Evolutionary “dwarfism” or “nanism” is the phenomenon in which a significant decrease in mean body size of a lineage (an ancestral-descendant sequence of populations) occurs through time. A detailed analysis of several Late Quaternary dwarfed marsupial lineages from Australia is given. Based on linear tooth dimensions of the dwarfed lineages, four points are considered: 1) percent dwarfing; 2) evolutionary rates of change of tooth dimensions; 3) variation within lineages before and after dwarfing; and 4) several aspects of multivariate dental allometry. [The lineages include Macropus titan (fossil)—M. giganteus (Recent) and Osphranter cooperi (fossil)—O. robustus (Recent) in the family Macropodidae (kangaroos), and Sarcophilus laniarius (fossil)—S. harrisii (Recent) in the family Dasyuridae (Tasmaniandevil).]

Dental measurements led to these conclusions: 1) Species with the largest body size show the greatest size reduction, and the species with the smallest body size change the least. 2) Evolutionary rates for this reduction in Australian lineages are similar to comparable Post-Pleistocene dwarfed lineages in Europe. 3) Tooth width, especially posterior width, changes more rapidly than length. The first molar changes relatively slowly, especially in length. 4) Variability is higher in the dwarfed forms than in the larger ancestors. 5) Multivariate allometric rates of dwarfing are consistent with results for rates of change calculated in darwins for the relation between change in length and maximum width (and less so for the relation between M1 and M4 reduction). This pattern of dwarfing allometry is broadly similar to within-species allometry, and is quite dissimilar to synchronous interspecific allometry.

Brief consideration is also given to taxonomy of dwarfing lineages and to problems of concurrent megafaunal extinctions. It is concluded that dwarfism is an adaptive process which is probably the result of a density-dependent factor(s) (i.e. a resource limited system).

The origin and evolutionary histories of the two most abundant and diverse genera of invertebrates (Prosserella: Brachiopoda and Syringostroma: Stromatoporoidea) in the Detroit River Group and associated rocks in the vicinity of the Michigan Basin appear to support the models of either allopatric speciation and punctuated equilibria (Eldredge and Gould, 1972) or quantum evolution (Simpson, 1944; 1953). Four morphotypes of Prosserella arose almost simultaneously (a “burst”) just below the base of the Detroit River in sandstone deposited near the axis of the Findlay Arch and persisted without evidence of significant progressive evolutionary change (a “trend”) until their almost simultaneous extinction by lineage termination near the top of the Detroit River. Neither ancestors nor descendants of Prosserella have been recognized and even the familial placement of the genus is uncertain. The genus, its species and morphotypes probably arose by means of very profound genetic or chromosomal “revolutions” that probably took place in small allopatric populations; each population quickly increased in abundance and geographic range and persisted without further significant morphologic modification until its extinction.

S. ristigouchense from the Lower Devonian of New Brunswick is the probable ancestor to six (or seven) species of Syringostroma that appear almost simultaneously (another “burst”) in lower Detroit River carbonate rocks deposited near the eastern margin of the Michigan Basin. Two of these new species are known only from reefs of early Detroit River age (a “crash”), four species persist without significant morphological change to at least the end of Detroit River deposition, and one of these was the probable ancestor to yet another newly evolved species that is abundantly represented in the conformably overlying Columbus Limestone.

The “founding fathers” of chronostratigraphy were pre-Darwinian and based their concepts and methods on assemblages of co-occurring taxa of unknown phylogenetic relations (assemblage-zones, concurrent range-zones and Oppel-zones) rather than on the range-zones of successional species in the same phylogenetic lineage (lineage-zones). Assumptions of these early biostratigraphers concerning the temporal relations of taxa are in close accord with the premises of punctuated equilibria and quantum evolution. The development of the chronostratigraphic system during the 19th century attests to the success of these early methods which depend for their precision on the number of taxa used and determination of the significance of morphologic differences among these taxa and their geographic and stratigraphic distributions.

