Recent theoretical results demonstrate that a phenotypic version of Wright's shifting balance theory generates the dynamical pattern of punctuated equilibria. Thus, classical mechanisms of random genetic drift and selection for multiple adaptive peaks produce geologically long periods of relative stasis interrupted occasionally by very brief intervals of rapid change. A simple extension of this theory is made here to encompass developmental constraints between quantitative characters, manifested as phenotypic and genetic correlations between characters. Developmental constraints do not qualitatively alter the dynamical pattern of phenotypic evolution produced by selection and random genetic drift. A quantitative definition of stasis is proposed, based on a common taxonomic practice for recognizing subspecies. From this it is concluded that stasis is not the rule for quantitative measurements of detailed sequences for fossil species throughout most of their existence. Instead, periods of relative stasis are interspersed with gradual fluctuating trends, short intervals of rapid change, and discontinuities of subspecific magnitude.

The evolution of body size in fossil horses is frequently depicted as a gradual, progressive trend toward increased body size (Cope's Law). Body size (actually body mass) was estimated for 40 species of fossil horses using dental and skeletal characters and regression equations derived from the same characters in extant species of Equus with known body mass. After body sizes were estimated, rates of morphological evolution, in darwins (d), were calculated between known ancestral and descendant fossil horse species. For the first half of horse evolution (from ca. 57 to 25 ma) body mass remained relatively static between about 25 and 50 kg with very slow evolutionary rates of 0.003–0.04 d. During the early–middle Miocene (from ca. 25 to 10 ma) there was a major diversification of body mass to about 75–400 kg and consistently higher evolutionary rates between 0.04 and 0.24 d. Since the late Miocene, body mass has generally increased with a maximum seen (in natural populations) in Equus scotti (ca. 500 kg) during the middle Pleistocene. Therefore, for horses, the traditional interpretation of gradual increase in body size through time is oversimplified because: (1) although the exception to the rule, 5 of 24 species lineages studied are characterized by dwarfism; and (2) the general trend seems to have been a long period (32 ma) of relative stasis followed by 25 ma of diversification and progressive (although not necessarily gradual) change in body size.

Body mass is estimated for extinct species of Sigmodon. These data are then used in appropriate equations derived among Recent mammals to estimate a suite of physiological and ecological variables which are followed through almost 4 ma of cotton rat history. A statistical trend towards large size is documented. Despite large swings in population size and other parameters, hypothetical values of population metabolism remain virtually constant, suggesting negligible population energetic benefit to size change in either direction. Studies of extant cotton rats and unrelated taxa sharing the same adaptive zone suggest that there is now, and has been in the past, negligible thermoregulatory advantage to modification of cotton rat body mass. Large size appears to be associated in Sigmodon with heightened aggression to the point that two species, particularly if of dichotomous size, cannot coexist in the same microhabitat. At least with regard to cotton rats, this conclusion represents a challenge to the comfortable hypotheses of coevolution and character displacement. The overall trend toward large size during Pleistocene time is considered then as the interplay of selection acting to favor large size in areas of sympatry and stochastic processes originating cotton rat populations of different body size. Because morphology and ecological strategies are stable within the cotton rat adaptive zone for millions of years, it is suggested that mammalian speciation events that result in exploitation of a new adaptive zone are uncommon and occasionally cross higher taxonomic categories. These events are defined as first-order speciation events, in contrast to second-order events that occur in clades within the same adaptive zone.

Numerical cladistic analysis of 73 cranial and postcranial characters has resulted in a highly corroborated hypothesis describing the phylogenetic pattern of early avian evolution. Using “non-avian theropod” dinosaurs as a comparative outgroup and root for the tree, the analysis confirmed Archaeopteryx to be the sister-group of all remaining avian taxa, or Ornithurae. This latter taxon is subdivided into two lineages, the Hesperornithiformes and the Carinatae. The carinates, in turn, were also resolved into two sister-groups, the Ichthyornithiformes and the modern birds, or Neornithes. This paper provides morphological data corroborating the divergence of the two basal clades of the Neornithes: the Palaeognathae (tinamous and ratites) and Neognathae (all other modern birds). The phylogenetic relationships of four important Cretaceous taxa were also investigated, but these fossil taxa were too fragmentary to determine their phylogenetic position unambiguously. Alexornis and Ambiortus are both carinates, but their relationships cannot be resolved in greater detail. The relationships of the Enantiornithes may lie within the Carinatae or these two taxa may be sister-groups. Gobipteryx is a neornithine and possibly the sister-group of the Palaeognathae.

This analysis indicates that major patterns of morphological change took place at the time of origin of the ancestors of the Ornithurae and the Carinatae. Ornithurine innovations included major changes throughout the skeleton, whereas those of the carinates, while substantial, were primarily restricted to the pectoral girdle and forelimb. The phylogenetic results, in conjunction with the known ages of fossil taxa, indicate that the early lineages of birds very likely arose in the Jurassic. The early cladistic events within the neornithine lineage are also more ancient than generally recognized, and may well extend back to the early Cretaceous.

Tiering is the vertical distribution of organisms within the benthic boundary layer. Primary tierers are suspension-feeding organisms with a body or burrow that intersects the seafloor. Secondary tierers are suspension-feeders that maintain positions above or below the sediment-water interface as either epizoans on primary tierers and plants or by living in the burrows of primary tierers. Different primary tierers from soft substrata, nonreef, shallow subtidal shelf and epicontinental sea settings have had different tiering histories, resulting largely from contrasting constructional and phylogenetic constraints. Primary colonial tierers generally occupied lower epifaunal tiers during the Paleozoic and Mesozoic, but since the Cretaceous they have been dominant in the highest tier (+ 20 to +50 cm). Primary echinoderm tierers have been almost exclusively epifaunal, and from the Paleozoic through the Jurassic they were present throughout the epifaunal tiered structure. Although primary bivalve tierers have been both epifaunal and infaunal, they have occupied only lower epifaunal tiers, whereas they have adapted to all levels of the infaunal tiering structure, particularly from the late Paleozoic through the Recent. Brachiopods have lived primarily in tiers directly above or below the water-sediment interface and have not contributed significantly to tiering complexity.

Of the numerous physical and biotic processes and constraints that affect shallow marine benthos, a few have contributed more significantly to changes in tiering patterns. Trends for increasing body size could have accounted for most of the development of tiering complexity up to +50 cm and down to –12 cm. Development of tiering above +50 cm could have been due to processes which would have yielded greater feeding capability, such as competitive interactions for a place from which to feed or adaptations to velocity gradients in the hydrodynamic boundary layer. The most significant process for development of infauanl tiering below –12 cm appears to have been as an adaptive response for predator avoidance.

Unlike infaunal tiering, which never declined after it developed, epifaunal tiering has undergone a general reduction twice. Reduction in epifaunal tiering at the end of the Paleozoic appears to have been the result of the mass extinction at this time, whereas long-term biotic processes seem to have been more important for the tiering decline at the end of the Mesozoic. Tiering structure through the Phanerozoic was thus produced through interactions of a number of physical and biotic factors, tempered by constructional and phylogenetic constraints of each primary tierer group.

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.

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.

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.

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.