During the latest Cretaceous and the Paleocene in western North America, disappearance rates for mammalian genera track appearance rates, both reaching their peak in the early Paleocene (Puercan) following the extinction of non-avian dinosaurs. Some of the disappearances during this time were pseudoextinctions that resulted when ancestral species disappeared during speciation.

Species-level cladistic analyses and a well-constrained biostratigraphic framework are required to study this form of pseudoextinction. Cladistic analyses show that monophyly cannot be established or rejected for some species because these species lack autapomorphies (uniquely derived character states) that unite their constituent members. Such taxa, termed metaspecies, are potential ancestors to species and higher clades with which they share a node in the cladogram.

A hypothetical species-level cladistic analysis coupled with three different hypothetical biostratigraphies shows how different models of speciation (bifurcation, budding, or anagenesis) result in very different patterns of true versus pseudoextinction. Depending on the speciation model, true extinction can be overestimated by as much as a factor of four, raising the specter of mass extinction. Species-level studies for three early Tertiary mammalian taxa—taeniodont eutherians, taeniolabidid multituberculates, and periptychid ungulates—use the same procedures. They show that almost 25% of disappearances during the early Paleocene (Puercan) for species in the analysis were pseudoextinctions of metaspecies. Budding and anagenetic-like peripatric speciation, but not bifurcation, are seen in the three examples.

Equating disappearance to true extinction can profoundly affect interpretations of faunal turnover, especially during mass extinctions or major faunal reorganizations. Some authors use pseudoextinction to describe the taxonomic rather than evolutionary disappearance of nonmonophyletic groups. Pseudoextinction, as used here refers only to the evolutionary disappearance of metaspecies via speciation. Both usages seem appropriate but should not be confounded.

Quantitative data on lycopsid and aquatic fern megaspore taxa recovered from Carboniferous, Mesozoic, and Tertiary strata have been compiled in order to analyze the changes in diversity of the two groups of fossil plants that produced them. Numbers of species of lycopsid megaspores are similar in the Carboniferous and Mesozoic, whereas the diversity of megafossils is much lower in post-Paleozoic deposits. Our data suggest that lycopsids were more diverse in the Mesozoic than previously thought and that there is a preservational bias against the megafossils, because the plants were probably mainly herbaceous. Heterosporous aquatic ferns first appeared in the Neocomian and gradually diversified until the early Late Cretaceous, after which their numbers remained relatively stable, whereas the variety of lycopsids declined dramatically during the Late Cretaceous. These changes occurred at a time of rapid angiosperm diversification. The reduced diversity of the lycopsids may have been caused by the invasion of their aquatic and damp forest-floor habitats by heterosporous ferns and by aquatic and herbaceous angiosperms. These diversity changes do not seem to be directly related to the global events at the Cretaceous-Tertiary boundary, but the relatively few samples available and the resulting range truncation would make detection of such correlations difficult.

A comparison is made between compilations of times of origination and extinction of fossil marine animal families published in 1982 and 1992. As a result of ten years of library research, half of the information in the compendia has changed: families have been added and deleted, low-resolution stratigraphic data have been improved, and intervals of origination and extinction have been altered. Despite these changes, apparent macroevolutionary patterns for the entire marine fauna have remained constant. Diversity curves compiled from the two data bases are very similar, with a goodness-of-fit of 99%; the principal difference is that the 1992 curve averages 13% higher than the older curve. Both numbers and percentages of origination and extinction also match well, with fits ranging from 83% to 95%. All major events of radiation and extinction are identical. Therefore, errors in large paleontological data bases and arbitrariness of included taxa are not necessarily impediments to the analysis of pattern in the fossil record, so long as the data are sufficiently numerous.

Cladistic analysis of osteological and dental characters in a monophyletic group of Miocene and younger tayassuids demonstrates a pattern of changes in the degree of sexual dimorphism in canine tooth diameter and zygomatic arch width, and in phenotypic correlations between these characters. Primitively, tayassuids have canine teeth that are sexually dimorphic and discretely bimodal in size, and zygomatic arches that are narrow in both sexes. Many late Miocene and Pliocene tayassuids have broad, winglike zygomatic processes. In some species, these processes are large in both sexes, but in others, those of females are much smaller than those of males. The presence of large processes in both sexes is primitive relative to the condition of strong sexual dimorphism. In five separate clades, the zygomatic processes of both sexes become reduced in size, and the degree of sexual dimorphism in canine size becomes reduced as well. The pattern is congruent with predictions derived from a theoretical model of the evolution of sexual dimorphism, and it further indicates the emergence of a new phenotypic correlation between two previously uncorrelated characters, canine size and zygoma size. The advent of this new correlation coincides with the advent of pronounced sexual dimorphism in zygomatic processes. Although such a pattern could be explained by genetically modifying phenotypic expression of homologous characters in one sex or the other, an epigenetic modification of expression is equally plausible: the evolution of sexual dimorphism in homologous characters could be accomplished by placing phenotypic expression of an originally monomorphic character under the control of steroid sex hormones. This hypothesis is consistent with evidence from many vertebrate groups, and it provides testable predictions.