A simple equilibrial model for the growth and maintenance of Phanerozoic global marine taxonomic diversity can be constructed from considerations of the behavior of origination and extinction rates with respect to diversity. An initial postulate that total rate of diversification is proportional to number of taxa extant leads to an exponential model for early phases of diversification. This model appears to describe adequately the “explosive” diversification of known metazoan orders across the Precambrian-Cambrian Boundary, suggesting that no special event, other than the initial appearance of Metazoa, is necessary to explain this phenomenon. As numbers of taxa increase, the rate of diversification should become “diversity dependent.” Ecological factors should cause the per taxon rate of origination to decline and the per taxon rate of extinction to increase. If these relationships are modeled as simple linear functions, a logistic description of the behavior of taxonomic diversity through time results. This model appears remarkably consistent with the known pattern of Phanerozoic marine ordinal diversity as a whole. Analysis of observed rates of ordinal origination also indicates these are to a large extent diversity dependent; however, diversity dependence is not immediately evident in rates of ordinal extinction. Possible explanations for this pattern are derived from considerations of the size of higher taxa and from simulations of their diversification. These suggest that both the standing diversity and the pattern of origination of orders may adequately reflect the behavior of species diversity through time; however, correspondence between rates of ordinal and species extinction may deteriorate with progressive loss of information resulting from incomplete sampling of the fossil record.

Cohort analysis provides an effective method of analysing taxonomic survivorship in the fossil record where large data sets are available. An analysis of the stratigraphic ranges of about 8,500 fossil genera and subgenera shows that survivorship patterns are substantially the same throughout the Phanerozoic. These patterns are used to calculate an average value for mean species duration among fossil invertebrates (11.1 Myr.). Also, the extra extinctions near the Permo-Triassic boundary are shown to be equivalent to about 85 Myr of normal, background extinction.

Both intensive analysis of Gryphaea and an extensive survey of Jurassic bivalves in Europe supports the punctuated equilibria model of speciation, with the exception of phyletic size increase, which appears within the limits of the data to be gradualistic and affects at least a significant minority of species. The size increase takes place both within species and between successive species in a lineage. In Liassic Gryphaea, a combination of allometry in the ontogenetic development of the ancestral species and paedomorphosis led also to changes of shape up the sequence. The production of evolutionary trends by species selection is not supported.

The favoured interpretation of phyletic size increase is a gradual delay in maturation time consequent upon a change in the organisms' adaptive strategy from the r-selected to the K-selected mode and is backed up by evidence on changing population numbers and distribution. An extinction/speciation model is proposed for neritic organisms based on fluctuating sea levels. Times of low sea level or regression correspond with times of high stress and hence high extinction among stenotopic organisms and increased rates of allopatric speciation, with r-selection as the dominant mode. Times of high sea level or transgression correspond with low extinction and speciation rates and increased freedom of migration. K-selection is the dominant mode and often leads to phyletic size increase provided the environment remains stable for a sufficiently long period.

Punctuated equilibrium and phyletic gradualism are alternative hypotheses that purport to explain the tempo and mode of evolution. We evaluate the two hypotheses, as they apply to the fossil record, on both theoretical and empirical grounds. Hidden randomness in data increases as a function of greater aggregation, and the hypothesis of punctuated equilibrium should not be applied to those examples where randomness is likely to occur. False stasis can result from a sustained pattern of emigration and immigration, and geographic variation must be studied in order to posit an unambiguous case of punctuated equilibrium. We describe a statistical method based on the general linear model for testing the relative fit of the alternative hypotheses to any set of temporally ordered metric data. Our method is hierarchical in the sense that subsets of the total explained variance can themselves be explained. The size of the first molar of the primate Pelycodus and of the condylarth Hyopsodus are analyzed. There are 17 tests in the two data sets, and we discover 12 instances of gradualism, four of punctuation and one of equilibrium.

Given estimates of the variation in total standing species richness through the periods of the Phanerozoic, mean species duration, and the relative intensity of the sampling of the fauna from each of the periods, the expected number of described species can be predicted for each period of the Phanerozoic using an analytic sampling model. This model is based on the assumption that the relative abundances of species in any geologic period can be approximated by the canonical (lognormal) species-abundance distribution.

