The origin and diversification of terrestrial communities has long been associated with the Devonian. The earliest known substantial assemblage of land plants and animals preserved in or close to their original habitats is still that of the Rhynie Chert peat bog, from Scotland, which is of Siegenian age.

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.

Paedomorphosis has played an important evolutionary role in several vertebrate lineages, yet there are few studies in which fossils have been used to document this evolutionary pattern directly. The unusually good fossil record of lungfishes offers an excellent opportunity to do this. An approach to recognizing paedomorphic character complexes in fossil lineages is formalized, and the interdependence of ontogenetic and phylogenetic studies is reemphasized. Concordant evolutionary changes in seven characters representing a variety of organ systems are discussed. For example, in Devonian lungfishes, the tail is heterocercal, endochondrally ossified bones are present, and the fin rays are similar to those of other Osteichthyes. Recent lungfishes have protocercal tails, suppression of endochondral ossification, and greatly reduced fin rays. The data suggest that paedomorphosis played a pervasive role in the evolution of lungfishes and give further evidence for the evolutionary importance of heterochrony.

Some statistical methods are described for the detection and estimation of variability in the rate of evolution, from biometrical data on single characters in fossil lineages. The methods were applied to some published examples of protistan and mammalian lineages, extending between 0.8 and 6.4 Myr in duration. Of six cases examined, only one failed to show evidence for significant variation in the rate of evolution, and only four deviated significantly from a random walk pattern, despite significant net evolutionary change in all cases but one.

Two rival models of evolution in multispecies systems are tested against empirical species-level data. The two models are the Red Queen model of Van Valen as reformulated by Stenseth and Maynard Smith, which assumes that evolution is driven principally by biotic interactions, and the Stationary model of Stenseth and Maynard Smith, which assumes that evolution is propelled primarily by abiotic factors and will cease in the absence of changes in abiotic parameters. Testing refers to the models' predictions regarding the behavior of extinction and origination rates, and assumptions regarding equilibrium diversity and a constant effective environment. The data set includes the dates of origination and extinction for all coccolith, planktic foraminifer, and radiolarian species recorded in the Oligocene through Holocene, and all planktic diatom and silicoflagellate and ebridian species recorded in the Middle Miocene through Holocene in 111 DSDP sites of the low- to mid-latitude Pacific Ocean.

The condition of stable specific age distribution over geologic time is met, which allows one to perform survivorship analysis on extinction rates. The best fit survivorship curve is a decreasing function of age for both coccolith and foraminifer species, and an increasing function of age for radiolarian species. Neither model predicts age dependence of the probability of extinction. The small disparity between these curves and age-independent curves for each group indicates, however, that an age-independent interpretation of extinction probability is a reasonable first approximation. Rates of origination are analyzed in terms of species accretion, introduced to represent the cumulative origination of species within a higher taxon as a function of the age or duration of the community. Accretion analysis indicates that the probability of accretion is both diversity-dependent and absolute time-dependent.

The assumption of a constant effective environment is tested by polycohort analysis and nonparametric logistic regression analysis of true species cohorts. Both techniques indicate considerable variation in extinction probability over geologic time. When the predictions of the two evolutionary models are adjusted to take this variation into account, the results of both survivorship and accretion analysis seem to conform more closely to the predictions of the Red Queen than to the Stationary model. However, as the speed with which the effective environment changes is increased relative to speciation-extinction rates, it becomes increasingly difficult to differentiate patterns predicted by the two models. The assumption of equilibrium diversity can be neither corroborated nor rejected, since the species-level data are compatible with both an equilibrium and a nonequilibrium view of diversity behavior. Reservations concerning the basic assumptions of both models indicate an ultimate test requires that both models be reformulated to make precise and distinctive predictions under a varying effective environment.

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.

