The genus Homo evolved its pronounced encephalization through postnatal extension of the high rate of brain growth that characterizes all primates in utero. Linked to this extension was delayed development, which represented an enormous ecological sacrifice because it produced the longest postnatal interval of physical helplessness in the Mammalia and forced mothers to carry infants.

Graphs relating brain growth to body growth indicate a pongid pattern of development for gracile australopithecines, implying that infants could cling to mothers whose forelimbs were occupied with climbing. Also present were several postcranial traits that would have made the adults more adept climbers than modern humans. Habitual use of these inherited traits is suggested by the fact that evolution failed to eliminate certain ones, such as short legs and long pedal phalanges, that impeded terrestrial locomotion. Moreover, the intensity of predation by large, swift, social carnivores must have compelled australopithecines to use trees as refuges, in the manner of chimpanzees and baboons; australopithecines probably also gathered some of their food in trees. Gracile australopithecines failed to expand their brain size, experiencing general evolutionary stasis for more than 1.5 m.y. I propose that this stability resulted from these animals' semiarboreal mode of life: First, their postcranial morphology remained compromised by selection pressures to maintain both terrestrial and arboreal adaptations. Second, by requiring that neonates be mature enough to cling to mothers, obligate arboreal activity precluded encephalization of the kind that characterizes Homo; this evolutionary constraint has previously been overlooked.

In contrast to australopithecines, early Homo approached H. erectus in pelvic configuration and brain size. A new brain-body growth curve for early Homo indicates extension of the fetal pattern well into the postnatal interval, implying that neonates were highly immature so that adults had to be fully terrestrial. Homo evolved shortly after the onset of the modern ice age about 2.5 Ma. Fossil pollen and carbon isotopes in paleosols record a contraction of forests in Africa at this time. I propose that this represented a crisis that led to the evolution of Homo by compelling some australopithecine populations to adopt a fully terrestrial existence. Although ecologically difficult, this behavioral restriction finally made possible encephalization through the evolution of delayed development. During the ecological crisis, a large brain evolved in at least one population of gracile australopithecines. An advanced tool industry and cunning behavior were of such great adaptive value for avoiding predators and expanding food resources on the ground that selection for encephalization soon overrode the problems imposed by helpless infants.

The fossil record of antelopes and micromammals provides a test of the idea that environmental forcing opened the way for the evolution of Homo: both of these groups experienced heavy extinction of forest-adapted species about 2.5–2.4 Ma and a rapid proliferation of species adapted to unforested habitats. The transformation of the hominid clade during Plio-Pleistocene time did not follow a simple pattern. Homo may have arisen either by anagenetic transformation of a “bottlenecked” species or by speciation, and it may not have evolved immediately with the onset of climatic change. Furthermore, just as a few forest-adapted antelope species survived the biotic crisis, a small-brained gracile taxon with arboreal adaptations may have persisted to the start of the Pleistocene. Robust australopithecines survived into the Pleistocene, perhaps because a broad vegetarian diet reduced their need to migrate frequently between home bases. With their extinction in mid-Pleistocene time, about the time that savannahs became widespread, only Homo remained.

The post-Paleozoic radiation of echinoids entailed a rapid diversification not only of adult morphology, but also of larval morphology. The timing, order, and phylogenetic distribution of evolutionary transformations in echinopluteus larvae are reconstructed here under maximum parsimony assumptions from a large neontological data base. Many echinoid larval apomorphies apparently evolved within the Paleozoic stem lineage and were subsequently retained during much of the crown-group radiation. This suite of apomorphies includes most (and perhaps all) of the skeletal elements and some features of soft anatomy such as vibratile lobes. Other features apparently arose or were modified during the post-Paleozoic radiation. These include skeletal features such as arm-rod structure and length, and soft structures such as epaulettes and skeletal muscles. Transformational hypotheses of this kind can be supported or rejected with further neontological data, and many can potentially be tested from fossil evidence. Many post-Paleozoic transformations in echinopluteus structure may be adaptive. For example, increases in arm length and flattening of arm ectoderm may increase feeding rate and efficiency, and both types of transformation have occurred several times within the crown group. Relational hypotheses of this nature can be tested through comparative functional studies in extant echinoplutei. Parallel evolutionary losses of feeding in echinoplutei are accompanied by loss or modification of characteristic structures. This suggests that developmental constraints do not fully explain the conservation of these structures in planktotrophic echinoplutei. Comparisons of larval and adult morphology over congruent time intervals demonstrate that the origin of many orders was accompanied by a suite of synapomorphies in larval morphology that was subsequently conserved. Many details of echinopluteus morphology are therefore of taxonomic significance. Intraordinal patterns of larval diversity, however, vary considerably. That larval morphology has diversified independently of adult morphology indicates that mosaic evolution has occurred within the life cycle and suggests that echinoid larvae and adults can and do respond to selection independently. Taken together, these findings underscore the complex ways in which complex life cycles can evolve.

Lengths of crinoid stalk segments consisting of multiple columnals (pluricolumnals) from the Fort Payne Formation of south-central Kentucky (Mississippian) were tested for uniformity using simulations. The results indicate that stalk segment lengths are too uniform to have been produced by a random process operating on homogeneous stalks.

The most parsimonious explanation for the uniformity of pluricolumnal lengths is based on analogy with stalks of living isocrinids. The stalks of these crinoids are organized into multicolumnal segments of approximately uniform length: columnals within each segment are connected by “through-going” ligament and “intercolumnal” ligament. The articulation between segments, however, consists of only intercolumnal ligament. Preliminary results from experiments on Recent carcasses indicate that the latter is the primary plane of disarticulation following death.