Three commonly cited models of standing species richness (Valentine, 1973; Gould et al., 1977; Bambach, 1977) each suggest different patterns of species richness in the Phanerozoic. By assuming that sampling of the fossil record is proportionate to sediment volume, it can be shown with the sampling model that the Empirical, Equilibrium, and Species-Richness Models each predict that the number of described species will be strongly correlated with sediment volume. Equally high correlations are predicted if it is assumed that sampling is proportionate to sediment area or to paleontological interest. The correlations predicted for each of the three models are remarkably similar. The impact of sampling effects is so strong that the variations in species richness postulated by these three models are almost completely obscured. Preservational biases will probably only further obscure the relationship between the number of described species and total species richness. Therefore, it seems likely that analysis of trends in the total number of described species will be of little use in determining trends in worldwide species richness in the Phanerozoic.

Comparison of the actual patterns of variation in the number of described species and the expected numbers of described species predicted by the sampling model reveals that more species are known from the Cenozoic than would be predicted from the abundance of Cenozoic sediments or from the amount of paleontological interest in the Cenozoic. This might have resulted from the Cenozoic sediments remaining relatively free of diagenetic effects which might have destroyed the fossils entombed in the sediments.

The rates and patterns of phyletic body size increase and decrease are discussed on the assumption that size change proceeds toward some limit due to selective advantage. Because the selection pressure must decrease as the average body size of an evolving population approaches the limit, some sigmoidal curve should be regarded as more appropriate for the model of size increase than an exponential curve as in the case of population growth. For phyletic size decrease, the predicted pattern is similar to a radiometric decay curve. The conventional unit of morphological rate, darwin, has been used on the assumption that the change is exponential, but actual lineages in the fossil record may represent only fractions of such sigmoidal curves. Some actual data on size increases in ceratopsian dinosaurs and Jurassic bivalves are examined, and it is concluded that sigmoidal size increase seems also empirically to be a more widespread pattern than exponential.

The accuracy of numerical summaries of data from the fossil record has been hotly contested in recent years. In this paper I present a computer simulation which mimics the process of preservation and the resultant loss of data in survivorship records. The simulation accepts as its input data a hypothetical “original” record and a time dependent model of preservational loss. These parameters are used to generate a “preserved” record, and the “original” and “preserved” data sets are then compared. Eleven hypothetical original records having different patterns of diversity and turnover were “preserved” in this manner. Indices of diversity, origination, extinction, turnover, and longevity were evaluated for each of five different models of preservational bias. All of the indices behaved in a single characteristic fashion. Taxonomic data that contained large fluctuations (of the order 100%) were preserved accurately. Similarly the large scale changes in preserved records matched original distributions. Records with small fluctuations (30%) were variably preserved and when such fluctuations were present in the preserved record they did not always correlate with events in the original record. Longevities were more accurately preserved than were other forms of taxonomic data. The results are believed to reflect the levels of accuracy obtainable from generic and familial data and suggest ways in which data sets warranting more detailed study may be singled out.

How much of a living marine fauna would be reflected in the fossil record? In order to answer this, I investigated the probable fate of 169 megascopic genera of the intertidal fauna of the Friday Harbor, Washington region. Three methods were used and these give very similar results. (I) From morphologic examination, 30% of the mud fauna, 32% of the sand fauna, and 29% of the rock fauna are predicted to yield many identifiable fossils; 38% of the mud fauna, 42% of the sand fauna, and 41% of the rock fauna are predicted to yield few identifiable fossils; and the remainder are predicted to yield no fossils. (II) In actual fact, 44% of the mud fauna, 32% of the sand fauna, and 39% of the rock fauna have a fossil record (data from Treatise on Invertebrate Paleontology). (III) The 16 sediment samples which were examined yielded 29% of the total fauna. I conclude (1) that the fossilization potential for the Friday Harbor intertidal fauna is largely independent of habitat, and (2) that 40% of the present megascopic fauna would be (and has been!) preserved in the fossil record.

