Functionally significant aspects of skull morphology were examined in Eocene miacids and in early members of the modern carnivore families to see if functional craniology might shed light on factors involved in the origin and early evolution of the modern carnivores. No key innovations are apparent at the beginning of the modern carnivore radiation, and an alternative hypothesis to account for that radiation is proposed: the radiation represents the filling of niches vacated by the extinction of several groups of archaic carnivores in the late Eocene. Differences that distinguish modern viverrids, canids, felids, and mustelids from each other today were less pronounced in the Oligocene, when these families first appeared, and provide no insights into possible ecological differences at the family level. However, body size differences among the early members of the modern carnivore families suggest that partitioning of prey resources by size may have been a factor in their initial radiation. Comparison of cranial morphology in Eocene miacids, early members of the modern carnivore families, and living carnivores allows reconstruction of the primitive conditions of carnivore skull morphology and determination of the pathways of morphological transformation that resulted in the diversity of skull morphology seen in modern carnivores.

The founders of North American vertebrate paleontology, F. V. Hayden, Joseph Leidy, E. D. Cope, O. C. Marsh, and their colleagues, collected and described the first suites of fossil mammals obtained from the rich Tertiary successions of the western United States. Among them were remains of fossil horses, and subsequent study of these resulted in an interpretation that supported the concept of Darwinian gradualism as the major mode of evolution. The fossil record of horses also contributed importantly to the demise of orthogenesis as an evolutionary pattern, and to the evaluation of evolutionary rates and long-term evolutionary trends in a major phyletic group of organisms.

Adaptation has been defined and recognized by two different criteria: historical genesis (features built by natural selection for their present role) and current utility (features now enhancing fitness no matter how they arose). Biologists have often failed to recognize the potential confusion between these different definitions because we have tended to view natural selection as so dominant among evolutionary mechanisms that historical process and current product become one. Yet if many features of organisms are non-adapted, but available for useful cooptation in descendants, then an important concept has no name in our lexicon (and unnamed ideas generally remain unconsidered): features that now enhance fitness but were not built by natural selection for their current role. We propose that such features be called exaptations and that adaptation be restricted, as Darwin suggested, to features built by selection for their current role. We present several examples of exaptation, indicating where a failure to conceptualize such an idea limited the range of hypotheses previously available. We explore several consequences of exaptation and propose a terminological solution to the problem of preadaptation.

Brinkmann (1929) argued that phyletic evolution in the Jurassic ammonite Kosmoceras is sufficiently linear that the amount of time missing at diastems can be estimated by the magnitude of morphologic jumps at those horizons. Brinkmann's unpublished raw data have been re-analyzed in order to evaluate the association between evolutionary jumps and physical breaks. All horizons in Brinkmann's 1400 centimeter stratigraphic section have been tested for morphological jumps. Up to 40 statistically significant morphological jumps exist in the section, the actual number depending on the sample interval used. A few of these correspond to the jumps found by Brinkmann and a few coincide with diastems. However, the correspondence between morphological jumps and physical breaks is not as consistent as was thought by Brinkmann. Furthermore, it is argued that the association between physical and biological discontinuities is not dependent on linear evolution but rather is the expected condition wherever phyletic evolution occurs. Due to the irregularity and unpredictability of evolutionary change in Kosmoceras, morphologic jumps cannot be used to assess the amount of time missing at diastems.

Few would argue with the assertion that within the practice of comparative morphology, the comparison of shapes is fundamental to any line of synthesis in paleontology. Yet the common act of such a comparison involves a great range of judgments concerning the homology, correspondence and divergence of form of various parts of any two related shapes. Considerations of similarity must include the scalar effects of size and material, the conversion of proportions as descriptive geometric dimensions to those useful numerical values that will be represented as ratios of measured distance (relative or compared against a standard), and, by no means insignificant, the judgment about the design or functional geometry implicit in the static as well as dynamic systems being measured. This review is concerned with these considerations and with quantitative comparisons of biological shape wherein some parts of specimens obviously have changed and others parts have not.

Using computer simulations and a quantitative method for describing bifurcating structures, the morphology of branching patterns seen in early land plants is analyzed. Four types or models of random branching (regular, geometric, binomial, and poisson) are shown to adequately describe the range of observed branching in most early land plants. Approximately 57% of all randomly generated computer patterns show reiterative branching events (=three successive identical modes of branching). Artificial canalization of reiterative events results in branching patterns structurally analogous with that of ancient fossil plants. Simulated phylogenetic changes among early land plant lineages, based on parsimonious transitions in branching patterns, indicate that most observed trends can be related directly to those seen in randomly generating branching patterns in which “size” is increased. The trimerophyte to progymnosperm trend in changing branching patterns is an exception, since the binomial model describing the progymnosperms has not been simulated by random processes.

