It is somewhat dismaying, if not surprising, that so many trained biologists know that dinosaurs may have been “hot-blooded,” yet are completely unaware of even the existence of the mammal-like reptiles, the Therapsida. The reasons for the dearth of widespread knowledge of the therapsids are probably fairly mundane: the mammal-like reptiles are less spectacular in appearance than dinosaurs, and good therapsid fossil localities are restricted primarily to the southern hemisphere. As a result, therapsids seldom become the subjects of museum displays or classroom student presentations. Moreover, biology textbooks, whether at the high school or college level, almost invariably devote more space for recounting the “dinosaur story” than to discussing the therapsid reptiles (1). Consequently, although application of biological systems data to the fossil record has become increasingly popular and useful in the last decade, many neontologists who might potentially make substantial contributions to our knowledge of therapsid biology remain oblivious to the significance of these animals. This is particularly unfortunate—therapsids are, of course, the direct predecessors of mammals, and, as such, the more we understand the mammal-like reptiles the more likely we are to understand the origins and functions of our own anatomical, physiological, and behavioral systems.

The review of current developments in paleobotany by Knoll and Rothwell (1981) prompts some supplementary observations on the state of marine paleobotany and, more generally, the problem of investigating groups with a poor fossil record. The marine angiosperms and the larger noncalcareous algae are such groups, being represented by very few fossils widely scattered in time and space (den Hartog 1970; Parker and Dawson 1965). Tropical freshwater macrophytes are likewise poorly known as fossils (Sculthorpe 1967).

A diffusion model of the distribution of a phenotypic character in a group of species is developed and analyzed. The model incorporates the combined effects of phyletic evolution, speciation and extinction. Directed speciation is modeled by assuming there is some bias to phenotypic changes during speciation. Species selection is modeled by assuming there is some dependence of either speciation or extinction rates on the phenotypic character. Three examples are analyzed to illustrate the use of the model. A model of completely random changes due to both phyletic evolution and speciation shows how between-species differences are established. A model of directed speciation due to multiplicative changes during speciation shows how a simple assumption about the speciation process can produce macroevolutionary trends. A model of species selection due to differences in extinction rates shows how the efficacy of species selection depends on the between-species variance produced both by speciation and by phyletic evolution.

A simple theoretical argument shows that the evolutionary process will look gradual if it is considered on too fine or too coarse a time scale. There exists an intermediate time scale in which the distribution of evolutionary rates will appear as bimodal.

A theoretical approach to the analysis of historical factors (Raup 1972) in evolutionary morphology is presented which addresses transformational hypotheses about structural systems. This (structural) approach to testing historical hypotheses about phylogenetic constraints on form and function and structural and functional versatility involves (1) the reconstruction of nested sets of structural features in monophyletic taxa, (2) the use of general or emergent organizational properties of structural and functional systems (as opposed to uniquely derived morphological features), and (3) the comparative examination of the consequences for structural and functional diversity of these general features in related monophyletic taxa.

Three examples of emergent organizational properties are considered: structural complexity, repetition of parts, and the decoupling of primitively constrained systems. Two classes of hypotheses about the evolution of design are proposed. Transformational hypotheses concern historical pathways of change in form as a consequence of general organizational features which are primitive for a lineage. Relational hypotheses involve correlations between structure-function networks primitive for a clade and morphological diversity both between and within terminal taxa. To the extent that transformational and relational hypotheses about form are corroborated, they provide evidence of underlying regularity in the transformation of organic design that may be a consequence of the hierarchical organization of structural and functional patterns in organisms.

Patterns of dental and cranial variation are analyzed in a unique sample of early Eocene equid Hyracotherium from one fossil quarry of late Wasatchian age in the Huerfano Basin of southern Colorado. The sample includes remains of two species differing principally in size: H. vasacciense and H. tapirinum. The larger species, H. tapirinum, is represented by 24 individual specimens that show marked bimodality in cranial size and robustness and in upper and lower canine size. These differences are attributed to sexual dimorphism. Males tend to be about 15% larger than females in cranial dimensions and 40% larger than females in canine dimensions suggesting, by comparison with modern ungulates, that early Eocene horses were probably polygynous. The appearance of Hyracotherium in North America and its rise to dominance in early Eocene mammalian communities coincides with development of an environmental mosaic including open park woodland and savanna habitats. The evolutionary success of Hyracotherium may reflect, in part, the adaptive superiority of a social structure in which females in open habitats are grouped for mutual protection from predators, for optimal foraging in patchy environments, or both.

Systematics and paleontology have had a long conceptual relationship, united by the common goal of reconstructing the history of life. Yet, with few exceptions, paleontologists have had little input into formulating systematic theory and methodology. The reasons for this apparently relate to two conceptual-philosophical traditions of post-Darwinian paleontology: (1) the widespread adoption of a species concept in which taxa are viewed as nondiscrete, arbitrarily designated segments of evolutionary continua, and (2) the belief that phylogenetic reconstruction is primarily an empirical matter of tracing evolutionary change through the stratigraphic record.