The fossil record would accurately (and preferentially) include the herbivore and filter feeding genera. The reason which I postulate for this is based on the suitability of heavily calcified exoskeletons to an essentially sessile mode of life, and the lack of suitability of such skeletons for readily mobile forms. 67% of the genera which essentially rest in one place are known as fossils. In contrast, only 16 to 27% of the burrowing detritus eaters (e.g., polychaetes) and roving carnivores (e.g., sea stars and crabs) are known as fossils. The percentage of herbivore and filter feeding genera in rocky environments is 39%, in sand 16% and in mud 34%. In the fossil record, deposits which were originally mud are likely to be most fossiliferous because (1) that environment has a high proportion of essentially sessile genera, (2) essentially sessile genera are far more likely to have a heavily calcified skeleton, and (3) few rock intertidal regions are buried.

Taxonomists working with late Cenozoic faunas tend to view living species as starting points for the evaluation of chronospecies (i.e., segments of evolutionary lineages subjectively designated as species) that extend backward in time from the Recent. This practice makes it possible to construct a survivorship curve for late Cenozoic chronospecies by evaluating all fossil lineages believed to have survived to the present day. A survivorship curve is produced by plotting the fraction of these lineages existing at any time that have not undergone enough phyletic evolution that their extant representatives are assigned to new species. This kind of surviviorship curve has been plotted for chronospecies of mammals using the beginning of the Würm, rather than the Recent, as an endpoint in order to avoid the effects of the Würm and post-Würm mass extinction. The survivorship curve reveals that all but a small fraction of established chronospecies have long durations relative to intervals of time during which distinctive higher taxa have arisen. Phyletic turnover of species has been remarkably slow. Most net evolutionary change must have been associated with saltational speciation. Even the large majority of genera must have arisen rapidly by one or more divergent speciation events. Estimates of rates of extinction suggest that the bottleneck effect, in which constriction of a lineage is followed by re-expansion as a distinct species, cannot be a major source of evolutionary change. These conclusions, based on the evaluation of mammalian phylogeny, seem also to apply to other taxa of animals, supporting the punctuational model of evolution. The long durations of hominid species imply that the evolution of humans, like that of other mammals, conforms to this model.

The shape of bryozoan taxonomic survivorship curves is strongly influenced both by grade of morphologic complexity and by mass extinction. Paleozoic bryozoan genera that are morphologically simple have linear taxonomic survivorship; morphologically intermediate taxa have slightly concave survivorship, and complex forms have very concave survivorship. Increasing morphologic complexity, and by inference, increasing specialization of adaptation appear to accompany a systematic departure from a stochastically constant extinction rate. However, the extinctions of the complex taxa are entirely concentrated during three mass extinction events, whereas the extinctions of the simple taxa are more uniformly distributed throughout the Paleozoic; the extinction pattern of the morphologically intermediate taxa is intermediate to those of the simple and complex groups. Exclusion of the genera affected by mass extinction increases the convexity of the survivorship curves, and reverses the apparent correlation of extinction rate with morphologic complexity. The macroevolutionary pattern of the complex genera resembles an r-strategy, whereas that of the simple taxa resembles a K-strategy.

This paper proposes a simple, unorthodox model for use in the study of vertebrate jaw mechanics. Central to the new “bifulcral model” is the assumption that the bite point may be regarded as a distinct and independent “occlusal fulcrum” equal in status to the jaw articulation or “joint fulcrum” of more traditional biomechanical models. The bifulcral model allows all mechanical forces acting on the feeding system to be evaluated in terms of their purely rotational and purely translational components defined relative to the occlusal and joint fulcra. The major benefit of this analytical approach is that it permits a new and substantially different perspective on the functional consequences of morpho-geometric organization in feeding systems. The bifulcral model clearly establishes the dynamic relationships among muscle alignment, bite point and the resultant patterns of mechanical stress at the craniomandibular joint (CMJ). It also reveals potentially important modes of competitive interaction between otherwise seemingly synergistic jaw muscles. Further, the bifulcral model encourages preliminary investigations into the possible contribution of intramuscular dynamics to the overall operational plasticity of the feeding machinery.