In this paper we analyze the laws of growth that control planktic foraminiferal shell morphology. We assume that isometry is the key toward the understanding of their ontogeny. Hence, our null hypothesis is that these organisms construct isometric shells. To test this hypothesis, geometric models of their shells have been generated with a personal computer. It is demonstrated that early chambers in log-spirally coiled structures cannot follow a strict isometric arrangement. In the real world, the centers of juvenile chambers deviate from the logarithmic growth curve. Juvenile stages are generally more planispiral and contain more chambers per whorl than adult stages. These traits are shown to be essential in order to keep volumes of consecutive chambers in geometric progression. We are convinced that the neanic stage marks the constructional bridge from a juvenile set of growth parameters to an adult one. The adult stage can be strictly isometric, that is, the effective shape is constant and the increase in volume after a chamber addition is proportional to the preexisting volume of the shell.

The shell volume is related to the biomass, the ratio of outer shell surface area to shell volume is related to the respiration rate and the ratio of the total shell surface area to shell volume is related to the total calcification effort. The influence of the parameters of the model on these relationships is investigated. Only the initial radius and the rate of radius increase affect the relationships between shell volume and surface area. The other shape parameters merely provide a fine tune-up of these relationships. Size itself plays a major role during foraminiferal development.

Immature specimens of the Late Cretaceous pterosaur Pteranodon were identified using three size-independent criteria: (1) fusion of various cranial and postcranial elements; (2) degree of epiphyseal ossification; and (3) bone grain or degree of ossification of limb-bone shafts. Immature individuals make up 15% of available specimens of Pteranodon and do not differ significantly in size from mature individuals. This and the extensive fusion of the mature skeleton suggest that Pteranodon had determinate growth. The bone of limb-bone shafts of immature individuals is fibro-lamellar bone, which suggests that they grew rapidly to adult size. The size-independent criteria can also be used to identify immature and mature individuals of other pterosaur taxa, and other large pterodactyloids also probably exhibited rapid determinate growth.

A series of experiments was carried out to investigate the nature and controls (oxygen, microbial populations, agitation) on the degradation of soft tissues. Decay was monitored in terms of morphological change, weight loss, and change in chemical composition in the polychaete Nereis virens. Polychaetes include a range of tissue types of differing chemical composition and preservation potential: muscle, cuticle, setae, and jaws. Regardless of conditions, all the muscle had broken down and fluid loss through the ruptured cuticle had reduced the carcass to two dimensions within 8 days at 20°C. In most cases some cuticle, in addition to the jaws and setae, remained after 30 days. Where oxygen was completely eliminated, the rate of decay of the more volatile issues was significantly reduced. The degree of both osmotic uptake of water by the carcass and changes in water pH differed depending on whether the system was open or closed to oxygen diffusion. Autolytic and chemical processes are not sufficient to fully degrade the carcass in the absence of bacteria. Where internal bacteria are present, the presence or absence of water column bacteria made little difference to decay rate. Initial degradation (in the first 3 days) affects mainly the lipid fraction and the collagen of the cuticle. Later decay reduces the nonsoluble protein and increases the relative proportion of refractory structural components (tanned chitin and collagen) to more than 95% by day 30. Thus, only the sclerotized tissues are likely to survive beyond 30 days in the absence of early diagenetic mineralization. The sequence of degradation predicted from the relative decay resistance of macromolecules in the sedimentary record (protein → carbohydrate → lipid) is not, therefore, a consistent indicator of the preservation potential of structural tissues which incorporate them.

The experiments reveal five stages in the decay of polychaete carcasses; whole/shriveled, flaccid, unsupported gut, cuticle sac, jaws and setae. All are represented in the fossil record. This allows an estimation of how far decay proceeded before it was halted by the fossilization process. The most complete preservations occur in the Cambrian where the Burgess Shale preserves evidence of muscle tissues. Traces of the gut and cuticle are more widely preserved, as at Mazon Creek, Grès à Voltzia, Solnhofen, and Hakel. Preservation varies within Konservat-Lagerstätten. The most common whole body preservation includes only the more recalcitrant tissues, jaws (where present) and setae, with an impression of the body outline. The stage of decay can be used as a taphonomic threshold, to provide an indication of how significantly the diversity of an exceptionally preserved biota is likely to have been reduced by taphonomic loss.