Using a conservative definition of the hominid taxon Homo erectus, the sample of specimens known at this time, 92 individuals in all, is examined to determine whether the species exhibited significant temporal trends or evolutionary stasis. The sample is divided into three low-resolution time spans, and it is shown that significant change characterizes a number of cranial, mandibular, and dental features. Rates of change are comparable to or above rates reported for other fossil vertebrate lineages sampled over a similar duration. The often repeated assertion that Homo erectus is an example of a species in evolutionary stasis is incorrect.

Three closely related species of the irregular echinoid Oligopygus succeed one another in the late Eocene deposits of Florida. This apparent lineage, where each species is similar but larger than the preceding species, offers an excellent opportunity to examine relationships between heterochrony and environmental change. Bivariate and multivariate techniques illustrate patterns of simple and complex allometry and indicate that simple size increase related to changes in the timing of maturation account for most of the morphological differences among species. Those differences that are not the by-products of size increase are due to heterochronic changes dissociated from the otherwise global (i.e., whole organism) hypermorphosis and were probably needed to increase the relative food intake of the larger forms. Unlike the vast majority of allometric studies, which show constant growth ratios, the ontogenies analyzed here often exhibit curvilinear trajectories. Thus, heterochronic events are seen as systematic alterations of already changing growth ratios. It is suggested that these heterochronic changes provided quick, efficient responses to selective pressures on a plurality of characters and behaviors. As the Florida carbonate environment became more stable, responses associated with K-type life history strategies were favored, resulting in “size selection.” Thus, the morphological differentia among these species may not have resulted from selection directed at those traits, but were either simple sequelae of size increase or of changes which occurred to permit the size increase.

A rate of extinction must specify both the amount of taxonomic change and the duration of that change. A consistent means of temporal measurement must be used to establish the duration. Terms used to describe the extinction, like catastrophic and gradual, should be quantitatively defined in relation to this framework.

Resolving rates of extinction in the stratigraphic record is limited by (1) the precision of chronostratigraphic correlations between individual sections and (2) the temporal completeness of litho- and biostratigraphic records in those sections. In this study, temporal completeness is estimated in eight of the best-known fluvial and pelagic sections spanning the K-T boundary. Temporal standardization is provided by correlations to the geomagnetic polarity time scale. A review of the literature documents lithologic criteria that might be used to infer the presence of a hiatus of some length at the biostratigraphically recognized K-T boundary. Rather than rejecting the presence of such gaps, this study argues that a hiatus of some length does exist and tries to estimate both a maximum limit for its duration and the probability that it represents a given duration below that limit.

Pelagic sections are probably more complete than fluvial ones. The section at Caravaca, Spain, can be expected to be the most complete, probably preserving sediment during each 10,000-yr interval of Chron 29R. The section in the San Juan Basin, New Mexico, is expected to be the most complete fluvial section, probably preserving sediment during each 100,000-yr interval of 29R.

Results indicate that our best-known marine and terrestrial sections spanning the K-T boundary should not be expected to document biologically catastrophic rates of extinction. A sufficiently precise means of global correlation that is independent of biostratigraphy and causal hypotheses is still unavailable, and it is very unlikely that the sections examined are complete at the 100-yr or biologically catastrophic level of precision. So, although catastrophic amounts of extinction might have occurred during the K-T transition, it seems unlikely that we can distinguish episodes of extinction lasting 100 yr or less from episodes lasting as long as 100,000 yr. Consequently, acceptance of catastrophic hypotheses based on these stratigraphic records seems improbably optimistic at this time.

By digitizing the enamel configuration and calculating the overall directional tendencies of the enamel edges in complex equid dental patterns, quantitative changes in functional attributes were traced through a phylogenetic series. Pronounced changes both in the directional emphasis of the enamel edges and in the chewing motion occurred and were interrelated. Two distinct trends were found. From early Eocene to Oligocene, edge alignment became increasingly confined to a single direction, but the teeth retained the phase I and phase II chewing angles characteristic of many primitive herbivorous and omnivorous mammals. The chewing direction in the horizontal plane also remained essentially unchanged. During the Oligocene and the early Miocene, both edge direction and chewing direction remained stable. However, the middle Miocene genus Merychippus represents a stage in which the enamel directional emphasis was lost in transition to a new anteroposterior emphasis, a trend that continued into the late Tertiary equine taxa. Concurrent with this transformation was a loss of all but vestiges of the phase I and II chewing angles in the vertical plane and a shift of the chewing direction in the horizontal plane to an almost transverse movement of the jaw.

