When asked how he might sum up his long life, an aged Benjamin Franklin replied that he had, at least, been useful to his fellows. Paleobiology is still young, as our journal has just completed 20 years and now begins this season of its majority, while the vibrancy of our subject certainly proclaims a vigorous incipiency. But academic generations are short, a mere 5 years or so, and Paleobiology has therefore enjoyed adequate time to demand judgment by accomplished results, not simple promise. When, fifteen years ago (6, 1, 80), I wrote a similar piece to celebrate the completion of this journal's first five volumes, I spoke in my title about “the promise of paleobiology”; now I must refer to “the task of paleobiology” and delineate what has been accomplished, and how much we have yet to do.

More than 5000 measurements were taken on over 1000 specimens of two species of brachiopods, Mediospirifer audaculus and Athyris spiriferoides, from the Middle Devonian Hamilton Group of New York state. Statistical analyses were performed on these data, with specimens partitioned by their occurrence in one of many paleoenvironments and stratigraphic horizons. Neither species showed substantial morphological departures between first appearance and extinction (the range of the Hamilton Group, roughly 5 m.y.). However, oscillations in morphology were discovered in both taxa.

For the two species we studied, groups of organisms occurring in a single paleoenvironment undergo moderate morphological change through time; however, the net sum of changes through time in all paleoenvironments in which these species occur is essentially zero. Therefore, stasis may be partly a property of the organization of species into different environmental populations. Different “environmental populations” may evolve, but they will typically do so in several different “directions,” generally producing no net change. The difference between the morphology of species in different environments over the whole interval of the Hamilton Group is also nil, thereby ruling out any major role that ecophenotypic effects could play in the patterns recognized herein.

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.

We analyze a new compilation of Neogene to Recent (22-0 Ma) Caribbean coral occurrences to determine how ecological and life history traits at the population level affect long-term evolutionary patterns. The compilation consists of occurrences of 175 species and 49 genera in one continuous (> 5 m.y.) sequence and 22 scattered sites across the Caribbean region. Previous study of evolutionary rates using these data has shown that both extinction and origination were accelerated between 4 and 1 Ma, resulting in large-scale faunal turnover. Categories for three morphological and two reproductive variables (colony size, colony shape, and corallite size; and sex, and mode of embryonic development; respectively) are assigned to each species in the compilation. Comparisons of the ecological variables with evolutionary rates using randomization procedures and modified analysis of variance show that only colony size was strongly related to rates of extinction and origination during either normal background times or times of accelerated extinction. Extinction rates were lower in species with large colonies, because species with small massive colonies tend to live in small, short-lived populations with highly fluctuating recruitment rates. During turnover, extinction rates increased disproportionately in species with small colonies. Origination rates are found to be less related to ecological variables, although species with small massive colonies originated at higher rates prior to turnover.

Accelerated turnover may have therefore involved an increase in local population extinction rates that caused increased rates of both species extinction and origination across the entire fauna. Since extinction rates accelerated disproportionately with respect to colony size, the overall result was a relative increase in species with large colonies. After severe disturbance, one might expect that populations of species with large colonies and high rates of fragmentation would be more likely to escape extinction, because of larger population sizes, longer generation times, and more constant rates of population increase. The modern Caribbean reef-coral fauna is therefore structured by large, long-lived colonies that are robust to regional environmental change. Many of the very taxa that allowed reef communities to escape collapse in the past are declining today in response to anthropogenic disturbances, suggesting that Caribbean reef communities may be less resilient in the future in response to ongoing environmental perturbations.

In phytogeographic data sets, the number of assemblages or floras from each interval may provide a test of the influence of sampling intensity on land-plant diversity. Using a data set of Silurian and Devonian compression-impression plant genera from Laurussia and the Acadian terrain, regression of five measures of land-plant diversity (total diversity, mean genus richness of floras, median assemblage diversity, most diverse assemblage, and standing diversity at interval boundaries) against the number assemblages or floras from thirteen intervals suggests that sampling bias influences all of the diversity measures to some extent, including within-habitat measures. The standing diversity of land plants at interval boundaries, the measure least influenced by sampling (r = 0.65, p = 0.05), increased steadily from the Middle Silurian to the late Givetian/early–middle Frasnian boundary, fell sharply in the early–middle Frasnian and remained low throughout the late Frasnian–middle Famennian. Standing diversity rose dramatically in the late Famennian and Strunian (latest Devonian): the Frasnian–Famennian extinction event may have affected land plants. The standing diversity of Silurian and Devonian microspore genera at interval boundaries mirrors that of compression-impression genera: neither record supports a land-plant diversity equilibrium during the Devonian.

In several increasingly realistic steps, a model of the stratigraphic distribution of fossils is presented. The first and simplest step assumes that if a taxon was extant it will have been preserved. The second step admits that if a taxon was extant, there is some probability less than one that it will have been preserved. This step produces randomly distributed gaps, and fossil ranges that are somewhat shorter than the span of time in which a taxon actually lived. The third step assumes facies-controlled taxa and parasequence-style cyclicity. This produces randomly and nonrandomly distributed gaps, including the anomalously long gaps recognized in confidence limit studies. The final model incorporates depositional sequences and indicates that first and last occurrences will cluster at sequence boundaries and at flooding surfaces in the transgressive systems tract. Across-shelf gradients in diversity, taphonomy, or eurytopy can control the strength, but not the stratigraphic position of these peaks. Comparison of the model to data from the Upper Ordovician suggests that these modeled features are present in the fossil record. Many previously studied paleobiologic patterns may be, at least in part, an artifact of facies control and sequence architecture. Such patterns include gradual or stepwise mass extinction, punctuated morphologic and taxonomic change, iterative evolution, and the replacement of shallow water faunas by deeper water faunas at biomere boundaries.

The mode of locomotion of any swimming animal is constrained by its size, architecture, and phylogenetic history. Considering these factors and the range of locomotory systems used by extant zooplankton, the range of possible modes of locomotion for graptoloids can be effectively limited. Three assumptions have been made: (1) graptoloids did not use a mode of locomotion unknown among modern organisms; (2) all graptoloids employed essentially the same mode of locomotion except, possibly, in their early growth stages; and (3) graptoloids did not rely entirely on passive buoyancy—no extant zooplankton groups in the size range of the graptoloids do. Structures that increase buoyancy or drag are often found in actively swimming zooplankton. They enhance feeding efficiency and reduce sinking rates during nonswimming periods.

The modes of locomotion utilized by extant zooplankton groups are ciliary propulsion, elongate body undulation, jet propulsion, rowing with skeletonized appendages, and rowing or undulation with muscular appendages. Of these, all but the last can be rejected for the graptoloids on the basis of scale or architecture. It is concluded that graptoloids probably used a rowing or undulatory motion with muscular appendages for swimming. Using a pterobranch model for the graptoloid zooids, the lophophore is considered an unlikely propulsive structure because the design requirements would conflict with those of a ciliarly suspension-feeding organ. Winglike, lateral extensions of the muscular cephalic shield, the same structure used for creeping locomotion in the benthic pterobranchs, is regarded as the most likely propulsive organ, analogous to the pteropod swimming wings.