Hierarchy is a central phenomenon of life. Yet it does not feature as such in traditional biological theory. The genealogical hierarchy is a nested organization of entities at ascending levels. There are phenomena common to all levels: (1) Entities such as genomic constituents, organisms, demes, and species are individuals. (2) They have aggregate characters (statistics of characters of subparts), but also emergent characters (arising from organization among subparts). Character variation changes by (3) introduction of novelty and (4) sorting by differential birth and death. Causation of introduction and sorting of variation at each level may be (5) upward from lower levels, (6) downward from higher levels, or (7) lodged at the focal level. The term “selection” applies to only one of the possible processes which cause sorting at a focal level. Neo-Darwinian explanations are too narrow, both in the levels (of genotypes and phenotypes) and in the directive process (selection) which are stressed. The acknowledgment of additional, hierarchical phenomena does not usually extend beyond lip service. We urge that interlevel causation should feature centrally in explanatory hypotheses of evolution. For instance, a ready explanation for divergence in populations is “selection of random mutants.” But upward causation from genome dynamics (or downward causation from the hierarchical organism) to the directed introduction of mutants may be more important in a given case. Similarly, a long-term trend is traditionally explained as additive evolution in populations. But sorting among species may be the cardinal factor, and the cause may not be species selection but upward causation from lower levels. A general theory of biology is a theory of hierarchical levels—how they arise and interact. This is a preliminary contribution mainly to the latter question.

A survey of all Cerion taxa and geographic variants reveals that a distinctive shape—a narrowly spired “smokestack” shell more than three times higher than wide—occurs only in dwarfs and giants and never in the much more common populations of normal size. The smokestack shape evolved once in giants (in the newly described species C. excelsior from Great Inagua and Mayaguana—the largest of all Cerion), but at least seven and probably eight times independently in dwarfs. A study of complex allometric patterns in Cerion's ontogeny, and of covariance sets in growth, indicates that smokestacks can easily evolve in dwarfs and giants by the common route of relative increase in whorl number during an allometric phase that adds height but no width to the shell. (Giants simply add more whorls to a shell with whorls of normal size; dwarfs grow a normal number of whorls in a shell with small whorls and reduced maximum width.) This pathway is not open to Cerion of normal size, and the restriction of smokestacks to both extremes of the size range records a channel set by Cerion's invariant allometries and the geometry of spiral growth in general, not an immediate adaptation conferring special advantages via the elevated spire itself.

Available data on the stratigraphic ranges of latest Silurian and Devonian vascular plant macro-fossils (sporophytes) and spores provide insights into the tempo and mode of early tracheophyte evolution. Patterns of diversification, origination, and extinction conform in general to the predictions of Sepkoski's kinetic model of diversification. Rates of generic origination and extinction vary not only through time but also between organ systems for a single time interval. This fact, coupled with data on longevity and turnover and comparative morphological observations, can be used to document mosaic evolution in early vascular plant history. Mosaic evolution is an important theme in plant evolution; indeed, what we recognize as macroevolutionary events often correlate with brief periods of pronounced mosaicism. Such evolutionary patterns reflect the developmental biology of tracheophytes in which individual organs often have life spans that are considerably shorter than the life of the whole plant. Under these conditions, individual organs or organ systems can respond to different sets of evolutionary pressures.

The major period of early vascular plant diversification occurred during the late Early and early Middle Devonian Period, 30 Myr or more after the origin of the group. Such lags in diversification are not uncommon in the fossil record. Sometimes they reflect extrinsic controls on diversification, but in other cases they appear to be a consequence of intrinsic rates of origination and extinction.

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.

Formulae are developed for the fraction of the population that must be eliminated by selection in each generation in order to account for a given rate of evolution in a metrical trait. A combination of directional and stabilizing selection is assumed. The effects of competition based on phenotypic value are also considered. The formulae are applied to data on evolutionary rates and their variances. It is concluded that most selective elimination is due to stabilizing selection, and that even very rapid evolutionary change in a single character usually involves low levels of additional elimination. It is suggested that long-sustained evolutionary trends are unlikely to be caused either by gradual change of the optimum under stabilizing selection or by the effects of competitive selection in favor of extreme individuals. It is possible that genetic correlations between traits under selection may limit the response of a single trait to directional selection and hence produce very gradual change.

