rapid and extensive evolutionary change between ~2.8 and 2.5 Ma. This time interval corresponds to the development of northern hemisphere glaciation, suggesting that climate-controlled paleoceanographic change may have played a significant role in the evolution of G. truncatulinoides.

The conductance of water vapor and respiratory gases by diffusion through the eggshells of Upper Cretaceous dinosaurs has been estimated from measurements of shell and pore geometry in fossil specimens. When compared to recent reptile and bird eggs for which nest environments are known, the highly porous eggshells of three dinosaur species indicate that the dinosaur nests were high in humidity and probably low in oxygen and high in carbon dioxide. Such conditions most likely occurred underground or within an incubation mound.

By isolating the eggs from the atmosphere, however, some large sauropods may have been forced to limit their clutch size to numbers small enough to prevent depletion of oxygen and elevation of carbon dioxide to intolerable levels in the nest. Fossil evidence supports this and suggests that one sauropod actually divided her large eggs into several clutches. Each small clutch probably had a metabolic rate similar to those of clutches produced by recent reptiles and mound nesting birds.

The growth rate of Nautilus pompilius in its natural environment has been determined from radioactive disequilibrium between 210Pb (half-life 22.3 yr) and its granddaughter 210Po (half-life 138 d) in septa of two juvenile specimens. 210Pb and 210Po data from the most recently formed shell material of both specimens indicate that 210Pb from sea water is incorporated into septa during septal formation and 210Po is excluded. Therefore the 210Po/210Pb activity ratio serves as a chronometer to estimate the age of each septum and the time between formation of septa. In the specimens studied the average time between sucessive points in septal deposition is 75 d for the nine most recent septa of one specimen and 23 d for the six most recent septa of the other specimen. These different growth rates, if representative of the ontogeny of each animal, suggest that the timing of septal deposition probably is dependent on the rate of shell and tissue growth coupled with buoyancy requirements and is not a unique period for all Nautilus. The habitat and ontogeny of Nautilus may be inferred from the pattern of stable isotopes of oxygen and carbon in the septa. Both specimens show a pronounced break in δ18O from nearly uniform light values in the first seven septa to heavier values (∼1%) after the seventh septum. We interpret this break to correspond to the hatching of Nautilus. A temperature (i.e. water depth) interpretation of the δ18O data for septa after the eighth is complicated by a positive correlation between δ18O and δ13C. This may reflect horizontal migration of the animal or a kinetically controlled fractionation of carbon and oxygen isotopes during septal formation.

Constrained seriation of a species-locality matrix of the Australian Cenozoic mammal record resolves a preliminary sixfold succession of land mammal ages apparently spanning the late Oligocene to the present. The applied conditions of local chronostratigraphic succession and inferences of relative stage-of-evolution biochronology lead to the expression of a continental geological timescale consisting of, from the base, the Etadunnan, Wipajirian, Camfieldian, Waitean, Tirarian, and Naracoortean land mammal ages. Approximately 99% of the 360 fossil assemblages analyzed are classifiable using this method. Each is characterized by a diagnostic suite of species. An interval of age magnitude may eventually be shown to lie between the Camfieldian and Waitean, but is currently insufficiently represented by fossils to diagnose. Development of a land mammal age framework marks a progressive step in Australian vertebrate biochronology, previously expressed only in terms of local faunas. Overall, however, the record remains poorly calibrated to the Standard Chronostratigraphic Scale. Codifying the empirical record as a land mammal age sequence provides an objective basis for expressing faunal succession without resort to standard chronostratigraphic terms with the attendant (and hitherto commonly taken) risks of miscorrelating poorly dated Australian events to well-dated global events.

The dominance of Paleozoic articulate brachiopods in once-muddy environments may be explained by an array of mechanisms and structures that reject nonfood particles, in some cases without interruption of feeding: (1) behavioral flexibility of the lophophore and its individual filaments; (2) persistent, variable-speed rejection currents on the mantle, which sometimes concentrate pseudofeces in topographically controlled vortices; (3) costae and alae (which have many other probable functions); (4) inhalant currents elevated above substrate; (5) marginal setae.

Some mantle currents parallel (and presumably augment) lophophore feeding currents; others diverge up to 90° to provide rejection while feeding continues. Contrary to previous reports, the lateral cilia seem to be involved in rejection and may reverse.

