Eocene lake beds of Horsefly, British Columbia, are preserved in varves, or discrete yearly layers representing seasonal changes in the lake. These varves allow study of temporal variation and rates of change in morphological and ecological characters on a very short time scale. One of the most sensitive indicators of the paleoenvironmental conditions on the floor of the lake may be the taphonomic condition of the fishes, which vary between perfectly articulated and completely disarticulated skeletons. Patterns of disarticulation correspond to those produced by scavengers. The taphonomy supports the hypothesis that the lake was warm monomictic, circulating in the winter, at which time scavengers could gain access to the bottom of the lake. Larger-scale environmental events (on the order of hundreds of years) are suggested by the fact that the proportion of well-preserved specimens reached two peaks within the seven centuries of deposition, one peak during the second century and another during the fifth and sixth centuries. These results clearly demonstrate two principles: that taphonomy can be a sensitive indicator of paleoenvironmental conditions, and that temporal averaging can affect the taphonomic properties of this fossil site, and presumably of others with equal or lower time resolution.

Analysis of the evolution of the Globorotalia (Fohsella) lineage of planktic foraminifera suggests that reproductive ecology and shell shape have evolved independently in this group. The silhouette of fohsellid shells displays a nearly unbroken anagenetic trend, yet isotopic data show that the fohsellids changed their depth of reproduction during the anagenetic evolution of their skeletons. Remarkably, there are no correlations between anagenesis in skeletal shape and the establishment of reproductive isolation. Apparently, anagenesis masks at least one speciation event that is apparent only in the isotopic evidence for a change in reproductive ecology. Although anagenetic trends have been widely cited as evidence for gradual speciation in planktic foraminifera and other microfossil groups, our data suggest that they should not always be considered to record either the tempo or mode of speciation.

Speciation was apparently uncoupled from morphological evolution in fohsellids because these evolutionary phenomena occurred in different phases of ontogeny. Gradual morphological changes were associated with the main phase of shell growth of both the ancestor and descendant species in the near-surface ocean. Reproductive isolation occurred when ancestral and descendant populations became established at different depths near the end of the life cycle. Morphological evolution may also be uncoupled from reproductive isolation in other organisms that experience very different selection pressures over the duration of their ontogenies, such as parasites with many hosts, species with multiple phases of metamorphosis, and organisms that broadcast their gametes.

The morphologic radiation of Early Jurassic ammonites following the near extinction at the end of the Triassic is analyzed from 436 species of 156 genera that form a representative sample of morphs occurring worldwide in the first three stages of the Jurassic (Hettangian, Sinemurian, Pliensbachian: 36 subzones, 24 m.y.). Morphologic diversity is analyzed independently of taxonomy by processing 18 shape parameters using multivariate analysis and clustering techniques. The morphospace thus defined indicates that morphs fall readily into two groups made up of four and five adjacent morpho-subsets. The temporal pattern of morphospace occupation in the 36 Lower Jurassic subzones displays diversification, depletion (sometimes total), and displacement of successive parts of the morphospace, reflecting a complex history in which morphologic radiation appears to be more than a process of diffusion. The history of the morphologic evolution is tentatively related to sea-level changes and there is a suggestion that morphologic diversity increases during second-order transgressive periods.

Phylogenetic patterns of trilobite clades were used to deduce biogeographic patterns during the Middle Devonian, a time of active plate collision between North America (Laurentia) and other plates, coincident with several major episodes of sea-level rise and fall. The mapping of biogeographic states onto phylogenies for asteropyginid and proetid trilobites indicated that during their history these trilobite clades often shifted the areas they occupied, and also underwent vicariant differentiation, followed by range expansion, followed by subsequent vicariance. Biogeographic patterns in these individual phylogenies were evaluated and synthesized using a modified version of Brooks Parsimony Analysis, which is discussed. This method makes it possible using cladistic methods to distinguish between episodes of vicariance and episodes of dispersal. Two types of dispersal are recognized herein: (1) the individualistic responses of certain taxa in a single clade that cannot be generalized, i.e., traditional ad hoc dispersal, and (2) those patterns of congruent range expansion that are replicated across several clades. The latter are not treated as true dispersal, expansion of a taxon's range over a barrier accompanied by diversification, but rather as a result of the temporary removal of barriers to marine taxa, due either to relative sea-level rise or to the collision of formerly disjunct plates. These are interpreted as changes in the structure of areas, and this type of dispersal is referred to as geo-dispersal. Geo-dispersal was found to have occurred in the Middle Devonian trilobite fauna of Eastern North America.