While the apparent phylogenetic changes among early land plant groups do not require deterministic explanations, the transition from regular to geometric branching and the “canalization” of reiterative branching patterns may represent a grade level response to selective pressures related to mechanical design and vegetative reproduction.

Barstovian (medial Miocene) mammalian faunas from the Texas Gulf Coastal Plain contained four apparently sympatric species of rhinoceroses: the common forms Aphelops megalodus and Teleoceras medicornutus, a dwarf Teleoceras, and a dwarf Peraceras. Previous work has suggested positive allometry in tooth area with respect to body size in several groups of mammals, i.e., larger mammals have relatively more tooth area. However, dwarfing lineages were shown to have relatively more tooth area for their body size. Our data show no significant allometry in post-canine tooth area of either artiodactyls or ceratomorphs. Similarly, dwarf rhinoceroses and hippopotami show no more tooth area than would be predicted for their size. Limbs are proportionately longer and more robust in larger living ceratomorphs (rhinos and tapirs) than predicted by previous authors. Limb proportions of both dwarf rhinoceroses and dwarf hippopotami are even more robust than in their living relatives.

The high rhinoceros diversity reflects the overall high diversity of Barstovian faunas from the Texas Gulf Coastal Plain. The first appearance of several High Plains mammals in these faunas indicates “ecotone”-like conditions as faunal composition changed. Study of living continental dwarfs shows that there is commonly an ecological separation between browsing forest dwarfs and their larger forebears, which are frequently savannah grazers. This suggests that the dwarf rhinoceroses might have been forest browsers which were sympatric with the larger grazing rhinos of the High Plains during the Barstovian invasion. The continental dwarf model also suggests that insular dwarfism may be explained by the browsing food resources that predominate on islands.

Calibrating levels of time resolution that are accessible in the fossil record is important in understanding what evolutionary phenomena can be profitably studied using fossils. A model for attritional bone assemblage formation in fluvial deposits, based on observations of taphonomic processes in modern environments, provides order-of-magnitude estimates for time intervals represented in single unit, ‘contemporaneous' vertebrate samples. In order to form units with adequate material for analysis of morphological variation or paleoecological associations, it appears that bones must be spatially concentrated or stratigraphically condensed by sedimentary processes or biological agencies. In many cases this means that significant periods of time will be represented by single unit assemblages. According to predictions from modern environments, carcasses contributed through normal attrition can accumulate in the soil to ‘fossiliferous' densities over time intervals of 102–104 yrs. Attritional channel assemblages include bones from three sources: floodplain land surfaces, floodplain deposits, and the active channel, and represent time intervals on the order of 102–104 yrs. Given additional limitations on the composition of the fossil sample imposed by circumstances of preservation, outcrop availability and collecting strategy, attritional fluvial assemblages probably can be resolved only to 103 years even under the best conditions. Time intervals represented by fossils are not necessarily the same as those represented by sedimentary events in fluvial systems because bones can continue to accumulate and may be concentrated during times of erosion or non-deposition. Fluvial vertebrate assemblages of comparable taphonomic history can be used to document evolutionary changes over periods longer than their finest level of time resolution. While they may not be applicable to questions of punctuated or gradual transitions over shorter time scales, the longer-term patterns should have their own evolutionary significance.

Colonies of the fossil bryozoan Archimedes and erect, spiralled species of the living bryozoan Bugula consist of wedge-shaped systems of radially diverging, bifurcated branches that extend from a helical axial margin. Morphology of these colonies may be simulated using few growth rules. These include 1) radius of the central helical margin (RAD), 2) rate of climb of central helical margin (ELEV), 3) radial angle between successive branches that originate from the central helical margin (ANG), 4) radial growth of all branches, 5) angle between branches and axis of central helical margin (BWANG), 6) distance between three adjacent, radially diverging branches at which the central branch bifurcates into two branches equally spaced between the two side branches (XMIN), and 7) placement of a spacing bar at base of newly bifurcated branches. In addition, size constraints on the simulations must be stipulated.

Simulations are begun at a proximal point along the central helix where a radial branch originates and are “grown” in repetitive steps by extending the central helical margin a distance distally, determined by ANG, then censusing established branches for XMIN in order to bifurcate appropriate branches and extend others in several short growth increments, etc. Growth of branches ceases at stipulated maximum width, and growth of the entire simulation ceases at stipulated maximum height.

The presence of a helical inner margin marked by uniformly spaced bifurcations generates the spiralled shape, i.e. ELEV must be a positive number. Values of RAD, ELEV (not zero), ANG and BWANG determine form of the spiral; the other growth rules apply to bifurcated unilaminate branch systems in general.

