We have calculated rates of evolution for 19 lineages of Neogene bivalves over time intervals ranging from 1 ma to 17 ma. Our morphometric comparisons are based on 24 variables, for which we have made more than 43,000 individual measurements normalized for shell size. We initially assessed evolutionary changes in shape for 19 early Pliocene (4 ma old) species of bivalves by comparing these forms to their closest living relatives, which in 12 cases have traditonally been assigned to the same species. To make our study unbiased and comprehensive, we included all species that met certain predetermined guidelines and that offered suitable fossil material for measurement. We compared early Pliocene and Recent populations using (1) all 24 variables treated separately, (2) multivariate distance (Mahalanobis' D), based on the full set of variables, and (3) eigenshapes for shell outlines. For these comparisons, we used as a yardstick the same measures of morphologic distance applied to pairs of geographically separated Recent populations that belong to eight of the living species to which the fossil populations were compared. As it turns out, with minor exceptions, the distribution of morphologic distances between 4 ma old and Recent populations resembled the distribution of distances between conspecific Recent populations.

We calculated net rates of evolution separating pairs of populations that belong to single lineages. For all intervals of time, the distribution of differences between population means for individual variables is remarkably similar to a comparable distribution representing the comparison of pairs of conspecific Recent populations from separate geographic regions. Because morphologic differences between populations do not vary greatly with evolutionary time, measured “rates” of evolution, on the average, decrease with interval of measurement. Because these differences resemble intraspecific variability, however, the rates do not represent significant evolution. Evolution has followed a weak zigzag course, yielding only trivial net trends.

The weak and reversible “trends” that we measured yield net rates averaging less than 10 millidarwins, which is much lower than most rates previously reported for marine invertebrates (average ~200 millidarwins for a 1 ma interval and ~60 millidarwins for a 10 ma interval). We attribute this disparity (1) to the fact that most previously published rates have been calculated when a significant amount of evolution was recognized in advance (often for a poorly documented lineage) and (2) to the fact that most measured variables have represented nothing more than some measure of body size. We conclude that shape, as opposed to size, has been highly stable in bivalve evolution over millions of years and 106–107 generations. We conclude that to characterize rates or evolution for any group of organisms, one must employ a large, unbiased sample of measurements for numerous well-documented lineages, and one must segregate data depicting shape from data depicting size.

This study addresses the question of evolutionary tempo and mode in a sequence of Upper Cretaceous bivalves in the genus Pleuriocardia from the Western Interior Basin of North America. Change between species was probably phyletic (without persistence of ancestors). There is some evidence for weak gradual change within the lineage, but most important change is concentrated in short intervals of time. Detailed examination of the differences among samples reveals pronounced geographic variation, whereas temporal variation within localities is generally minor. The relatively rapid episodes of change fit the model of punctuated equilibrium, but the phyletic nature of species-level change does not.

The value of the debate about the punctuated and gradualistic models has been to force a more criticial examination of fossil sequences, but these sequences should not be forced into narrowly defined categories. A variety of evolutionary patterns may exist in the fossil record, and it is this variation in pattern that will inform us of the underlying processes.

A diversity and turnover analysis has been undertaken for a number of invertebrate groups in the Liassic of northwest Europe. There is a more or less steady rise in diversity from the early Hettangian through to the Pliensbachian, followed by a marked decline into the early Toarcian, after which it tends once more to increase. Ammonites stand out from the other invertebrates as having had an exceptionally high rate of turnover, with very short species durations.

Increase of neritic habitat area due to rise of sea level, and recolonization following the end-Triassic mass extinction event appear to be the promoters of diversity increase or radiation. Severe reductions of neritic habitat area with associated environmental deterioration, related either to episodic marine regressions or spreads of anoxic bottom waters, and bound up respectively with sea-level fall and rise, are seen as the prime factors responsible for increase of extinction rate. While the environmentally sensitive ammonites were affected by even minor regressions, the other, more eurytopic groups were evidently more resistant to these. The only event that warrants the term mass extinction, affecting nearly all the benthos and nekton but not the plankton, correlates precisely with the early Toarcian anoxic event. Several episodes can be recognized of migrations of organisms into Europe following extinctions.

Species duration data for living benthic foraminifera derived from an extensive literature search have been compiled and analyzed to investigate rates and patterns of species origination. The same data subjected to taxonomic standardization through examination of many specimens lodged in museum collections indicate strikingly different, and more realistic, rates and patterns.

Evolutionary generalizations based on data generated from the literature only are often unreliable and may be directly in opposition to reality. Extensive attempts at taxonomic standardization should be the norm in paleobiological investigations.

The multibrachiate grade of organization in camerate crinoids was principally attained through pinnule differentiation: the accelerated development of a proximal pinnule that ultimately became a pinnule-bearing arm. Although pinnule differentiation is not the only means of achieving a multibrachiate organization in modern crinoids, it was the primary mode in producing both dichotomous and exotomous branching patterns in multibrachiate camerates. Whereas most of the proximal branchings, commonly fixed in the cup of camerates, were due to pinnule differentiaton, the first dichotomy and distal dichotomies above the cup apparently developed from direct modification of a brachial plate at the growing tip of the immature crinoid arm.