Specific application of the new model to structural and functional problems concerning the origin of the mammalian feeding complex lead to the following tentative conclusions. (1) Contrary to current opinion, the CMJ of most cynodont therapsids probably did not experience positive vertical loads (= compressive) when the cheek teeth were utilized for mastication. Instead, net CMJ loads were either neutral or somewhat negative (= tensile). (2) The development of a pronounced coronoid process in cynodonts was more directly related to promoting the differentiation of the masseter complex than to either improving the mechanical advantage or prehensile capacity of the temporalis muscle. (3) Differential motor activity within the complex temporalis musculature of cynodonts could have resulted in “derived lines of action” markedly different from the reconstructed lines of action employed in previous analyses of feeding mechanics in these reptiles. Such derived lines of action may have been optimal configurations for the integrated activity of the temporalis and masseter musculature. (4) Selection in cynodonts favored the evolution of a superficial masseter rather than the elaboration of the preexisting and geometrically similar pterygoideus musculature. This occurred because the masseter held the greater potential for improving bite force, motor control and facilitating the reduction of the postdentary bones of the mandible while still preserving the basic spatial economy of the cranial region. (5) The rearward growth of the condylar process of the dentary in cynodonts promoted the reduction of the postdentary unit primarily by shifting the point of load application between the dentary and postdentary units, thereby reducing bending stresses in the postdentary unit. (6) The enlarged squamosal sulcus of cynodonts was occupied mainly by a hypertrophied depressor mandibulae muscle; an auditory tube was also present in the sulcus deep to the muscle. (7) The depressor mandibulae played a major role in CMJ stabilization in therapsids; this was possibly its only function in later cynodonts. The peculiar downturned retroarticular process of the mandible in therapsids appears to have been related to this same function.

Measurement of the compressive strength and elastic modulus of the skeletal material of three common Caribbean corals suggests that the mechanical properties of coral skeleton are an important factor in the adaptive repertoire of these animals. The strength (stress at fracture) of the specimens tested is 12–81 meganewtons/meter2, with material from branched colonies being generally stronger than material from massive colonies. These values are lower than the strength of most other carbonate skeletal materials, but higher than that of carbonate engineering materials like concrete and limestone. The comparatively low strength of coral skeleton may be the result of architectural properties produced by the requirements of competing adaptive factors, such as polyp phototropism, or it may reflect the low probability that a colony will be broken, and therefore need to be stronger, before it achieves reproductive parity. The skeleton of the three species tested here is strongest when stress is applied parallel to the growth direction of the polyps. Strength varies inversely with skeletal porosity. Decreasing porosity in highly stressed colonies represents a potentially valuable adaptation for enhancing strength. The adaptive value of porosity modification may explain differences in porosity and strength between highly stressed branched growth forms and more moderately stressed massive growth forms. Boring organisms reduce the strength of coral skeleton by increasing its porosity. Only minor amounts of boring can produce strength reductions of up to 50%. Specialized, stress-minimizing branch arrangements help maximize resistance of coral structures to mechanical degradation in situations where colony size is unusually large or hydraulic energy dangerously high.

The pattern observed in nature has often served as a compelling motivation for seeking the underlying processes which are assumed to control the pattern. Spatial and temporal patterns of species diversity are the most commonly observed and described from the study of recent and paleocommunities. These patterns include those found within a local or discrete region such as succession and changes along an environmental gradient and interregional or global patterns such as latitudinal diversity gradients and comparisons of the deep-sea and the continental shelf faunas. Many hypotheses have been advanced to explain these patterns in terms of differences in the biology of species adapted to different or changing environments. However, a more simple explanation is possible: that these patterns do not result from any specific biologic processes.

We examine this possibility and show that succession can result from probabilistic immigration and local extinction of species, diversity gradients can result from probabilistic disturbance in naturally patchy environments, and that latitudinal gradients can be simply a function of the shape of the earth. As long as such diversity patterns can be explained independent of any specific biologic processes, they offer no test of the importance of these processes, either ecologically or evolutionarily.

Bones of recent mammals in the Amboseli Basin, southern Kenya, exhibit distinctive weathering characteristics that can be related to the time since death and to the local conditions of temperature, humidity and soil chemistry. A categorization of weathering characteristics into six stages, recognizable on descriptive criteria, provides a basis for investigation of weathering rates and processes. The time necessary to achieve each successive weathering stage has been calibrated using known-age carcasses. Most bones decompose beyond recognition in 10 to 15 yr. Bones of animals under 100 kg and juveniles appear to weather more rapidly than bones of large animals or adults. Small-scale rather than widespread environmental factors seem to have greatest influence on weathering characteristics and rates. Bone weathering is potentially valuable as evidence for the period of time represented in recent or fossil bone assemblages, including those on archeological sites, and may also be an important tool in censusing populations of animals in modern ecosystems.