The range of observed colony forms and hypothetical morphospace of Archimedes may be simulated by varying BWANG, XMIN, and ELEV. RAD was kept constant, as its variation would be redundant with XMIN and ELEV. Variation in ANG affects near-helix morphology, but its influence is undetectable beyond this zone. Variability within colonies may be simulated by assigning to each variable a standard deviation with mean-centered randomly chosen values for each decision.

Fine-scale sampling and analysis of fossiliferous sequences have been used in the debate over gradual vs. punctuated evolutionary transitions between species. The time scale and completeness of these studied sequences can be evaluated using criteria based on compilations of sedimentation rates over different time spans. A set of procedures, herein termed “resolution analysis,” provides the means for estimating the time scale and quality of sequences from which evolutionary patterns are distilled. Seven such published studies are evaluated with these procedures. In general, most fossil sequences are too incomplete on fine time scales to show changes operating within a standing population. Short segments of some sequences have the potential to document nearly complete morphological histories on time scales approaching generation-to-generation processes. Resolution analysis is a necessary step in inferences regarding fossil evidence of evolutionary tempo and mode.

Stratigraphic incompleteness necessarily results from the episodic nature of sedimentation. Many stratigraphic gaps result from minor, temporary shifts in sediment distribution, though other, more profound gaps result from changes in habitat conditions that must have had an effect on local biotic distribution. Analyzing paleontological patterns involves not only the positive evidence provided by fossiliferous strata, but also the negative evidence left as gaps by shifting habitat conditions. Thus, incomplete sequences may not be “flawed” records of continuous populations. Incomplete sequences may be a faithful, literal record of separate populations separated in time by local extinctions and re-invasions.

It is shown that the abundant Jurassic oyster Gryphaea provides material adequate to test biogeographic and evolutionary hypotheses involving speciation. Phyletic size increase is widespread if not characteristic and tends to be marked by a sharp initial increase followed by a longer period of stability. The larger size of phyletically younger forms is apparently because of greater age of individuals rather than a higher growth rate. Species durations fall within the range of the majority of Jurassic bivalve species. The detailed study of European Gryphaea indicates a pattern of punctuated equilibria allied with morphological trends, some of which are paedomorphic. Gradualistic, species selection and cladistic models of change are not supported. World-wide analysis of Gryphaea species supports centres of origin and migration rather than vicariance, and a relationship exists between migrational and evolutionary events and changes in the physical environment.

Field and laboratory data indicate that all four species of Thais (Gastropoda, Muricacea) from rocky shores of the northeastern Pacific selectively attack barnacles at the margins of parietal (lateral) and opercular plates. Attacks are also more likely to be successful at plate margins. Such preferential attack and differential attack success may account for the evolutionary reduction in the number of parietal plates exhibited within seven of the eight families of balanomorph barnacles. Additional evidence suggests that predation by drilling gastropods may have also favored the evolution of strong external ribbing on these plates within some balanomorph lineages.

Radiation and plate reduction in the Balanomorpha occurred in concert with extensive radiation in muricacean gastropods, starting in the Late Cretaceous. Of the three skeletal conformations exhibited by open-surface dwelling barnacles (8, 6 and 4 parietal plates), only genera of the four-plated form have become proportionally more common in the Recent. Further, in a rather striking evolutionary experiment, a lineage of barnacles evolving free from gastropod predation retained primitive skeletal features. This supports the interpretation that predation by drilling gastropods has been an important selective force behind the widespread, parallel evolutionary tendency towards plate reduction in the Balanomorpha; a trend recognized by Darwin (1854), but for which he offered no explanation.

Alleles, individuals, and species are all examples of entities possessing variation in the properties that underlie natural selection: branching (reproduction), persistence (survivorship), and heritability of characters. This suggests that the logic embodied in the theory of natural selection can be abstracted from its usual application to the level of individuals to encompass selection operating among any biological entities for which these essential properties can be meaningfully defined. This approach leads to a unified perspective of adaptation, selection, and fitness at all levels. Expanded versions of the Price covariance selection equations provide a convenient and useful conceptual vehicle for this discussion. The advantages of a hierarchical approach are twofold: it permits exploration of concepts and ideas across levels by analogy, and it focuses attention upon the mechanisms that account for different evolutionary dynamics at each level rather than obscuring these biologically unique properties with argument by extension from a single “special” level.

We point out that the choice of a single measure of evolutionary change restricts the context in which “other level” processes will be perceived. We illustrate the limited forms in which higher and lower level selection can be recognized from the unique perspective provided by any given level through extensions of Price's formula.