Repeated claims for the superiority of the gill of suspension-feeding bivalves over the “weak” individual filaments of the lophophore are probably false. In suspension-feeding bivalves, simultaneous feeding and rejection are likely to be hindered by fused gill elements and mucus-trapping of food. The energetically efficient articulates are predicted to have a competitive advantage over suspension-feeding bivalves when oxygen or food is limiting, as, for example, after a bolide impact.

Both the body fossils and trackways of sauropod dinosaurs indicate that they inhabited a range of inland and coastal environments during their 160-Myr evolutionary history. Quantitative paleoecological analyses of a large data set of sauropod occurrences reveal a statistically significant positive association between non-titanosaurs and coastal environments, and between titanosaurs and inland environments. Similarly, “narrow-gauge” trackways are positively associated with coastal environments and “wide-gauge” trackways are associated with inland environments. The statistical support for these associations suggests that this is a genuine ecological signal: non-titanosaur sauropods preferred coastal environments such as carbonate platforms, whereas titanosaurs preferred inland environments such as fluvio-lacustrine systems. These results remain robust when the data set is time sliced and jackknifed in various ways. When the analyses are repeated using the more inclusive groupings of titanosauriforms and Macronaria, the signal is weakened or lost. These results reinforce the hypothesis that “wide-gauge” trackways were produced by titanosaurs. It is commonly assumed that the trackway and body fossil records will give different results, with the former providing a more reliable guide to the habitats occupied by extinct organisms because footprints are produced during life, whereas carcasses can be transported to different environments prior to burial. However, this view is challenged by our observation that separate body fossil and trackway data sets independently support the same conclusions regarding environmental preferences in sauropod dinosaurs. Similarly, analyzing localities and individuals independently results in the same environmental associations. We demonstrate that conclusions about environmental patterns among fossil taxa can be highly sensitive to an investigator's choices regarding analytical protocols. In particular, decisions regarding the taxonomic groupings used for comparison, the time range represented by the data set, and the criteria used to identify the number of localities can all have a marked effect on conclusions regarding the existence and nature of putative environmental associations. We recommend that large data sets be explored for such associations at a variety of different taxonomic and temporal scales.

The number of diatom valves and fragments per gram of surface sediment in the Atlantic Ocean accurately reflects diatom abundance in the overlying waters, without any evidence of significant lateral drift during settling to the sea bottom. The distribution pattern of the number of resting spores per gram is similar to that for the whole and fragmented diatom valves. Fresh water diatoms and opal phytoliths are abundant in the sediments off the west coast of Africa where they have been deposited by the Trade Winds.

Despite the relatively small number of core tops analyzed, the abundance and distribution patterns of diatoms in the sediments exhibit striking similarities to the patterns of primary productivity, phosphates, and annual production of silica in suspension in surface waters. Areas with high phosphate values and primary productivity and, therefore, areas of upwelling can be inferred from the quantitative distribution of diatoms in the sediments. Q-mode factor analysis, based on the abundance of forty-two species in thirty-seven core tops, produced six diatom species assemblages whose distributions provide additional information on the positions of certain water masses and major currents.

Since the data on the quantitative distribution of diatom valves as well as on the diatom assemblages in the sediments of the Atlantic Ocean today allow prediction of certain water mass characteristics and circulation patterns of the overlying waters, they therefore, permit the reconstruction of paleoceanographic circulation patterns in ancient Atlantic Oceans, using the diatom distribution in sediments from dated horizons.

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.

The problem of gradual versus punctuated change within phyletic lineages can be understood in terms of the homogeneity of evolutionary dynamics. Hypotheses of punctuated change imply that the rules governing evolutionary change shift over time such that the normal dynamics of stasis are temporarily suspended, permitting a period of net evolutionary change. Such explanations are members of a larger class of models in which evolutionary dynamics are in some way heterogeneous over time. In this paper, I develop a likelihood-based statistical framework to evaluate the support for this kind of evolutionary model. This approach divides evolutionary sequences into nonoverlapping segments, each of which is fit to a separate evolutionary model. Models with heterogeneous dynamics are generally more complex—they require more parameters to specify—than uniform evolutionary models such as random walks and stasis. The Akaike Information Criterion can be used to judge whether the greater complexity of punctuational models is offset by a sufficient gain in log-likelihood for these models to be preferred.