Morphologic analysis of 281 species of ammonoids from Great Britain, the North American mid-continent, and the South Urals, at eight successive levels within the Namurian Series (ca. 18 Myr duration), using bivariate plots and principal-components analysis, permits definition of morphologic diversity and identification of morphotypic patterns in time and space. Namurian ammonoids exhibit the same general range of shell geometry that characterizes ammonoids as a whole; there were few post-Namurian innovations in the basic geometry of planispiral ammonoids. Within this overall range of geometry, there are eight preferred morphotypes: two were phylogenetically monopolized by long-ranging forms; three were generalized and reoccur in successive horizons; two others were homeomorphically utilized at different times by different lineages; and one represents morphologic innovation followed by radiation. Such patterns seem to represent combined effects of function, phylogeny, and ecology. Synchronous variations in isolated successions suggest global controls such as eustatic sea-level fluctuations, whereas provincial differences in diversity may be attributable to paleogeographic and ecologic factors. We predict that the Namurian record of ammonoid morphologic diversity and change will be found to be distinctive and differentiable from earlier and later intervals.

Faunal stasis among Indo-Pacific reef-building corals is explained as the result of chronic environmental disruptions preventing evolutionary processes from approaching completion since the Late Pliocene. The model assumes shallow reefal habitats (<20 m) on the continental shelves are major sites of scleractinian evolution and explores ecological and evolutionary consequences of high-frequency sea-level fluctuations and their associated transgression-regression cycles. Because single generations, dominated by a few large clonal genotypes, may persist indefinitely, local populations may not have experienced enough generations to approach evolutionary equilibrium with their environments during the estimated average duration (≈3200 yr) of existence of shallow habitats. Persistent consequences of chronic evolutionary disturbance may be the extensive intraspecific variation so characteristic of the dominant genera of shallow Indo-Pacific corals and the apparent paucity of recently evolved endemic species. The same disturbances may have accelerated speciation rates among reefal organisms with much shorter generation times.

Several important questions in evolutionary biology and paleobiology involve sources of variation in extinction rates. In all cases of which we are aware, extinction rates have been estimated from data in which the probability that an observation (e.g., a fossil taxon) will occur is related both to extinction rates and to what we term encounter probabilities. Any statistical method for analyzing fossil data should at a minimum permit separate inferences on these two components. We develop a method for estimating taxonomic extinction rates from stratigraphic range data and for testing hypotheses about variability in these rates. We use this method to estimate extinction rates and to test the hypothesis of constant extinction rates for several sets of stratigraphic range data. The results of our tests support the hypothesis that extinction rates varied over the geologic time periods examined. We also present a test that can be used to identify periods of high or low extinction probabilities and provide an example using Phanerozoic invertebrate data. Extinction rates should be analyzed using stochastic models, in which it is recognized that stratigraphic samples are random variates and that sampling is imperfect.

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.

Surprisingly, there is a relationship between rates of sediment accumulation and the time spans for which they have been calculated. This relationship can be used to estimate expected rates for specific sedimentary environments and time spans. The most probable completeness of a given sedimentary section at a given short time span can be calculated by the ratio of the measured long-term rate of sediment accumulation to the expected short-term rate. Although the measured time span is usually based on radiometric and paleomagnetic data, the cumulative time of formation estimated from fossil soils in a sequence may also be used to calculate rates and may be useful in comparing the completeness and rate of accumulation of different sequences. By both kinds of estimates, terrestrial sedimentary successions are disappointingly incomplete. Some reasons for incompleteness are illustrated with a simple model of episodic flooding, exceeding a threshold for destruction and sedimentation over a particular kind of vegetation, and thus initiating a new cycle of soil formation. In such a model, rock record is lost to erosion during cutting and filling cycles, to overprinting of weakly developed soils by later, better-developed soils, and to continued development, near steady state, of the soils preserved.