Biogeographic analysis indicated that Eastern North America is a strongly supported area, with the Appalachian and Michigan Basins as sister areas. Armorica and the Canadian Arctic are also sister areas. Congruence was found between area cladograms produced by vicariance and dispersal analyses for Middle Devonian trilobites, suggesting that in some cases the geological processes governing vicariance, such as sea-level changes, were the same as those that caused dispersal.

Statistical inference about the upper and lower endpoints of the stratigraphic ranges of fossil taxa can be based on the pattern of finds. Strauss and Sadler (1989) described a test and confidence interval for a common upper or lower endpoint in two or more taxa. This approach is conservative, in the sense that it provides only an upper bound on the significance level. This paper describes and illustrates a test and confidence interval for which the significance level is known.

We compare the preservation (taphonomic grade) and age of Chione (bivalve) and foraminifera from modern siliciclastic tidal flat sediments of Bahia la Choya, Sonora, Mexico (northern Gulf of California). Disarticulated shells of Chione collected from the sediment-water interface of Choya Bay exhibit a substantial range in taphonomic grade and age, several hundred years to ∼80–125 ka based on Accelerator Mass Spectrometer 14C dates and D-Alloisoleucine/L-Isoleucine values. There is not, however, a one-to-one correspondence between age and taphonomic alteration of Chione: old (or young) valves may be highly altered or they may be relatively pristine. In contrast to Chione, most foraminiferal tests at Choya Bay are quite pristine, which suggests a quite young age, but tests are surprisingly old (up to ∼2,000 calendar years based on Accelerator Mass Spectrometer 14C dates).

We suggest that following seasonal pulses in reproduction, some foraminiferal tests are rapidly incorporated into a subsurface shell layer by “Conveyor Belt” deposit feeders and preserved there, while the rest of the reproductive pulse rapidly dissolves. Ultimately, some of these buried tests, along with Chione, are transported back to the surface by biological activity and storms. The much greater range of taphonomic grades and ages among Chione shells suggests that they, unlike foraminifera, are sufficiently large and preservable (low surface/volume ratio and chemical reactivity) to undergo many cycles of degradation, burial, and exhumation before complete destruction. The age of foraminiferal tests indicates that time-averaging of microfossil assemblages at Choya Bay is much more insidious than would be expected considering the relatively pristine state of the tests alone.

Based on our studies, the lower limit of temporal resolution of shallow shelf microfossil assemblages appears to be ∼1000 years. We caution, however, that each depositional setting (taphofacies) should be evaluated on a case-by-case basis before gross generalizations are made. Indeed, the discrepancy between age and taphonomic grade of fossil assemblages at Choya Bay suggests that neither hardpart size or taphonomic grade are infallible indicators of test preservability or likely temporal resolution of the host assemblage, and that the dynamics of hardpart input and loss must also be evaluated.

Heterochrony, change in developmental rate and timing, is widely recognized as an agent of evolutionary change. Heterotopy, evolutionary change in spatial patterning of development, is less widely known or understood. Although Haeckel coined the term as a complement to heterochrony in 1866, few studies have detected heterotopy or even considered the possibility that it might play a role in morphological evolution. We here review the roles of heterochrony and heterotopy in evolution and discuss how they can be detected. Heterochrony is of interest in part because it can produce novelties constrained along ancestral ontogenies, and hence result in parallelism between ontogeny and phylogeny. Heterotopy can produce new morphologies along trajectories different from those that generated the forms of ancestors. We argue that the study of heterochrony has been bound to an analytical formalism that virtually precludes the recognition of heterotopy, so we provide a new framework for the construction of ontogenetic trajectories and illustrate their analysis in a phylogenetic context. The study of development of form needs tools that capture not only rates of development but the space in which the changes are manifest. The framework outlined here provides tools applicable to both. When appropriate tools are used and the necessary steps are taken, a more comprehensive interpretation of evolutionary change in development becomes possible. We suspect that there will be very few cases of change solely in developmental rate and timing or change solely in spatial patterning; most ontogenies evolve by changes of spatiotemporal pattern.