I use this approach to analyze three case studies for which punctuational explanations have been proposed. In the first, a model of punctuated evolution best accounted for changes in pygidial morphology within a lineage of the trilobite Flexicalymene, but the uniform model of an unbiased random walk remains a plausible alternative. Body size evolution in the radiolarian Pseudocubus vema was neither purely gradual nor completely pulsed. Instead, the best-supported explanation posited a single, pulsed increase, followed later by a shift to an unbiased random walk. Finally, for the much-analyzed claim of “punctuated gradualism“ in the foraminifera Globorotalia, the best-supported model implied two periods of stasis separated by a period of elevated but not inherently directional evolution. Although the conclusions supported by these analyses generally refined rather than overturned previous views, the present approach differs from those prior in that all competing interpretations were formalized into explicit statistical models, allowing their relative support to be unambiguously compared.

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.

Using a geometric model of shell morphology, it is demonstrated that biconvex brachiopods occupy only a small region of the potential geometric space available to organisms with planispiral exoskeletons composed of two articulated valves. Measurements taken for a sample of 324 genera of the articulate orders Pentamerida, Rhynchonellida, Spiriferida, and Terebratulida were analyzed using a simple geometric model of shell form and ontogeny. The frequency distribution of brachiopod shell morphologies exhibited by the four orders represents the biological optimization of the spatial relationships between area and volume. Biconvex brachiopods develop shells which are designed to minimize shell surface area while maximizing internal shell volume. The means by which optimization is achieved is related directly to the effects of increase in absolute size during ontogeny. The boundaries upon shell geometries utilizable by biconvex brachiopods are determined by (1) limitations of articulation, and (2) limitations of surface and volume.

Mesozoic marine ecosystems were dominated by several clades of reptiles, including sauropterygians, ichthyosaurs, crocodylomorphs, turtles, and mosasaurs, that repeatedly invaded ocean ecosystems. Previous research has shown that marine reptiles achieved great taxonomic diversity in the Middle Triassic, as they broadly diversified into many feeding modes in the aftermath of the Permo-Triassic mass extinction, but it is not known whether this initial phase of evolution was exceptional in the context of the entire Mesozoic. Here, we use a broad array of disparity, morphospace, and comparative phylogenetic analyses to test this. Metrics of ecomorphology, including functional disparity in the jaws and dentition and skull-size diversity, show that the Middle to early Late Triassic represented a time of pronounced phenotypic diversification in marine reptile evolution. Following the Late Triassic extinctions, diversity recovered, but disparity did not, and it took over 100 Myr for comparable variation to recover in the Campanian and Maastrichtian. Jurassic marine reptiles generally failed to radiate into vacated functional roles. The signatures of adaptive radiation are not seen in all marine reptile groups. Clades that diversified during the Triassic biotic recovery, the sauropterygians and ichthyosauromorphs, do show early diversifications, early high disparity, and early burst, while less support for these models is found in thalattosuchian crocodylomorphs and mosasaurs. Overall, the Triassic represented a special interval in marine reptile evolution, as a number of groups radiated into new adaptive zones.

The genus Homo evolved its pronounced encephalization through postnatal extension of the high rate of brain growth that characterizes all primates in utero. Linked to this extension was delayed development, which represented an enormous ecological sacrifice because it produced the longest postnatal interval of physical helplessness in the Mammalia and forced mothers to carry infants.

Graphs relating brain growth to body growth indicate a pongid pattern of development for gracile australopithecines, implying that infants could cling to mothers whose forelimbs were occupied with climbing. Also present were several postcranial traits that would have made the adults more adept climbers than modern humans. Habitual use of these inherited traits is suggested by the fact that evolution failed to eliminate certain ones, such as short legs and long pedal phalanges, that impeded terrestrial locomotion. Moreover, the intensity of predation by large, swift, social carnivores must have compelled australopithecines to use trees as refuges, in the manner of chimpanzees and baboons; australopithecines probably also gathered some of their food in trees. Gracile australopithecines failed to expand their brain size, experiencing general evolutionary stasis for more than 1.5 m.y. I propose that this stability resulted from these animals' semiarboreal mode of life: First, their postcranial morphology remained compromised by selection pressures to maintain both terrestrial and arboreal adaptations. Second, by requiring that neonates be mature enough to cling to mothers, obligate arboreal activity precluded encephalization of the kind that characterizes Homo; this evolutionary constraint has previously been overlooked.

