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.

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.

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.

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.

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.

Using queueing theory and the fast Fourier transform, we develop a new approach to analyzing the diversity paths of fossil taxa. The methods employed focus on calculating the probabilities, for each taxonomic group, of arriving at particular values of the diversity at particular times, assuming that diversity paths are random walks. These probabilities, which are represented in probability matrices, allow us to establish a number of null hypotheses that can be used to test for randomness in the diversity paths of the taxonomic groups themselves. As the model employed here is conceptually similar to that employed by Gilinsky and Bambach (1986) in bootstrapping, one consequence of the approach is, in effect, an analytic solution of the bootstrap. A comparison of bootstrapping and analytic results in 15 taxa of marine invertebrates gives nearly identical results. The analytic approach is more general, however, and allows for the calculation of probability matrices under other model assumptions not treated here. We have also applied the probability matrices to a series of tests for nonrandomness as a whole, in contrast to the more usual method of testing particular clade statistics. The familial diversity paths of all 36 extinct orders of marine invertebrates with ten or more families were analyzed. In no case was the null hypothesis of randomness rejected at the 0.05 level. Given the model, and assuming that extinct taxa are representative of all taxa, the familial diversity paths of marine orders appear to behave as random walks.