Because fossil soils are evidence of ancient environments and ecosystems independent of the fossil record, they may provide evidence of expected kinds of fossils, such as silica phytoliths, calcareous phytoliths, pollen, leaves, fruits, seeds, charcoal, land snails, coprolites, and bones. The degree to which the kinds of fossils actually found fail to meet these expectations is a crude measure of the completeness of representation of a former ecosystem in the fossil record. Some of the discrepancy between expected and actual occurrence of fossils can be related to the original Eh and pH of a fossil soil, as approximated by the oxidation state of iron in its minerals (for Eh) and by carbonate or zeolite content (for pH). Different kinds of fossils can be envisaged as having a characteristic Eh-pH stability field within which they can be expected to have been preserved if originally present. Even under ideal conditions of preservation, it takes some time for fossils to accumulate in soils to levels at which representative collections can be made. Estimates of this temporal control on preservation can be gained by comparing fossil occurrences with the degree of development of fossil soils. Neither these chemical nor temporal factors account fully for the degree of incompleteness observed because original abundance, trampling, predation and many other factors are also important determinants of fossil occurrence.

These considerations can be used as guidelines for choosing stratigraphic sections appropriate for particular paleobiological and geological problems. For example, a study of speciation of terrestrial vertebrates would best be in a sequence of weakly developed, calcareous fossil soils (Entisols and Inceptisols), of near-uniform texture and yellow to brown color, formed under an extraordinarily high long-term rate of sediment accumulation. On the other hand, a study of coevolution of vertebrates and plants would best be based on a sequence of weakly to moderately developed, calcareous fossil soils of predominantly drab (gray, green, and blue) color, with interbedded carbonaceous shales.

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.

The mid-Paleozoic was punctuated by a rapid radiation of durophagous (shell-crushing) predators. These new predators were primarily placoderm and chondrichthyan fishes but probably also included phyllocarid and eumalacostracan arthropods. Coincident with the radiation of these durophages, beginning in the mid-Devonian, there was an increase in the frequency of predation-resistant morphologies in a variety of marine invertebrate taxa. Among bellerophontid molluscs, disjunct coiling disappeared and umbilici became less common while the frequency of genera with sculpture increased. The abundance of brachiopod genera with spines on one or both valves increased dramatically. Sculpture became more pronounced and common among genera of coiled nautiloids. Inadunate and camerate crinoids showed a marked increase in spinosity, and all three crinoid subclasses tended to develop thicker thecal plates.

Trends toward increasing relative frequencies of predation-resistant features were formed in different ways. Bellerophontid genera lacking predation-resistant features tended to go extinct, leaving the sculptured, tighdy coiled forms as the predominant forms. Among Brachiopoda, the radiation of productids provided the tremendous increase in numbers of spinose genera. Among crinoids, predation-resistant features were acquired through evolution within established clades.

These observations suggest that predation by shell-crushing predators has been an important control on the morphology and composition of the marine invertebrate fauna since at least the Middle Devonian. The mid-Paleozoic radiation of durophages and response of the marine fauna was in many respects similar to events of the Mesozoic Marine Revolution, in effect, the Paleozoic precursor to that event.

Taphonomic analysis of several late Pleistocene mastodon (Mammut americanum) skeletons excavated in southern Michigan provides compelling evidence of mastodon butchery by Paleo-Indians. The occurrence of butchery and details of butchering technique are inferred primarily from patterns of bone modification. An important aspect of butchering practice was production and use of tools fashioned from bones of the animal being butchered. Evidence for butchery and bone tool use includes matching marks on the conarticular surfaces of disarticulated pairs of bones; cutmarks on bones; green bone fracturing; use wear, secondary flaking, and impact features on bone fragments; and burned bone. Interpretation of these features is facilitated by information on patterns of bone distribution and disarticulation preserved in a primary depositional context. Preliminary comparisons among nine sites indicate that putative butchering sites differ consistently and in a variety of ways from sites that appear to record no human involvement. Although based on a small sample of sites, the apparent frequency of butchered individuals relative to those that were not butchered is unexpectedly high. These findings provide new evidence of a well-developed “bone technology” employed by the late Pleistocene human inhabitants of eastern North America. In addition, these data offer circumstantial support for the hypothesis that human hunting was an important factor in the late Pleistocene extinction of mastodons.