Available systematic evidence supports the hypothesis that species are real, discrete units in space and time and that, unless they are postulated to be directly ancestral to another species, they can be defined by the possession of one or more evolutionary novelties (derived characters). Species beginnings are delineated by speciation (vicariance) events and their terminations by subsequent speciation events or by extinctions.

Natural groups are composed of taxa that have shared a common genealogical history. Cladistic analysis is a method to construct and test hypotheses of monophyly and thereby define natural groups. Cladistic hypotheses are necessary to investigate many of the major questions within contemporary paleobiology. Virtually no studies of evolutionary rates, patterns of taxonomic diversity, modes of taxic evolution, and patterns of morphological diversification can be undertaken without reference to cladistic hypotheses about the composition of natural groups.

Because paleobiology is historical in its content, paleontologists are greatly limited in their ability to use paleontological data to investigate questions about the evolutionary process. According to current evolutionary theory, the concepts of adaptation and natural selection relate to genetic and ecological processes that take place within local populations (microevolution). If so, then data relevant to examining these phenomena are likely to be lacking in paleontological samples. Consequently, explanations of paleontological pattern that include process-related concepts such as adaptation and natural selection are axiomatic in their logical structure and thus cannot be falsified or critically evaluated by that paleontological pattern.

The growth rate of Nautilus pompilius in its natural environment has been determined from radioactive disequilibrium between 210Pb (half-life 22.3 yr) and its granddaughter 210Po (half-life 138 d) in septa of two juvenile specimens. 210Pb and 210Po data from the most recently formed shell material of both specimens indicate that 210Pb from sea water is incorporated into septa during septal formation and 210Po is excluded. Therefore the 210Po/210Pb activity ratio serves as a chronometer to estimate the age of each septum and the time between formation of septa. In the specimens studied the average time between sucessive points in septal deposition is 75 d for the nine most recent septa of one specimen and 23 d for the six most recent septa of the other specimen. These different growth rates, if representative of the ontogeny of each animal, suggest that the timing of septal deposition probably is dependent on the rate of shell and tissue growth coupled with buoyancy requirements and is not a unique period for all Nautilus. The habitat and ontogeny of Nautilus may be inferred from the pattern of stable isotopes of oxygen and carbon in the septa. Both specimens show a pronounced break in δ18O from nearly uniform light values in the first seven septa to heavier values (∼1%) after the seventh septum. We interpret this break to correspond to the hatching of Nautilus. A temperature (i.e. water depth) interpretation of the δ18O data for septa after the eighth is complicated by a positive correlation between δ18O and δ13C. This may reflect horizontal migration of the animal or a kinetically controlled fractionation of carbon and oxygen isotopes during septal formation.

The chamber formation cycle in externally shelled, chambered cephalopods consists of mural ridge formation, secretion of the siphuncular connecting ring, septal calcification, and cameral liquid removal. Radiographic observation of the chamber formation cycle in specimens of Nautilus macromphalus allows direct observation of the various processes of the chamber formation cycle in a chambered cephalopod, and gives direct measures of rates. New chamber formation in N. macromphalus initiates when slightly more than half of the cameral liquid has been removed from the last formed chamber. At this volume, the liquid within the chamber drops from direct contact with the permeable connecting ring to a level where it is no longer in direct contact and must move onto the connecting ring due to wettable properties of the septal face and septal neck. This change from “coupled” to “decoupled” emptying coincides with the formation of a mural ridge at the rear of the body chamber, in front of the last formed septum. With completion of the mural ridge, the septal mantle moves forward from its position against the face of the last formed septum and attaches to the new mural ridge, where it begins calcifying a new septum in front of the newly created, liquid-filled space. Emptying of the new cameral liquid from this space commences when the calcifying septum has reached from one-third to two-thirds of its final thickness. The cessation of calcification of the septum coincides with a liquid volume in the new chamber of approximately 50%, at which point the cycle begins anew. During the chamber formation cycle apertural shell growth appears to be continuous. Since apertural shell growth is the prime factor leading to increased density in seawater, and hence decreased buoyancy, the period in the chamber formation cycle between the onset of septal calcification and the onset of emptying would be a time of greatly decreasing buoyancy. This is avoided by the removal of decoupled liquid from previously produced chambers. In this way constant neutral buoyancy is maintained. The time between chamber formation events in aquarium maintained N. macromphalus appears to be between 70 and 120 d.

Analysis of post-mortem buoyancy loss in Nautilus shells suggests that extensive nekroplanktonic drifting occurs infrequently. Most shells do not reach the surface but settle to the sea floor, after a short period of ascent. This occurs because the rate of water influx into the phragmocone due to ambient hydrostatic pressure is sufficiently rapid in most cases to overcome positive buoyancy before the shell reaches the surface. The resulting geographic distribution of Nautilus shells would therefore mirror the distribution of the live animals. Thus, post-mortem drift in Nautilus cannot be used as a basis for questioning the validity of cephalopod paleobiogeography. Estimate of influx rates in ammonoid siphuncles indicates that many, if not most, ammonoid shells also would not become nekroplanktonic. This is especially true for small (<5 cm diameter) shells. Cephalopod paleobiogeographic investigation appears less subject to criticism stemming from the supposed obfuscating effects of post-mortem drift than previously thought.