The preservational pattern of the Mississippian stalks suggests that their ligamentary organization was similar to that of the isocrinids. Since the plane of primary disarticulation in isocrinids corresponds to the autotomy plane, Mississippian crinoids also may have been capable of autotomizing their stalks.

Faunal similarity among regions is often used as a means of identifying regions of endemism in fossil faunas. At least two large-scale taphonomic effects can affect apparent faunal similarity: stratigraphic and facies mismatching. In stratigraphic mismatching, an unconformity represents removal of most or all of a complete assemblage zone in one region, and the constituent taxa are mistakenly interpreted as having never inhabited that region. In facies mismatching, environmental differences between two regions (possibly unrecognized) cause the inference of a barrier that never existed. The two types of mismatching can work in concert if a facies was originally represented in a single stratigraphic interval that has been completely removed from one region. Analysis of faunal similarity via multivariate analysis of individual localities, coupled with comparison of the regions as single samples, may indicate mismatching if the results differ significantly.

We view these two problems as part of a suite of taphonomic effects that are not evident in paleobiological analyses of smaller geographic scope. First, there is ambiguity in the notion of “barrier,” even when a candidate is obvious. Second, barriers in paleobiogeography are often hidden and must be inferred from their effects rather than observed. Third, stratigraphic and facies mismatching produce effects on regional faunas similar to those produced by barriers. Anyone using barriers to explain faunal disruptions should address these three points.

Upper Eocene faunas of central Florida seem taxonomically distinct from those of the remainder of the Gulf Coastal Plain. This has historically been attributed to a known paleogeographic feature, the Suwannee Strait, which acts as a barrier. The amount of dissimilarity of the echinoid faunas is greater than the amount predicted as a result of sampling problems. Comparison of the results of multivariate and whole-region analyses suggests that mismatching of the two faunas, rather than a true barrier, causes the distinction. Principally facies, but also strata are mismatched. Early Late Eocene faunas inhabited terrigenous sands to the north of the strait and carbonate sands to the south and show the highest distinctiveness. Middle Late Eocene faunas inhabited primarily carbonate sands to the south and both carbonate sands and muds to the north. Overall similarity is higher for both local and regional analyses, and the faunas of northern sands are more similar to those of the southern region than to the northern mud faunas. Overall similarity across the strait is highest in the late Late Eocene strata when both regions had carbonate mud and sand facies. The faunas exhibit greater similarity within facies than they do within regions. Upper Late Eocene strata are poorly preserved north of the strait because of post-Eocene erosion.

This paper presents a new means of interpreting the distribution of taxa among taxonomic lists. Traditionally, “similarity” indices have been used to compare lists, and “association” measures have been used to compare taxonomic distributions. It is argued that when sampling regimes are poorly understood, all similarity and association indices are unjustifiable. However, observations that taxa have overlapping (conjunct) or nonoverlapping (disjunct) distributions are universally meaningful. Because greater sampling can only increase the number of known conjunctions and because long lists serve as conjunctional Rosetta Stones, conjunction data sets can be far more reliable than the lists that generate them. One way to account for patterns of conjunction and disjunction is to create theoretical spaces composed of two distributional boundaries or edges for each taxon in each dimension. A parsimonious arrangement of edges implies distributions that always overlap when taxa are conjunct and infrequently overlap when taxa are disjunct. A procedure involving correspondence analysis is shown to minimize the number of implied disjunctions. The method is used to analyze a set of 271 Miocene large-mammal genus lists from the Great Plains region. Biostratigraphic and radiometric data demonstrate that the “best” arrangement of distributional edges is temporal and corresponds to an age-range chart. The edge sequence is calibrated to the radiometric time scale and used to compute a detailed genus-level diversity curve and to redefine the boundaries between Miocene Land-Mammal Ages and Subages in the Great Plains. Each of the Miocene Ages can be divided into two or three Subages. Diversity levels are apparently low throughout the Early Miocene and rise sharply during the Middle Miocene. Significant drops in diversity occurred at about 17.4, 11.6, 8.7, and 6.5 Ma. The base of the Barstovian corresponds to the base of the Middle Miocene, and the Clarendonian straddles the Middle-Late Miocene boundary.

Evolutionary patterns in the articulate brachiopod hinge are investigated and this empirical example is used as a model to examine evolutionary trends and evolutionary constraints. Several features of the hinge-system geometry exhibit persistent directional change from the Cambrian to the Recent and result in increased mechanical advantage in opening the valves. These geometric changes are reflected also in general features of shell morphology and growth and in patterns of familial diversity through time. Specific, identifiable constraints on brachiopod morphology and function related to the position of the pedicle and muscles and nature of the hinge line and hinge structures may be said to direct the observed trends. The pattern of evolutionary change among all articulate brachiopods is most satisfactorily accommodated by a diffusion model of morphological change. Examined independently, the deltidiodonts (with noninterlocking hinge structures) appear to reflect a process of directional selection through time, whereas change in the cyrtomatodont hinge mechanism (with interlocking teeth and sockets) may reflect the incidental effects of selection for increasing mantle-cavity volume rather than increasing diductor (opening) muscle moment. The “transition” from deltidiodont to cyrtomatodont hinge structures over the course of brachiopod evolutionary history may be interpreted from at least two distinctly different perspectives. Paleoecologically, this transition appears to reflect a habitat shift from soft sediment to hard, rocky substrates. Phylogenetically, the transition reflects the process of evolution: deltidiodonts persist as their modified descendants, the cyrtomatodonts. Investigating the acquisition of evolutionary novelties over time, or the transition from primitive to derived morphological features, may be the most informative approach to follow in studying evolutionary trends.