Geographical restriction to refuges implies the regional extinction of taxa in areas of the previous range falling outside the refuge. A comparison of the circumstances in the refuge with those in areas from which the taxa were eliminated is potentially informative for pinpointing the causes of extinction. A synthesis of data on the geographical and stratigraphical distributions of cool-water molluscs of the North Pacific and North Atlantic Oceans during the late Neogene reveals four patterns of geographical restriction, at least two of which imply that climatic cooling was not the only cause of extinction during the last several million years. These four patterns are (1) the northwestern Pacific restriction, involving 15 taxa whose amphi-Pacific distributions during the late Neogene became subsequently restricted to the Asian side of the Pacific; (2) the northwestern Atlantic restriction, involving six taxa whose early Pleistocene distribution is inferred to have been amphi-Atlantic, but whose present-day and late Pleistocene ranges are confined to the northwestern Atlantic; (3) a vicariant Pacific pattern, in which many ancestral amphi-Pacific taxa gave rise to separate eastern and western descendants; and (4) the circumboreal restriction, involving six taxa whose early Pleistocene distribution, encompassing both the Atlantic and Pacific Oceans, became subsequently limited to the North Pacific. Like the Pliocene extinctions in the Atlantic, previously studied by Stanley and others, the vicariant Pacific pattern is most reasonably interpreted as having resulted from regional extinction of northern populations in response to cooling. The northwestern Pacific and Atlantic restrictions, however, cannot be accounted for in this way. In contrast to the northeastern margins of the Pacific and Atlantic, the northwestern margins are today characterized by wide temperature fluctuations and by extensive development of shore ice in winter. Northeastern, rather than northwestern, restriction would be expected if cooling were the overriding cause of regional extinction. Among the other possible causes of extinction, only a decrease in primary productivity can account for the observed northwestern and circumboreal patterns of restriction. Geographical patterns of body size and the distribution of siliceous deposits provide supporting evidence that primary productivity declined after the Miocene in the northeastern Pacific, but remained high in the northwestern Pacific, and that productivity in the Pacific is generally higher than it is in the Atlantic. The patterns of geographical restriction in the northern oceans thus provide additional support to previous inferences that reductions in primary productivity have played a significant role in marine extinctions.

Nonparametric randomization tests were used to determine whether fossil marine families classified in different higher taxa are significantly different in their durations, their rates of origination and extinction, and their proportion of originations. For each of these properties, significant differences among orders and classes were found that could not be explained by differences at other taxonomic ranks, but no such differences were found among phyla. Thus, orders and classes appear to possess an evolutionary coherence that phyla lack.

Functional consequences of the variation in geometry and morphology of the articulate brachiopod hinge mechanism are poorly understood, despite the fact that hinge structures have considerable importance in brachiopod taxonomy. Jaanusson (1971) proposed that the ability to resorb shell material during growth, particularly in the hinge structures, can be used to distinguish two groups within the articulates, the deltidiodonts and the cyrtomatodonts. He considered the two groups to be morphologically distinct, “natural” phylogenetic groups, separated by a “functional discontinuity.” In order to test the morphological, functional, and phylogenetic implications of shell resorption, comparisons of the hinge-system geometry and diductor muscle moment are made here between deltidiodont and cyrtomatodont brachiopods. A truss network composed of landmarks relevant to the valve opening mechanism is constructed to characterize hinge-system geometry. The function of the hinge mechanism is analyzed in the context of valve opening, and diductor muscle force, effort lever arm, and moment are compared between deltidiodonts and cyrtomatodonts. The distribution of resorption among brachiopods is investigated with respect to a phylogenetic hypothesis proposed by Williams and Rowell (1965a).