Physical models of six species of the Mesozoic bivalve genera Gryphaea, Exogyra, and Ilymatogyra and hollow hemispheres of comparable size were manufactured, the former from molds of fossil specimens. The survival (maintenance of a position within 1 cm of the sediment-water interface) of populations of these models on 3 sediment types in the lower intertidal of the Gulf of Mexico was monitored over an 18 mo period. Bulk sediment density had no detectable influence on the mean mass density (g cm−3) of the population of models which maintained position at the sediment-water interface. Increased intensity of sediment reworking strongly decreased the probability of survival of the models. Model shells took up orientations consistent with previous laboratory studies and observations of fossils preserved in situ. Some of the models were attacked and broken open by blue crabs and stone crabs, and crab predation was an important source of “mortality” of the models. These results are consistent with the hypothesis that increased sediment reworking and the evolution of durophagous predators in the Cretaceous caused the extinction of exogyrid and gryphaeid bivalves and the near-total elimination of the reclining mode of life in post-Cretaceous times.

The threespot damselfish, Eupomacentrus planifrons, is an algal-gardener and a major cause of biological disturbance on modern Caribbean coral reefs. The hermatypic coral Acropora cervicornis produces characteristic gall-like growths called “chimneys,” in response to damage by the threespot. Chimneys have been discovered on coral fragments excavated from the Jamaican Falmouth Formation, an Upper Pleistocene reef assemblage. This indicates that algal gardening, and thus biological disturbance, has been an integral aspect of Caribbean coral reefs since at least the last sea-level stand, some 125,000 yr BP.

Because predation by drilling gastropods is uniquely preservable in the fossil record, it represents important evidence for the study of coevolution. Previous studies of drilling gastropod predation have been largely descriptive and sometimes contradictory. We formulate and test a model of prey selection by naticid drilling gastropods. The model adequately predicts both prey species selection and prey size selection. Prey preferences parallel prey profitabilities, determined by calculating prey species-specific and predator size-specific cost-benefit functions. The model also specifically suggests the evolution of potential refugia from predation and the evolution of potential predatory attributes. Application of the model to several Miocene and Pliocene assemblages studied by Thomas (1976) corroborates the feasibility and utility of this approach in examining the evolutionary record of naticid predation, which extends from the Late Mesozoic. Apparent evolutionary stasis and convergent morphological trends among prey species may be consistent with continuous selection pressures against predation.

The view that interspecific competition with balanoid barnacles has caused the evolutionary decline of chthamaloids (Stanley and Newman 1980), is challenged on 3 grounds. First, as a phyletic stock, chthamaloid size, as indicated by the mean adult basal diameter of extant taxa, has been decreasing, probably since the end of the Cretaceous. Small organisms will lose in interspecific competition to larger ones. However small body size reduces the susceptibility to predation and various forms of disturbance, and therefore allows chthamaloids to inhabit intertidal areas denied to balanoids. Second, although many species of Chthamalus occupy a high intertidal “refuge,” there is little evidence that competition is the primary ecological force maintaining them there. Predation is the most significant influence in tropical areas. In many temperate zone communities characterized by intense invertebrate predation or physical disturbance, chthamaloids achieve their maximum density lower in the intertidal than do balanoids or coronuloids. Last, the fossil evidence supporting a decline in chthamaloids is missing. The difficulty of seeking a single mechanism to explain global, multispecific phenomena is discussed.

A number of facts indicate that the diversity of chthamaloid barnacles is much lower now than it has been in the past. Among these are the relict, often disjunct geographic distributions of distinctive chthamaloid higher taxa and their representation by only a few living species. The living chthamaloid barnacles also have a disjunct bathymetric distribution: as we will document here in detail, they are concentrated in the intertidal zone and in deep water. In his rejoinder to our paper, Paine makes no cogent argument against our interpretations of this powerful evidence.

Our case that the balanoids have competitively displaced the more primitive chthamaloids is based on several facts: (1) the Chthamaloidea have dwindled during the Cenozoic Era, so that most have relict or refugial distributions, whereas the Balanoidea have undergone a remarkable adaptive radiation; (2) balanoid species are known to defeat chthamaloid species in competition for space; (3) the competitive success of the balanoids can be attributed to an inherent biological feature—a tubiferous skeleton, which fosters rapid growth; (4) the peak diversity of living balanoids coincides precisely with the bathymetric gap in the chthamaloids’ distribution.

We argue that the radiation of balanoids during the past 40 Myr or so has caused a decline in chthamaloid diversity by reducing and destabilizing low intertidal and shallow subtidal populations, thus elevating average rate of extinction and depressing average rate of speciation. This model allows for exceptions to the rule of balanoid competitive dominance, but contrary to Paine's claim, the exceptions that exist are either trivial or support our view.

Paine offers no alternative to our competition hypothesis. He suggests that an increase in predation has caused a decline in chthamaloid body size, but neither a Cenozoic trend in body size nor one in predation pressure has empirical support; furthermore, the body-size hypothesis has no direct bearing on the case we have presented.