In contrast to australopithecines, early Homo approached H. erectus in pelvic configuration and brain size. A new brain-body growth curve for early Homo indicates extension of the fetal pattern well into the postnatal interval, implying that neonates were highly immature so that adults had to be fully terrestrial. Homo evolved shortly after the onset of the modern ice age about 2.5 Ma. Fossil pollen and carbon isotopes in paleosols record a contraction of forests in Africa at this time. I propose that this represented a crisis that led to the evolution of Homo by compelling some australopithecine populations to adopt a fully terrestrial existence. Although ecologically difficult, this behavioral restriction finally made possible encephalization through the evolution of delayed development. During the ecological crisis, a large brain evolved in at least one population of gracile australopithecines. An advanced tool industry and cunning behavior were of such great adaptive value for avoiding predators and expanding food resources on the ground that selection for encephalization soon overrode the problems imposed by helpless infants.

The fossil record of antelopes and micromammals provides a test of the idea that environmental forcing opened the way for the evolution of Homo: both of these groups experienced heavy extinction of forest-adapted species about 2.5–2.4 Ma and a rapid proliferation of species adapted to unforested habitats. The transformation of the hominid clade during Plio-Pleistocene time did not follow a simple pattern. Homo may have arisen either by anagenetic transformation of a “bottlenecked” species or by speciation, and it may not have evolved immediately with the onset of climatic change. Furthermore, just as a few forest-adapted antelope species survived the biotic crisis, a small-brained gracile taxon with arboreal adaptations may have persisted to the start of the Pleistocene. Robust australopithecines survived into the Pleistocene, perhaps because a broad vegetarian diet reduced their need to migrate frequently between home bases. With their extinction in mid-Pleistocene time, about the time that savannahs became widespread, only Homo remained.

(1) Hydrodynamic principles and experiments with empty shells predict that pressure distributions around brachiopod shells generated by ambient currents should, depending on the orientation of the shell relative to the current, either augment or oppose the ciliary-driven flow of water through the lophophore. For living articulate brachiopods with plectolophes or spirolophes, orientations where the anterior-posterior axis of the shell is parallel to the current direction should result in pressure distributions which oppose active pumping. This effect should be strongest when the excurrent region of the shell faces into the current. Orientations where the anterior-posterior axis is perpendicular to the current direction should result in pressure distributions which act in concert with active pumping, most strongly when one of the incurrent regions is directed into the current. These effects are independent of specific shell shape.

(2) Laqueus californianus and Terebratulina unguicula actively reorient to currents in the laboratory, preferring orientations where the anterior-posterior axis of the shell is perpendicular to the current and the right-left axis is parallel to the current. Both species may traverse an arc as great as 120° to achieve their final orientation. Hemithyris psittacea also will actively reorient to currents, moving towards orientations where the anterior-posterior axis is perpendicular to the current. The maximum rotation observed for H. psittacea was 45°. Terebratalia transversa never reoriented in the laboratory.

(3) Using epifaunal hydroid colonies as indicators of current direction, both Hemithyris psittacea and Terebratalia transversa are oriented in nature with the anterior-posterior axis of the shell perpendicular to the prevailing currents. While scuba diving, I confirmed this orientation phenomenon for T. transversa by direct measurement of the orientation of the brachiopods relative to prevailing currents.

(4) Larval Terebratalia transversa avoid areas with current speeds greater than about 0.2 cm/s during metamorphosis and show no orientation to the ambient currents immediately after metamorphosis. Post-metamorphic T. transversa can actively reorient on the pedicle. The orientation observed in adults is probably achieved by active reorientation to local currents of post-zygolophe juveniles.

(5) Threshold current speeds for reorientation in Laqueus californianus and Terebratulina unguicula are low and approximately equal to the excurrent pumping speeds of each species; dynamic pressure rather than viscous entrainment is probably the relevant factor determining reorientation behavior.

Physical models of six species of the Mesozoic bivalve genera Gryphaea, Exogyra, and Ilymatogyra and hollow hemispheres of comparable size were manufactured, the former from molds of fossil specimens. The survival (maintenance of a position within 1 cm of the sediment-water interface) of populations of these models on 3 sediment types in the lower intertidal of the Gulf of Mexico was monitored over an 18 mo period. Bulk sediment density had no detectable influence on the mean mass density (g cm−3) of the population of models which maintained position at the sediment-water interface. Increased intensity of sediment reworking strongly decreased the probability of survival of the models. Model shells took up orientations consistent with previous laboratory studies and observations of fossils preserved in situ. Some of the models were attacked and broken open by blue crabs and stone crabs, and crab predation was an important source of “mortality” of the models. These results are consistent with the hypothesis that increased sediment reworking and the evolution of durophagous predators in the Cretaceous caused the extinction of exogyrid and gryphaeid bivalves and the near-total elimination of the reclining mode of life in post-Cretaceous times.