The modern gorgonocephalid ophiuroid Astrophyton, a basketstar, has feeding habits which may be analogous to those of some Paleozoic crinoids that lack pinnulate arms (Meyer and Lane 1975). Field study has demonstrated the very quick response of Astrophyton when plankton impinge upon its arms. Study of the skeletal microstructure by scanning electron microscopy has shown that the quick response is facilitated by the development of relatively large ventral muscle fossae and articular surfaces whose shape follows a catenary curve. Prey are impaled by spines which articulate with the ventral surface of vertebral ossicles. Stereomic microstructure of vertebral ossicles is galleried where ligaments are present and labyrinthic where muscular tissue is present. The same microstructures are found in muscular articulations of modern crinoids. Future work on the arm structure of Paleozoic taxocrinids and cyathocrinids may demonstrate structural analogies and suggest similarities in food capture between such crinoids and modern baskets tars.

Consideration of the physics of sinking of hollow, rigid bodies leads to equations of motion for sinking cephalopod shells. We have derived equations of motion for three post-mortem sinking situations: sinking with a fixed amount of water in the phragmocone; rapid phragmocone filling (no siphuncular tube); and slow phragmocone filling (siphuncular tube intact). In all three cases sinking speed can be closely approximated by the terminal velocity calculated from the total weight, buoyancy, and drag parameters of the shell.

Experiments on modern Nautilus shells yield sinking velocities in agreement with calculated values. The experiments also show that orientation of a sinking Nautilus shell varies as the phragmocone fills with water. With small negative buoyancy the shell sinks with its plane of symmetry upright, but as it fills, it begins to rock from side to side and leans over and sinks with its plane of symmetry horizontal when the camerae are about 55% full.

The maximum sinking speed of upright adult Nautilus shells is approximately 30 cm/sec, which appears to be too small for embedding in the bottom upon impact. The maximum depth to which Nautilus sinks in the upright position ranges from about 7 m for rapidly filling shells to as much as 600 m for slowly filling shells. In the latter case, the shell will continue to fill after coming to rest on the bottom, and the stability of the vertical orientation will be removed within 1 or 2 days. Thus, primary vertical preservation of cephalopod shells indicates water depths less than about 10 m.

A new account of the mechanical action of the heterocercal tail in fishes is given, developed from study of the swimming of sharks. The heterocercal tail is capable of delivering a thrust that can be oriented in a wide range of angles in the vertical plane. The orientation results from a balance of forces acting in the tail. Normally the heterocercal tail delivers neither an epibatic nor a hypobatic thrust, but rather one directly through the center of gravity of the fish. On the basis of the new account, predictions concerning the shape and proportions of the heterocercal tail are made and tested.

Ecological investigations were conducted on rodent faunas in the Sonoran Desert of the southwestern United States and the Monte Desert of northwestern Argentina. Both study areas are physiognomically quite similar and the rodents are only distantly related. Such conditions are ideal for an assessment of possible evolutionary convergence. Multivariate analyses of morphoecological characters support the subjective interpretation that ecological equivalents between the deserts differ in their degree of similarity. The data indicate that the Monte rodent fauna is composed of two distinct groups: a) the caviomorph rodents, which are highly desert adapted; and b) the murids, which range widely beyond the desert's boundaries and are probably not as highly desert adapted, as a group, as are the caviomorphs. When the Monte and Sonoran faunas are compared with a North American coniferous forest rodent fauna, data show that both desert groups have converged toward one another. The different degrees of desert adaptation and different distributional patterns indicate that the Monte Desert was colonized in two different periods (one in the early Tertiary and one in the late Pliocene or early Pleistocene) and can be interpreted as supporting theories regarding South America's colonization by mammals which were first proposed by Simpson (1951).

Growth processes and patterns set functional limits on the adaptive range of organisms that have evolved a particular “Bauplan.” The design of the elastic ligament has played a large part in determining the scope of the evolutionary radiation of the Arcoida. Tensional lamellar and compressional fibrous materials of this ligament, which appear to be similar in their elastic properties to those of other bivalves, are not segregated in positions where they could best perform their different mechanical functions. The growth of this unspecialized ligament exhibits strong positive allometry, with respect to shell size, in many fossil and living arcoids. A simple mechanical model shows that this allometry is essential if the strength of the ligament is to keep up with the weight of the animal, during ontogeny. Inherent disadvantages of the allometry include increasing dorsal breakage of the ligament itself, interference with the function of the hinge teeth and reduction of the closing moment exerted by the adductor muscles. Direct measurements of living arcoids show that allometric growth just maintains a linear relationship between ligament strength and animal volume. Major differences in ligament strength are related to shell growth patterns and substantially different environmental adaptations. Ligament strength is highly variable within individual populations; between populations, it is correlated with shell thickness and local environmental conditions. Most arcoids are adapted for shallow-burrowing or epifaunal modes of life in physically unstable environments, where they are frequently disturbed. The large, muscular foot enables these mobile animals to regain suitable living positions after such disturbances. The weak ligament has limited the potential specialization of the arcoids for either deeper burrowing or permanent epibyssal attachment, also requiring the retention of the foot in epifaunal forms. This ligament has made diverse adaptations possible, while preventing extreme specialization, except in isolated taxa. As such, it is largely responsible for the evolutionary reversals documented by Stanley (1972). This is konstrukionsmorphologie No. 46.

A computer simulation was developed which estimates the effects of differential preservation on the value of taxonomic diversity in fossil assemblages. Taxonomic diversity was measured with the Shannon-Weaver index, H′ and with evenness, J. Preservation markedly alters quantitative measurements of taxonomic diversity. The degree of alteration can, however, be predicted on the basis of the species-abundance distribution and on the distributions of species of varying preservability. The indices of diversity H′ and J measure different aspects of diversity, but respond to differential preservation in a similar manner. Evenness, J, is recommended for paleoecologic use. The computer model can be used to revise fossil diversity data. A simple form of such a model has been used, and suggests that some previous interpretations of paleoecologic diversity may be in error.