Study of the assemblage of encrusting organisms on co-occurring disarticulated valves of the bivalves Crassostrea virginica and Mercenaria mercenaria in Bogue Sound, North Carolina, indicates that there is little or no substrate specificity among the encrusting organisms but that the shape of the shells has an important influence on how extensively members of each higher taxon collectively inhabit the shells. Encrusting bryozoans, a dense low mat composed of many species from diverse phyla, and a unicellular film cover most of the area of both exterior and interior surfaces. The encrusting bryozoans most extensively cover both surfaces of C. virginica but are in second place behind the multispecies mat on exterior surfaces of M. mercenaria and behind the unicellular film on its interior surfaces. These differences are inferred to result from different physical stability of valves of the two bivalve species, which exhibit different frequencies of circumrotatory growth.

Degradation of the assemblage by sodium hypochlorite, to simulate loss of organic matter during fossilization, results in the complete loss of encrusting sponges, erect hydrozoans, erect bryozoans, and ascidians. Loss of these taxa results in overexposure and more apparently uniform distribution of skeletal taxa with respect to their surface representation in living assemblages and also in complete loss of the higher tiers present in the living assemblage. However, indications of the original structural organization of the living assemblage is indicated by preservation of the most abundant taxa in the lower tiers and by the retention in the reduced treated assemblage of the patterns of distribution that characterized the living assemblage.

I examine the late Pleistocene megafaunal extinctions by testing the only extinction model with strong a priori predictions, the blitzkrieg model (Martin 1973; Mosimann and Martin 1975). I first test an assumption of the blitzkrieg and other extinction models that the megafaunal extinctions occurred in the terminal Wisconsin (12-10 Ka). This assumption has been disputed by Grayson (1989, 1991), but I find that both a reanalysis of Grayson's data and an analysis of new reliable data support a terminal Wisconsin extinction.

The blitzkrieg model predicts that the ranges of megafauna in North America were constricted as the semicircular front of hunters moved southeastward; hence the extinctions should be time-transgressive from northwest to southeast. I test this prediction in three separate analyses that examine (1) the location of terminal sites for each taxon relative to all their other late Wisconsin fossil sites, (2) the location of terminal sites for each taxon relative to all their other reliably dated late Wisconsin fossil sites, and (3) the spatio-temporal pattern of all the reliably dated terminal Wisconsin sites without regard to taxonomy. The geographic distribution of the megafaunal remains does not support the blitzkrieg hypothesis in any of the three analyses. Moreover, all of the patterns in the data are in a direction opposite to that predicted by the blitzkrieg hypothesis. I examine how these conclusions affect both climatic and predation models, particularly in relation to the testability of other extinction hypotheses.

During land plant evolution, a change in stelar architecture, i.e., in the geometric arrangement of the water-conducting tissue inside the plant axis, can be observed. In the most primitive stele type, the protostele, the vascular tissue is organized as a simple central strand. Further evolutionary changes led to more peripherally arranged vascular tissues. In the siphonostele, for example, the vascular tissue forms a hollow cylinder filled with pith. A functional explanation of this early stelar evolution is provided in the present paper. Using a numerical simulation approach, we analyze the water transport properties of various protostelic and siphonostelic model axes. The results indicate that several geometric parameters are relevant for understanding the water transport properties of various stele types and for explaining the early stelar evolution: the parenchymatic path lengths (i.e., the distance between the xylem surface and the transpiring plant surface), the ratio of xylem surface over transpiring surface, and the ratio of cross-sectional area of xylem to cross-sectional area of the parenchyma outside of the xylem. As a whole, the evolution of early stele types may be viewed as a size-related multi-criteria optimization process in which the xylem volume as well as the fluid pressure gradients in the parenchyma and in the xylem are minimized. For slender plant axes, a protostele appears to be the optimal stelar architecture. In wider plant axes, however, other stelar architectures (such as a siphonostele) prove to be more efficient than a protostele.