An exploration of the implications of such an approach leads us to the assertion that the development of a unified theory of evolution demands the recognition and incorporation of hierarchical structure as a conceptual foundation.

Paleoecologists studying Paleozoic reef-builders have interpreted their growth forms as responses to conditions of depth and turbulence in reef complexes. Comparison of the shapes of Paleozoic stromatoporoids and corals with the growth forms of modern scleractinians has been used to reconstruct Paleozoic conditions. A review of shape zonation on modern reefs indicates that no general pattern is applicable to all reefs and variations in shape are the result of the interaction of many environmental factors with the genetically dictated growth pattern of the coral. In most zones of a reef a wide range of shapes co-exist. The growth forms of corals on modern reefs are not a simple vegetative response to the many environmental parameters that have been shown to influence form, but are constrained by phylogenetic and developmental influences as well as functional ones.

Interpretations of the environments of western Canadian and other mid-Paleozoic reefs have been based on the growth forms of stromatoporoids. The environmental significance of the shapes has been deduced from comparison with the shapes of modern scleractinians, functional morphology, nature of the enclosing sediment, position of growth, position within the reef, and diversity gradients. The validity of these criteria is open to question and considerable doubt remains concerning the significance of the growth forms. The shapes of reef animals are not specific guides to environments of modern reefs and should not be expected to be guides for ancient ones.

Species diversity is a product of both biological interactions and physical-geographic factors. A model which eliminates biological interactions as a determinant of diversity can be used to assess the potential impact of physical and geographic factors operating alone on temporal trends in diversity (succession) and on spatial differences along latitudinal or environmental gradients. If a biologically neutral model can generate diversity patterns similar to those we observe, it can be argued that traditional hypotheses which depend on biological interactions are not the most parsimonious explanations for diversity patterns. If the neutral model approach supports a spectrum of possible hypotheses, perhaps the entire concept of species diversity can be discarded as a useful measure of community structure.

An earlier analysis of this type supported the notion that diversity patterns can be adequately explained by physical disturbance, global geography gradients, and stochastic immigration and extinction events. Our neutral model analysis does not affirm this view. We argue that the coarse level at which simple colonization models operate does not permit strong conclusions regarding the underlying mechanisms. Detailed community analyses may provide the best evidence for the nature of structuring processes. Based on the evidence available in the literature, species diversity is likely to be generated by a small number of biological processes interacting with physical-geographic gradients.

The function of lunules in sand dollars is reviewed and it is argued that ambulacral lunules and the anal lunule evolved for quite different purposes. Whereas ambulacral lunules evolved to increase the perimeter of the test for food gathering, this was clearly not the original function of the anal lunule, although in some genera it has become secondarily adapted to this role. Many of the unique features of the anal lunule can be explained if it evolved as an outlet for ciliary feeding currents drawn centripetally into the mouth, without disrupting either oral or aboral currents. This allowed the entire margin of the test to be used for collecting the fine particles sieved through the aboral spine canopy. Anal lunules probably evolved only once, in the Miocene, and the Monophorasteridae and Mellitidae are probably sister groups.

A model is proposed, based on examples that have been interpreted as phylogenetic trends, to explain how directional morphological evolution at the species level can arise by heterochrony. The examples illustrated are of Tertiary to Recent rhynchonellide brachiopods, Cambrian olenellid trilobites, living spatangoid echinoids, Tertiary to Recent schizasterid echinoids, Cenomanian ammonites and Silurian monograptids. Morphological discontinuities between species along morphological gradients (which can be recognised both spatially and/or temporally), and temporal morphological stasis within species, are both consistent with the punctuated equilibria model of macroevolution. It is argued that morphological discontinuities have arisen by selection of morphological novelties produced by heterochronic processes. These novelties are preadaptations which allow ecological and, consequently, genetic isolation from ancestral species. Establishment of a heterochronic morphological gradient is only possible given a suitable environmental gradient. The terms “paedomorphocline” and “peramorphocline” are proposed for these heterochronic morphological gradients. Paedomorphoclines and peramorphoclines each comprise a number of species occupying a series of adaptive peaks, which have evolved sequentially through time by selection along an environmental gradient.

A survey was undertaken of the developmental types of the neogastropod families Volutidae, Fasciolariidae, Nassariidae, Mitridae, Buccinidae and Olividae in the Palaeocene and Eocene series of the Gulf of Mexico. Although the families were dominated by planktotrophic development in the Paleocene, most show a general increase in numbers of species with nonplanktotrophic development through the Eocene. Present day environmental correlations with larval development do not adequately explain the observed shift in developmental types in the Early Tertiary. It is proposed that higher speciation rates for nonplanktotrophs and a unidirectional trend in developmental change may account for the progressive increase of nonplanktotrophic development.