The frequency of sublethal damage was examined in two ophiuroid species, Palaeocoma milleri and ?Ophioderma tenuibrachiata (Ophiodermatidae), from a shallow-water Jurassic “starfish bed.” None of 60 specimens was regenerating arms, a result that agrees with four previous studies of Paleozoic and early Mesozoic ophiuroids. By contrast, 66.1 percent of a living Ophioderma brevispinum population from Belize were regenerating one or more arms. For living populations of Ophiothrix oerstedi, the natural injury frequency was high at sites where the field mortality of tethered individuals was high. All predators that have been observed feeding on ophiuroids cause sublethal injuries, which would appear as regenerating arms in the fossil record. These results support the hypothesis that predation pressure on ancient ophiuroid populations was low and increased after the Jurassic.

A computer model of background and mass extinctions in a taxonomic hierarchy has been used to study the effects of different extinction patterns in a search for clues as to the causes of actual extinction events. Model taxa at four levels were built up from speciation events in adaptive space according to rules of origination which seem plausible biologically. The frequency distribution of species among the three higher taxonomic levels in the model is similar to that in living marine taxa which have good fossil records. Three mass extinction patterns were imposed on the model after species diversity had attained equilibrium (i.e., when speciation = background extinction): random; bloc (contiguous niches were cleared); and clade (all members of selected higher taxa were removed). Effects on the taxonomic profile varied with pattern. Four of the five historical mass extinctions resemble the effects of the random pattern. End-Permian families were harder hit than those in the random model, but this may be a result of an extremely high species extinction level. It is concluded that the effect of extinctions on the taxonomic hierarchy provides a tool to help in understanding extinction causes.

Primary vascular architecture of members of the Paleozoic Aneurophytales (Progymnosper-mopsida) is described. This architecture is somewhat more complex but fundamentally similar (homologous) to that of members of the Trimerophytina, putative ancestors of aneurophytes. It is suggested that the presence of complex stelar morphology in aneurophytes was epiphenomenal, a passive result of changes in growth and development in a trimerophyte-like ancestor. Specifically, I suggest that the evolutionary transformation in primary vascular architecture from haplostele to ribbed protostele was a direct consequence of changes that affected the vertical spacing and degree of organization of lateral appendages in early vascular plants. This view is in sharp contrast to adaptationist explanations of change in stelar morphology expressed by other authors and provides an example of non-adaptive change in evolution.

Deciduousness in mesic, broad-leaved plants occurred in disturbed, middle-latitude environments during the Late Cretaceous. Only in polar environments in the Late Cretaceous was the deciduous element dominant, although of low diversity. The terminal Cretaceous event resulted in wide-spread selection for plants of deciduous habit and diversification of deciduous taxa, thus leaving a lasting imprint on Northern Hemisphere vegetation. Various environmental factors have played important roles in subsequent diversification of mesic, broad-leaved deciduous taxa and in origination and decline of broad-leaved deciduous forests. Low diversity and rarity of mesic deciduous plants in the post-Cretaceous of the Southern Hemisphere indicate that the inferred “impact winter” of the terminal Cretaceous event had little effect on Southern Hemisphere vegetation and climate.

Six drainage basins at the margins of Trinity Lake were analyzed to determine the relations of source vegetation (largely coniferous forest) to plant debris in deltaic deposits that represent high-energy depositional environments. Thirty-one samples containing an estimated 1,043,000 identifiable plant fragments were subjected to a multivariate statistical (correspondence) analysis; in the resulting ordination, samples from the more mesic side of the lake clustered separately from samples from the drier side of the lake. The distribution of a species in the source vegetation is generally paralleled by the distribution of the same species in the samples, and all major forest trees are present in the samples. However, relative abundances in the source vegetation have no direct relation to relative abundances in the samples. Plants growing far away from depositional sites are poorly represented, even if these plants produce organs suited to long-distance transport; this occurs by dilution by material derived from plants in proximity to depositional sites and suggests that megafossil assemblages that contain putative mixtures of high-altitude “temperate” and low-altitude “thermophilic” taxa represent true biotic associations. The information gained from the Trinity analysis is compared to information gained from analysis of plant debris in low-energy depositional environments; each environment contains different kinds of information that is significant in paleoecological reconstructions of regional vegetation. Whereas high-energy environments contain the best taxonomic representation of source vegetation, low-energy environments retain information on spatial distributions of taxa within source vegetation. High-energy environments typically also contain the best representation of different organs of a given taxon.

The study of tracks is often regarded as a fringe subdiscipline outside the scientific mainstream. Papers dealing with footprints traditionally appear either in popular magazines like Natural History (Bird 1939, 1944; Brown 1938), or in very obscure publications (Sarjeant 1974). Usually it is only the spectacular discoveries, which are directly applicable to topical debates, that rouse much scientific interest. For example, the discovery of Pliocene hominid tracks (Leakey and Hay 1979) provided unequivocal evidence for the antiquity of bipedalism (cf., Napier 1967). Similarly the tracks of a running theropod allowed for direct estimates of the speed attained by dinosaurs (Farlow 1981), and the tracks of a herd of running theropods fueled debate about gregariousness and stampede behavior (Thulborn and Wade 1984).