Size and shape are analyzed for Pliocene lineages of the rodent genus Stephanomys Schaub 1938. Previous phylogenetic studies were based mainly on size variation and descriptive comparisons, without any attempt to quantify shape changes. Hence, on the basis of regular size increase, Stephanomys has been considered a prime example of phyletic gradualism. In order to quantify morphological variation within the lineage, a method for analyzing complex outlines, the elliptic Fourier transform, was applied to tooth contour (upper and lower first molars). It was then possible to compare evolution in size, estimated by tooth area, as well as evolution of shape, represented by Fourier coefficients.

While size seems to change gradually through time, morphology gives a rather discontinuous evolutionary pattern for both the upper and lower molar. Such a discrepancy between the evolution of size and shape of a single structure suggests that different genetic determinisms and mechanical constraints may act on size and shape. Hence it may be misleading to infer generalized evolutionary processes from either size or shape alone.

It has been suggested that a sudden event at the end of the Cretaceous period caused a major extinction that was felt disproportionately by creatures in the water column. It also has been argued that benthic deposit feeders, being relatively independent of abundance of organic particles in the water column, should have survived the crisis more readily than suspension feeders, which depended more upon feeding upon phytoplankton. I argue that the hypothesis of relative immunity of deposit feeders is insufficient, because deposit feeders by and large depend upon a supply of organic matter from the water column and would have succumbed to food shortage nearly as rapidly as suspension feeders, possibly within a maximum of three to six months. This near simultaneity of extinction would have been especially true of continental shelf environments. Even in some parts of the deep sea, it is likely that a dependence upon the water column above might have caused deep-sea deposit feeders to succumb rapidly. Therefore, deposit feeders would not necessarily have outsurvived suspension feeders during a crisis of depleted water-column phytoplankton, increased shading by inert particles, or poisoning of the water column and killing of phytoplankton. The relatively lower rate of extinction of nuculoid bivalves may relate instead to their presence in deeper-water refuge habitats, their apparent relative ability to diversify in higher latitudes, or their resistance to factors other than food shortage.

The evolutionary origin of Morozovella angulata from its immediate ancestor, Praemurica uncinata, is preserved in Paleocene sediments from the Gulf of Mexico. This event represents the beginning of the morozovellid radiation and marks the first appearance of keeled planktonic foraminifera after the Cretaceous/Tertiary extinction. Parallel biometric and isotopic analyses were performed on size-segregated specimens from a succession of stratigraphic horizons. The biometric data reveal a temporal pattern of variation consistent with paedomorphosis. The appearance of angulose juvenile chambers in the otherwise rounded ancestral form (Praemurica uncinata) results in an allometry that becomes more pronounced upsection. At the origin of M. angulata, the juvenile morphology of the ancestor is retained throughout the entire ontogeny. Isotopic analysis of this sequence reveals the gradual acquisition of an increasingly heavy adult δ13C signal relative to that of the juvenile, while the δ18O data display no temporal or size-related trends. The temporal increase seen in the slope of the δ13C/size relationship may reflect the evolution of an increased dependency on photosymbionts.

Faunal abundance and fossil size and shape data from microvertebrate localities in the Upper Cretaceous Judith River Formation of south-central Montana illustrate that even when located in similar sedimentary facies, concentrations of vertebrate microfossils may exhibit strikingly different taphonomic profiles. Degrees of microfossil size and shape sorting may vary even among sites from the same sedimentary facies. In some instances, such variations may make it impossible to disregard taphonomic causes for differences sites exhibit in paleofaunal abundances. To limit the possibility that taphonomically generated faunal differences might be mistaken for true differences in paleoecology, comparisons of paleofaunal abundances should be restricted to microvertebrate sites both from similar sedimentary facies and with similar profiles of fossil sizes and shapes.