In the Oligocene equids, the edges of the labial cusps did attain perpendicularity to the phase I chewing direction, which was oblique to the horizontal plane, but the chewing direction in the horizontal plane was far from perpendicular to the lingual enamel edge direction because the lingual cusps were configured for efficiency in compressive, not shearing, motion that dominated the phase II surfaces. In the later Tertiary the chewing direction and edges attained essentially a perpendicular relationship, which represents the theoretical optimum for a shearing system. The radical change in edge direction in the Miocene probably occurred as a result of an increase in translative movement (longer glide) between the surfaces, making the lingual edges subject to selection for perpendicularity and flattening of the occlusal surface, resulting in a condition where a single edge direction became optimal for all cusps. The extended chewing stroke translation was perhaps the key event initiating the other changes and would have resulted from increased chewing effort accompanying a shift to a diet of mainly grass. The loss of the dual (shearing and compressive) function of the occlusal surface suggests that the earlier diet may have contained significant proportions of other plant parts, such as fruits, seeds, or tender leaves that might be optimally chewed by compressive movements.

The proposition is presented that the large thecae of many Paleozoic crinoids housed gonads, unlike modern crinoids that have the gonads on the arms or pinnules. Early in their history, inadunate crinoid gonads migrated into a voluminous anal sac, effectively separating them from other vital organs. Excision of the sac by predatory fishes and cephalopods would have been less traumatic than an attack on the theca, and the sac could be more readily regenerated. The pores and slits between plates on inadunate sacs are interpreted as gonopores. Some anal sacs may also have served as brood chambers. The traditional explanation of the sac as a special respiratory structure is discounted.

C. M. Yonge's idea that differences in habitat distribution and species diversity between archaeogastropods and caenogastropods are caused by differences in their respective abilities to obtain oxygen in turbid water is tested experimentally for the first time. Of 11 species of marine prosobranchs studied—six archaeogastropods and five caenogastropods—three archaeogastropods and two caenogastropods show significant reductions of respiration in response to turbidity, as shown using one-way analysis of covariance (ANCOVA). Two-way nested ANCOVA, which goes beyond one-way ANCOVA by incorporating all data into a single analysis, was also unable to detect any significant difference between archaeogastropods and caenogastropods in their respiratory responses to turbidity. Though the number of species studied was limited, the data and statistical tests suggest that there may be no biologically important systematic difference between archaeogastropods and caenogastropods in their respiratory responses to turbid conditions. Results of this study cast doubt on the validity of Yonge's hypothesis and suggest the need for new research into the controls of gastropod distribution and species diversity.

Bottom site remote camera photosequences at depths of 73–538 m on forereef slopes in Palau show that Nautilus belauensis is a highly mobile, chemosensitive, epibenthic scavenger and opportunistic predator. The overall depth range of this species is ca. 70–500 m, but photosequences indicate a preferred range of 150–300 m. Nautilus is active both nocturnally and diurnally, locating bait sites within 1–2 h. Associated macrofauna includes caridean shrimps, crabs, and eels; teleosts are rare below 100 m, but sharks are recorded in most photosequences below 250 m. Summarily, Nautilus exhibits a combination of characters that typify deep-sea strategy, including reproductive tactics, growth rate, and population dynamics. This and other evidence suggest that fossil Nautilidae may have been deep-water forms, in contrast to the typically shallower water ammonoids, and that Nautilus is a normal component of the deep forereef rather than a late Cretaceous refugee from shallow water.

A survey of the primary xylem anatomy and dimensions of axes (=“stems”) of Upper Devonian plant petrifaction/compression fossils reveals the following: among genera, the primary xylem strand (1) increasingly deviates from a cylindrical geometry (=haplostele) as both the axis diameter and the nonxylem (“ground”) tissue volume increase and (2) increases in geologically younger specimens. (3) The perimeter length to area ratio of the primary xylem strand (or the primary xylem surface area to volume ratio) is dependent upon axis diameter or the area of the ground tissue. This ratio (4) decreases in evolutionary lineages that involve the appearance of larger plants that maintain a haplostelar anatomy (e.g., the evolution of trimerophytes from a rhyniophyte ancestor) or increases in evolutionary lineages involving the appearance of nonhaplostele anatomy (e.g., the evolution of progymnosperms and pteridophytes from trimerophyte ancestors) and (5) the relative amount of primary xylem to ground tissue increases in stratigraphically younger specimens. These trends are interpreted within the context of (1) correlations between the diameter of axes and the complexity of the primary xylem anatomy reported for some extant pteridophytes and (2) changes associated with the evolution of new primary xylem configurations and organographic relationships. Based on the appearance of new ontogenetic patterns and concomitant changes in the anatomy of the primary xylem strand associated with the evolution of derived plant groups, a stria extrapolation of a size to complexity correlation based on ontogenetic studies to the phyletic level is considered suspect.