The lack of tools for teasing genetic information out of the fossil record has been a source of frustration to both neontologists and paleontologists, both of whom would like to know more about what was happening genetically in association with such evolutionarily significant events as cladogenesis and extinction. The dearth of such information has been a factor contributing to the current schism between population geneticists and paleontologists over interpretations of historical patterns of evolution (Gould and Eldredge 1977; Stanley 1979; Charlesworth et al. 1982; Levinton 1983; Turner 1983), a schism which seems unbridgeable without at least some insight into paleontological patterns of genetic variation. Several recent papers, however, have documented a negative correlation between the level of fluctuating asymmetry (small, random, right-left differences between otherwise bilaterally symmetrical characters) and the level of heterozygosity in a variety of organisms. Whatever its underlying cause may be, this correlation raises a flicker of hope that a tool may exist for inferring whether some populations of fossil taxa were more variable genetically than others.

Four basic types of skeletal concentrations are modeled in terms of changes in sedimentation rate alone. The model categorizes fossil concentrations on the relatively objective basis of their bed contacts, and uses this criterion to infer directional shifts in net sedimentation. This radical simplification of accumulation histories, in which hardpart input is held constant, yields a surprisingly powerful model capable of predicting a broad spectrum of taphonomic and paleobiologic phenomena. Type I concentrations grade from less fossiliferous sediments and terminate in omission surfaces; if hardpart supply is held constant, they record a slowdown from positive to zero net sedimentation. Type II concentrations are the same as Type I but terminate in erosion surfaces (slowdown to negative net sedimentation), and Type III and IV concentrations are characterized by basal erosion or omission surfaces, respectively, grade upward into less fossiliferous sediments, and record increases in net sedimentation from negative or zero rates to positive rates. According to the model, samples collected from successive horizons within any of these shell beds will differ in the degree and type of post-mortem bias owing to differing histories of hardpart exposure at the depositional interface. Moreover, because rates of sediment accumulation govern the abundance of hardparts at the depositional interface and thus many of the physical characteristics of the benthic habitat, the dynamics of fossil accumulation have direct consequences for the structure of benthic communities (taphonomic feedback) and for ecologically controlled species morphometry.

The model is highly robust to fluctuations in hardpart input, as judged by its ability to correctly infer modes of formation of concentrations in synthetic stratigraphic sections. In addition, field examples of Type I–IV concentrations show independent evidence of formation during intervals of reduced net sedimentation, and many exhibit trends in taphonomic and paleobiologic features expected from the postulated changes in net sedimentation. The model thus provides a testable working hypothesis for the accumulation of fossil material in a wide range of environments, and should be applicable to concentrations of any taxonomic composition, state of preservation, or geologic age. The power and robustness of this heuristic model in fact argue that fossil-rich and fossil-poor strata provide fundamentally different records of past conditions, and that sedimentation rather than hardpart input is the primary control on the nature of the fossil record.

Bones of mammals exhibit progressive stages of weathering during their time of subaerial exposure. Consequently, the study of bone weathering in fossil assemblages may help to assess the period represented by an accumulation of bones. Stages of bone decomposition due to subaerial weathering have been identified in assemblages of fossil macromammals from Olduvai Gorge, Tanzania. A modern bone assemblage collected by spotted hyenas is used to devise a method for recognizing attritional accumulations of bones from weathering characteristics. This method, which involves study of long bone diaphyses, is applied to Plio-Pleistocene faunal assemblages from Olduvai, 1.70–1.85 ma old. Previous work indicates that early hominids had an important role in the collection of fauna at five of the six sites studied. It is shown that animal bones were accumulated at each site over a period of probably 5–10 yr or more. The length of this period, along with other taphonomic evidence, suggests that the processes of bone aggregation at these sites differed from those at the short-term campsites of modern, tropical hunter-gatherers.

Symbiotic relationships involving physical contact between worms and solitary rugosan polyps are recorded by the following structures in North American Late Ordovician corals: (1) Trypanites borings enclosed within septal swellings in two specimens, (2) vermiform grooves and openings along the external wall of one corallum, and (3) a chamber containing a unique brown tube within one individual. These features are indicative, respectively, of commensal boring polychaete annelids that penetrated through coralla, commensal epizoic worms of unknown taxonomic affinity that attached to the side of a polyp, and a tubicolous worm (possibly a polychaete) that was likely a parasitic endozoan. Symbionts comparable to the latter two types are also known from two specimens of Devonian solitary rugose corals.

Indirect evidence suggests that symbioses between solitary rugosans and the worms that produced Trypanites borings as dwelling structures in the sides of coralla were relatively common. However, direct evidence that the hosts were alive has been found in only two corals. In both cases, worms bored through septa within the calices and came into contact with basal surfaces of the polyps, which secreted skeletal material that sealed off the intruders. The rarity of such structures suggests that the encounters were inadvertent. If boring worms favored upcurrent portions of objects in order to maximize feeding benefits and avoid sedimentation, their locations indicate that the concave sides of curved coralla faced toward prevailing currents when in life positions.

“Opportunistic” worms are known to have attached to the sides of polyps only in rare instances when the hosts became temporarily exposed as a result of accidents or abnormalities. This indicates that coralla normally served to shield polyps from colonization by nonboring epizoans.

Worms that apparently extended up through openings in basal surfaces of polyps likely obtained sustenance parasitically within the central cavities. They could have entered the hosts through their mouths, or via the calices when parts of the polyps detached from their coralla and contracted radially. The rarity of this type of relationship in solitary Rugosa suggests that the worms entered inadvertently.

Symbioses involving physical contact between worms and polyps seem to have been rare throughout the history of solitary rugose corals. Both groups apparently tolerated such associations when they did occur, although the rugosans secreted structures in their coralla that served to isolate the symbionts. In doing so, they recorded the presence of worms not likely to be preserved as body fossils. The interpretation of such features provides information on the physiology and ethology of both organisms, on the history of symbiotic relationships, and on the diversity of soft-bodied organisms in ancient environments.

A range of specialized burrowing sculptures evolved convergently in at least one lingulid and in several obolid and lingulasmatid genera. The sculptures in Lingula punctata (Ordovician) indicate a burrowing process with the pedicle trailing behind the shell, similar to that of Recent Lingulidae. In the Obolidae and Lingulasmatidae, the orientation of the sculptures indicates a burrowing process with the pedicle oriented downward in the sediment. A burrowing mechanism in which the valves alternated in functioning as anchors, without active participation of the pedicle, is proposed for these forms. Infaunal life habits for several obolid genera are further indicated by the shell morphology and by the distribution of repaired shell damage.

The burrowing Obolidae are likely to have been adapted to relatively high-energy environments, which required frequent reburrowing in addition to escape (upward) burrowing. The increase in bioturbation in these environments in the Middle Paleozoic may have contributed to their extinction. The Lingulidae, instead, being mostly adapted to extreme environmental conditions, could survive in marginal niches which were not intensively exploited by actively burrowing organisms.

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.

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.

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.