Oxygen and carbon isotopic analyses have been conducted on Miocene to Pliocene (6.0 to 2.9 Ma) members of the gradually evolving, deep-dwelling planktonic foraminiferal clade, Globoconella, in temperate waters of the southwest Pacific, Deep Sea Drilling Project (DSDP) Site 593. In the late Miocene the clade began with Globoconella conoidea, and continued through G. conomiozea, G. sphericomiozea, and G. puncticulata to the extant form G. inflata. Isotopic analyses were performed on ancestor-descendant species within the clade to determine if isotopic differences exist between these species which would, in turn, suggest depth and/or seasonal habitat differences and perhaps segregation, as well as ecological changes in the clade. Isotopic analyses were also conducted on the relatively shallow-dwelling planktonic foraminifer Orbulina universa and the benthic form Cibicidoides wuellerstorfi to determine if any relationships exist between the evolution of Globoconella and paleoceanographic/paleoclimatic change.

Small (usually 0.1–0.15 ‰ but up to 0.3 ‰) oxygen isotopic differences exist between ancestor and descendant forms that we believe represent small (∼1°) temperature differences. These temperature differences are inferred to indicate depth and/or seasonal habitat differences and possible segregation between the species during the gradual evolution. The largest differences in oxygen isotopic values occur between ancestor and descendant forms during the most conspicuous morphological transition within the clade near the Miocene/Pliocene boundary. During this interval, the clade underwent a transformation from conical to spherical forms and there was a loss of the keel. No consistent differences were observed between ancestor and descendant carbon isotopic values.

Both morphological and ecological evolution appear to have been associated with paleoceanographic/paleoclimatic changes. Intervals marked by warming of surface-to-upper intermediate waters are associated with evolution of forms with a spherical test and inferred adaptation to cooler waters relative to ancestral forms. We propose two alternative models for the evolution of the Globoconella clade. In the first model, we assume that depth and/or seasonal segregation between ancestor and descendant forms provided a partial barrier to gene flow and that evolution resulted from genetic drift and/or differential selective pressures acting on each morphotype. In the second model, we assume that coeval members of Globoconella formed a vertical and/or seasonal morphocline in the water column and that segregation did not provide an effective barrier to gene flow. Evolution proceeded as directional selection acted on morphotypes of Globoconella inhabiting selectively advantageous positions in the water column.

The macroevolutionary processes underlying large-scale biotic replacements are still poorly understood. Opinion remains divided regarding the roles of mass extinction, biotic interaction, and environmental perturbations in these replacement events. Previous attempts to test replacement hypotheses have largely focused on taxonomic diversity patterns. Taxonomic data alone, however, provide little insight about ecological interactions and hence other approaches are needed to understand mechanics of biotic replacements. Here I propose a conceptual model of replacement based on predation-mediated biotic interactions, and attempt a test using analysis of the Cenozoic replacement of the gastropod family Aporrhaidae by a closely related group, the Strombidae.

Taxonomic, morphologic, and geographic data analyzed in this study all suggest a replacement of aporrhaids by strombids following the end-Cretaceous mass extinction. While most of the taxonomic replacement was associated with a mass extinction, some replacement also occurred during background times and was mediated by higher origination rates in strombids rather than by higher extinction rates in aporrhaids. Morphologically, the replacement was largely confined to the portion of the morphospace unaffected by the end-Cretaceous extinction. At a global scale, the geographic overlap between the two groups declined through the Cenozoic, reflecting increasing restriction of aporrhaids to colder, temperate waters while strombids flourished in the tropics. However, at a finer geographic scale a more mosaic pattern of replacement is evident and coincides with Eocene and Oligocene climatic fluctuations.

The results of this study suggest that mass extinction, long-term biotic interaction, and environmental change can all play significant roles in biotic replacements. Since the relative importance of each factor would vary from one event to another, an understanding of the general nature of large-scale biotic replacements requires a knowledge of the relative intensities of each of these processes.