Deltidiodont brachiopods are morphologically more variable than cyrtomatodonts, and a greater proportion of the variability is correlated with size. Deltidiodonts and cyrtomatodonts employ different strategies to open the valves; deltidiodont lever arms are relatively longer, whereas cyrtomatodont diductor muscles have relatively larger cross-sectional areas. The greatest muscle moment in deltidiodont hinge systems is realized in the maintenance of a gape angle; in the cyrtomatodont system, it is achieved at the initiation of a gape. Although they are morphologically and functionally distinct, it is doubtful that the two groups are separated by a “functional discontinuity.” Because the phylogenetic relationships among brachiopod orders are not yet resolved, the status of shell resorption as a homologue is still unclear. Resorption is manifest in at least some members of each major group of articulates (except the pentameraceans); it is likely that resorption has evolved independently several times in brachiopod evolution, in part because of the increased morphological flexibility it confers.

The borophagine canids were bone-cracking scavengers in the Miocene-Pleistocene of North America. In this they parallel the Recent hyenas. This paper analyzes the borophagine adaptation in relation to that of hyaenids, using Osteoborus cyonoides as an example. The emphasis during canid evolution on the posterior molars, which is a derived condition, created a constraint on the adaptation of borophagines. This constraint meant that the borophagines used P4/4 as bone-cracking teeth, whereas hyaenids use P3/3. The latter adaptation has the advantage of separating the bone-cracking teeth from the meat-cutting portion of the dentition, thereby allowing a dual purpose dentition in hyaenids. In borophagines, no such dual purpose was possible, and it is suggested that they were closer to obligate bone-cracking scavengers than Recent hyaenids. Other than the evolution of a specialized bone-cracking tooth, the borophagines adapted to bone cracking by evolving a vaulted and strengthened skull for the dissipation of the strong forces generated during bone cracking. In this they again parallel the hyaenids. Evolution within borophagines involved an elaboration of patterns already set at the group's inception, creating an evolutionary trend which was mediated by the constraint on the bone-cracking morphology. This trend may be due to selection or sorting, or may, under certain assumptions, be stochastic. Other evolutionary trends may also be epiphenomena of constraints that lock morphological evolution.

Physical models of graptolites have been constructed for a range of morphologies, with emphasis on planar, multiramous forms. The models are life size and have the density of a living graptolite, based on the now-established collagenous nature of the periderm and unavoidable assumptions about the amount of extrathecal tissue present in the living colony. These models have been used to test the two main hypotheses of graptolite life habit developed by Bulman, Rickards, Kirk, and others. Testing of graptoloid models in water suggests that many rhabdosome shapes were designed for passive rotation within the water column. This is caused in the models by a variety of modifications, including changes in thecal and stipe orientation, alterations of colony shape and the addition of vanes and hooks. Rotation would only have been useful when the rhabdosome was in directional motion and the frequency of such modifications seems anomalous if no such movement occurred. Thus movement by some means is required, either passively, by changes in buoyancy, or by automobility. Spiralling action would increase the harvesting path of an individual living on a planar, multiramous colony, making this a theoretically advantageous mode of life for the morphology. It would prevent the individual zooids of scandent biserial and uniserial colonies from feeding from the same narrow band of water.

Sublethal injuries recorded in the shells of five Upper Mississippian ammonoid species from the Imo Formation of northwest Arkansas are manifested as repaired shell breaks, which can be categorized as minor, moderate, massive, deep-acute, or as perforations. Overall, 15% of the ammonoids exhibit some form of repaired break. The injuries are distributed as follows: Anthracoceras discus 9%; Fayettevillea bransoni 13%; Fayettevillea friscoense 21%; Rhadinites miseri 24%; Richardsonites mapesi 38%. These figures are substantially lower than in living Nautilus, in which more than one-half of adult specimens exhibit repaired shell breaks. The different frequencies of injuries may reflect species-selective predation, differential abilities to sustain and to repair shell damage, or they may be due to interspecific differences in physical parameters such as shell thickness and body chamber length. The most likely possible perpetrators of the injuries include sharks, other fishes, and cephalopods.

The tailbend, a downward flexure of the vertebral column, is a prominent feature of post-Triassic ichthyosaurs. However, due to the vagaries of preservation and of preparation, its presence cannot be verified in all skeletons. Computed tomography offers a solution to the problem by identifying the wedge-shaped centra that contribute to the tailbend. Leptopterygius tenuirostris appears to have a tailbend, albeit slightly downturned. There is also evidence that its close relative Eurhinosaurus longirostris had a tailbend, too.