Specific conductance was calculated for secondary xylem in seven Carboniferous stem taxa utilizing an equation derived from the Hagen-Poiseuille relation. Arborescent and lianoid representatives of major pteridophytic (Calamitaceae, Lepidodenraceae, Sphenophyllaceae) and gymnospermous (Cordaitaceae, Medullosaceae) groups were examined. In the calamite Arthropitys communis and the seed plant Cordaites (Cordaixylon sp. and Mesoxylon sp.), conductance corresponded approximately to the low end of the range for both extant conifers and angiosperms. A substantially higher conductance was determined for the wood of Arthropitys deltoides, conforming to the high end of the range for conifers and the low-middle part of the range for angiosperms. The highest conductance values were found in Sphenophyllum plurifoliatum, Medullosa noei, and Paralycopodites brevifolius and corresponded to the middle-high portion of the range for vessel-containing angiosperms. This outcome is particularly significant in light of the fact that tracheary elements in the fossils are imperforate. The results indicate that conductance in secondary xylem of some of the most ancient, woody groups was comparable to that in extant plants and that highly effective conducting tissue developed relatively early in plant evolution. Moreover, it is suggested that the general relationship between wood anatomy, growth habit, and ecology demonstrated for living plants can also be extended back in time to include fossil plants.

The European Plio-Pleistocene Mimomys (Hintonia) occitanus-ostramosensis lineage of arvicolid rodents appears to present a genuine case of gradual evolution. Analysis of numerous dental samples recorded from 3 to 1.5 ma by several biometrical methods led to the following conclusions: samples from the whole geographical area order into a morphological continuum (chronomorphocline); the successive populations in the trend show important overlap of their variation; and the absence of stasis or any interruption in the trend, combined with its unidirectionality, suggests genuine phyletic gradualism. The chronomorphocline is characterized by an important increase in tooth hypsodonty, by the progressive and continuous development of lateral enamel tracks. Four phases of evolution can be recognized in the lineage, the rates of morphological changes being variable and related to climatic fluctuations (Reuverian, Pretiglian, Tiglian, Eburonian). Morphological changes are initiated by environmental fluctuations, although the polarity of climatic change is not a determining factor. Environmental changes act as stimulus and release drift processes. Phyletic gradualism occurring over such a wide area implies extensive gene flow, and suggests that only general, not local, environmental components act to produce the observed morphological changes.

The buoyancy, stability, and orientation of a shelled cephalopod in water are the predictable products of shell geometry, body chamber length, and such physical parameters as shell, tissue, and water densities. Given such physical characteristics as shell geometry, shell, tissue, and water densities, and shell thickness, the hydrostatic characteristics of planispiral shelled cephalopods, including orientation, centers of mass and buoyancy, stability, and neutrally buoyant body chamber length, can be calculated and simulated using microcomputer-based techniques. Individual variables such as geometry, body chamber length, and shell thickness are linked in a calculable manner to orientation, neutral buoyancy, and stability. Living Nautilus provides a means of testing the model and for making hydrostatic comparisons between ammonoids and nautiloids. The close agreement between calculated versus observed body chamber lengths in five species of Mississippian ammonoids shows that neutral buoyancy, and (with one exception) Nautilus-like orientations, were at least feasible for these species.

We have recently shown how capture-recapture models can be used in conjunction with stratigraphic range data to estimate taxonomic extinction rates and taxonomic diversity. Here we present a new method that can be used to estimate taxonomic turnover (defined here as the proportion of taxa extant at time i, that originated in the interval i – 1 to i). We used these methods in conjunction with stratigraphic range data for families in five phyla of Paleozoic marine invertebrates. We estimated fossil encounter probabilities, extinction rates, diversity, and turnover and used these estimates to test hypotheses about variation among phyla and geologic series. Encounter probabilities varied among taxa and showed evidence of a decrease over time for the geologic series examined. The number of families varied substantially among the five phyla and showed some evidence of an increase over the series examined. There was no evidence of variation in extinction probabilities among the phyla. Although there was evidence of temporal variation in extinction probabilities within phyla, there was no evidence of a linear decrease in extinction probabilities over time, as has been reported by others. We did find evidence of high extinction probabilities for the two intervals that had been identified by others as periods of mass extinction. We found no evidence of variation in turnover among the five phyla. There was evidence of temporal variation in turnover, with greater turnover occurring in the older series.

It has been shown in laboratory experiments that viruses can transfer DNA from the host cell to other cells and even to those of another species. In nature such transfers could facilitate convergent evolution. Studied cases of convergent evolution indicate that a gene might be successfully transferred to another species to produce a similar phenotypic effect as frequently as once per 1–10 ma and the effects of gene transfer on evolution in general are likely to be considerable. There are many constraints on the cases of convergent evolution that might be considered as possible examples of gene transfer. First, both co-occurrence in space and time of the taxa in question and presence of complete reproductive isolation must be assessed. Further, both the limited space available in a virus particle and the necessity that the transferred DNA functions well in both mitosis and meiosis in heterozygotic individuals indicate that only rather short pieces of DNA would be expected to be transferred. This limits the possible effects of any gene transfer. Examples of convergent evolution in five Siluro-Devonian conodont lineages are briefly presented and assessed in terms of possible mechanisms of convergence.