The intensity of drilling predation was studied on samples of fossil and Recent species of Turritella, a soft-bottom mesogastropod mollusc. Our data and records in the literature show that the frequency of drilling has remained about the same from the Eocene to the present. There may have been less predation by drilling during the Late Cretaceous. Among living Turritella, there is a sharp increase in intensity of drilling predation from the temperate zones to the tropics. This latitudinal trend is paralleled by an equatorward increase in number of species of drilling gastropods. Strong spiral ribs of some Eocene, Miocene, and Recent species of Turritella confer protection against drilling, but the mechanism of this immunity remains unclear.

The feces of the Shasta ground sloth (Nothrotheriops shastense), preserved by the arid climate of the lower Grand Canyon, were collected at various levels and examined by microhistological analyses to identify and quantify plant taxa in the diet. Over 500 pieces of different Shasta sloth coprolites were examined. Sloth dung from the nearby Muav Caves was examined and compared with that from Rampart Cave.

Seventy-two genera of plants were identified in the sloth dung deposited discontinuously from over 40,000 to about 11,000 yr BP. The major plant taxa in the Rampart Cave sloth diets were desert globemallow (Sphaeralcea ambigua = 52%), Nevada mormontea (Ephedra nevadensis = 18%), saltbushes (Atriplex spp. = 7%), catclaw acacia (Acacia greggii = 6%), Cactaceae spp. (= 3%), common reed (Phragmites communis = 5%), and yucca (Yucca spp. = 2%).

Six of the most abundant plants in sloth diets were collected in the environs of Rampart Cave and were analyzed for their energy, fiber and nutrient values. The simulated diets of Rampart Cave sloths averaged 1679 cal/g in digestible gross energy and 7.9% for digestible protein. Apart from a substantial increase in digestible energy and in mormontea there was no unusual change in the sloth diet immediately prior to the time of their extinction.

The ecological role of Nothrotheriops shastense is less dramatically different from that of extant desert herbivores than was previously believed.

Much new empirical evidence on the levels of Phanerozoic paleoprovinciality and of species diversity within paleocommunities now permits a reevaluation of marine diversity patterns. Data on paleoprovincial patterns are assembled from the literature and evaluated by means of a stochastic computer simulation model. The simulation is based on the statistics of modern patterns of diversity and endemism extrapolated conservatively to the paleoprovincial patterns and on estimates of species duration from the fossil record. The species diversities associated with the paleoprovincial patterns are then corrected for temporal changes in species packing in communities as determined by Bambach (1977) from studies of paleocommunities. The model thus has an empirical basis throughout. Furthermore it is free of biases that can arise due to the differential preservation of taxa in space and time.

The Paleozoic and Mesozoic were characterized by low provinciality and low average species diversity, on the order of 38,000 to 40,000 species, although there were significant fluctuations in standing diversities. In the Cenozoic, provinciality rose markedly, primarily through the appearance of latitudinal provincial chains, and average species diversity rose to about 240,000. Today it stands over 350,000; this is an order of magnitude greater than the Paleozoic average.

The traces of macroboring organisms are known throughout the Phanerozoic, with diversification and exploitation of the macroboring niche paralleling variations in the development of skeletal metazoa. The oldest macroboring biota is an abundant yet low diversity fauna in hardgrounds and reefs of Lower Cambrian age. Following the extinction of archaeocyathids at the end of the Lower Cambrian (and thus the demise of skeletal reefs until the Middle Ordovician), boring organisms appear to be restricted to submarine hardgrounds. With the development of skeletal reefs in the Middle Ordovician the macroboring fauna shows a rapid speciation and a dramatic increase in diversity. This same pattern occurs again in the Devonian. This record appears to represent refuge of the fauna in low stress, hardground environments when skeletal reefs were not present and radiation in the high stress environment of the reef when large skeletal metazoa were abundant and diverse.