Under conditions of normal calcium metabolism, strontium/calcium ratios (Sr/Ca) have been shown to reflect the trophic level of contemporary and recent terrestrial fauna. These ratios therefore offer a potential means of studying fossil ecosystems and the diet of prehistoric humans. In cases in which suitable controls have demonstrated the preservation of biogenic Sr/Ca, it has been possible to investigate the proportionate importance of meat vs. vegetable foods in the diets of prehistoric humans. However, diagenetic change after interment has made it impossible to discern biogenic Sr/Ca in faunal and human skeletons over 15,000 y b.p. A procedure is investigated for the analysis of biogenic and diagenetic apatite in vertebrate fossils, on the basis of solubility differences among carbonate, hydroxy-, and fluorapatites. When applied to the 2 ma b.p. fauna of the Omo Basin (Ethiopia), distinct characterization of the herbivore, omnivore, and carnivore fauna in conformity with trophism was discerned, in spite of anomalous Sr/Ca of one highly specialized carnivore, Homotherium. Possible metabolic and/or taphonomic explanations of this anomaly are discussed, and future basic research into the solubility profile procedure is outlined.

The geometric regularity of oblique sculptures in pectinids and some other bivalves was examined both theoretically and empirically. Camptonectes microsculpture occurs exclusively on the external surface of foliated calcite, and is well characterized by an invariably orthogonal relation to growth increments and almost uniform density over the surface. It is genetically unrelated to radial and other sculpturing. According to our SEM observations, its divergent orientation seems to be controlled primarily by the growth of calcite laths. In contrast, the shagreen microsculpture of pectinids is characterized by a quincunx arrangement of scales on the diagonal network. Shagreen microsculpture can be regarded as a variant of diagonal sculpture together with the simple oblique, divaricate, V-shaped, and zigzag patterns that are widespread in other bivalves. The geometric regularity of these variations as well as their mutual relationship can be readily recognized by computer simulations. All of these diagonal sculptures may result from interference between radial and commarginal elements. The formation of diagonal sculptures seems to occur by the periodic activation of regularly arranged sculpture-producing cells on the mantle margin.

In a nonhierarchical world, where selection on organisms regulated all nonrandom evolutionary change, the traditional equation of selection (a cause of sorting) with sorting itself (differential birth and death among varying organisms within a population) would rarely lead to error, even though the phenomena are logically distinct (for sorting is a simple description of differential “success,” and selection a causal process). But in a hierarchical world, with entities acting as evolutionary individuals (genes, organisms, and species among them) at several levels of ascending inclusion, sorting among entities at one level has a great range of potential causes. Direct selection upon entities themselves is but one possibility among many. This paper discusses why hierarchy demands that sorting and selection be disentangled. It then presents and illustrates an expanded taxonomy of sorting for a hierarchical world. For each of three levels (genes, organisms, and species), we show how sorting can arise from selection at the focal level itself, and as a consequence either of downward causation from processes acting on individuals at higher levels or upward causation from lower levels. We then discuss how hierarchy might illuminate a range of evolutionary questions based on both the logical structure of hierarchy and the historical pathways of its construction—for hierarchy is a property of nature, not only a conceptual scheme for organization.

The extinction of a species represents reduction of both geographic range and population size to zero. Most workers have focused on geographic range as a variable strongly affecting the vulnerability of established species to extinction, but Lyellian percentages for Neogene bivalve faunas of California and Japan suggest that population size is a more important variable along continental shelves. The data employed to reach this conclusion are Lyellian percentages for latest Pliocene (∼2 ma old) bivalve faunas of California and Japan (N = 245 species). These regions did not suffer heavy extinction during the recent Ice Age, and for each region the Lyellian percentage is 70%–71%.

Discrepancies in population size appear to explain the following differences in survivorship to the Recent (Lyellian percentage) for three pairs of subgroups: (1) burrowing nonsiphonate species (42%) versus burrowing siphonate species (84%), which suffer less heavy predation; (2) burrowing nonsiphonate species of small size (73%) versus burrowing nonsiphonate species of large body size (96%); (3) Pectinacea (30%) versus other epifauna (71%), which suffer less heavy predation. During the Mesozoic Era, when predation was less effective in benthic settings, mean species duration for the Pectinacea was much greater (∼20 ma).

Along the west coast of North and Central America, mean geographic range is greater for siphonate species of large body size than for siphonate species of small body size and greater still for pectinacean species. These ranges are inversely related to mean species longevity for the three groups, which indicates that geographic range is not of first-order importance in influencing species longevity. Species with nonplanktotrophic development neither exhibit narrow geographic ranges along the west coast of North and Central America nor have experienced high rates of extinction in California and Japan.

Rates of extinction are so high for Neogene pectinaceans and nonsiphonate burrowers that without enjoying high rates of speciation these groups could not exist at the diversities they have maintained during the Neogene Period. They are apparently speciating rapidly because of the fission effect: the relatively frequent generation of new species from populations that are fragmented by heavy predation. Thus, ironically, there may be a tendency for high rates of speciation to be approximately offset by high rates of extinction. Only if mean population size for species in a particular group becomes extremely small is it likely to result in a high rate of extinction and a low rate of speciation—and hence a dramatic decline of the group. The fission effect may contribute to the general correlation in the animal world between rate of speciation and rate of extinction.

Wolpoff's (1984) recent discussion of evolutionary rates in Homo erectus deserves careful study. Whether Homo erectus or other hominid species exhibit gradual, continuous change in key characters or whether instead there is evidence for morphological stasis in the fossil record is an important question. By allocating all Homo erectus specimens to three groups of early, intermediate, and later geological age and by comparing group means for 13 measurements, Wolpoff attempts to show that gradualism is the rule for this mid-Pleistocene taxon. This method is straightforward, but it is crucial that the samples be composed in a manner which is biologically reasonable. I argue here that Wolpoff has not done this. While there may be legitimate doubt concerning sorting of the fossils, especially where specimens are incomplete, several of the individuals said to be representative of Homo erectus are simply inappropriate for use in this analysis. Wolpoff insists that he has employed a “conservative” definition of the species, but instead he has measured everything in sight. This approach to the record does influence his results.

Standard techniques of energy exchange analysis are applied to modelling the thermal regimes of several species of sailback pelycosaurs. Of particular interest is the role played by the sail in thermoregulation. Although the sail did increase the rate at which the model could warm up in the morning, its effectiveness fell short of previous estimates. The sail increased the daytime internal temperature by typically 3°C to 6°C. The more massive subjects had the highest temperature increases owing partly to their higher thermal inertia and partly to their disproportionately large sails. The sail had no impact upon the nocturnal minimums in temperature.

Overheating does not appear to have been a significant problem in the Permian environment, particularly in view of the high thermal inertia of the subjects modeled. When overheating occurred, the sail was of limited value for dumping excess heat.

Histological examination of Stegosaurus dorsal bony plates shows a thin wall of incompletely remodeled bone surrounding a large cancellous region containing some large “pipes” delineated by thin bony walls. Growth proceeded mainly from the basal region of the plates. Histological observations provide a test against which several functional interpretations of the plates are checked. While some interpretations (e.g., plates as armor) are clearly falsified, others (linked to thermoregulatory functions) seem to be more robust, although by no means can they be viewed as clearly vindicated on the basis of the evidence presently available.

Focusing on the question of trends in Homo erectus cranial capacity evolution, Rightmire contends to have reanalyzed the data and demonstrated “a pattern of change … quite different from that described by Wolpoff.” Accepting the procedures used in this earlier analysis (Wolpoff 1984), he does this by changing the composition of the Homo erectus sample.

A comparison of the morphology and wear of the chewing surfaces among archaic Paleocene herbivores suggests that a compressive type of interaction between opposing chewing surfaces was becoming a dominant mode of chewing, unlike the translative (grinding or shearing) mechanisms common in Eocene and later herbivores. In part, the evidence for this conclusion is that most of the wear occurs on horizontal surfaces centered at the tips of the cusps and pronounced unidirectional striae tend to be lacking in these areas. In the diverse early Paleocene periptychid condylarths, multidirectional striae occur on the wear surfaces and there is a measured loss of directionality of the crests and enamel edges from the condition in the earliest condylarth, the Late Cretaceous Protungulatum. The patterns are even more random in the later Paleocene phenacodontids, suggesting that this was a broad characteristic for the larger Paleocene condylarths.

Efficiency (that is, the volume of food processed per stroke) in compressional chewing systems is theoretically proportional to the area of the opposing surfaces, with large, relatively flat surfaces optimal, whereas in translational systems linear structures are dominant. The reduction in alignment of striae and in linearity of topographic features of the chewing surfaces is consistent with an increasing dominance of compressive chewing from the condition characteristic of palaeoryctid and probably other Cretaceous insectivores in which both compression and shearing occurred between opposing surfaces. The change was brought about by expanding surface areas at the expense of edge length, which was accomplished independently and differently by the phenacodontids and the periptychids, one by increasing both the cylindrical shape and size of the cusps, the other by increasing the cylindrical shape and converging the cusps. The retention of large paraconules and metaconules in the upper molars, cusps essentially lost in the periptychids, preadapted the phenacodontid pattern for later development of the oblique lophodonty and translatory chewing component characteristic of perissodactyls (horses and their relatives), the dominant herbivores of the Eocene.