The Precambrian-Cambrian Boundary traditionally has been viewed as a fundamental discontinuity in the historical record of life. Almost from the birth of stratigraphic geology, it was recognized that the richly fossiliferous strata of Cambrian and younger periods were underlain by sedimentary rocks largely devoid of macroscopic fossils. Darwin (1872) was well aware of this dichotomy and of the “sudden manner in which several groups of species first appear” in the rock record. In the sixth edition of the Origin of Species (1872), he wrote, “The case [for this sudden appearance] at present must remain inexplicable … and may be truly urged as a valid argument against the views [on evolution] here entertained.” Careful biostratigraphic work by Walcott and others in the closing decades of the nineteenth century accentuated rather than dispelled the problem. Diverse Early Cambrian invertebrate assemblages were documented from North America and Scandinavia, but repeated searches for comparable fossils in Precambrian strata produced only a few, enigmatic remains (see review by Yochelson [1979]).

For paleobiologists interested in extinct carnivorous reptiles, publication of Walter Auffenberg's (1981) 13–month field study of the Komodo dragon (Varanus komodoensis), or ora, as he prefers to call it, provides a useful portrait of a reptile as the top predator of a terrestrial community. A great deal of information about the ora's habitat utilization, activity patterns, diet, social behavior, and reproduction is presented.

Rancho La Brea is unquestionably the most famous fossil deposit in North America, and its fauna occupies a key position in vertebrate paleontology as the basis for the Rancholabrean Land Mammal Age. The 23 acres now known as Hancock Park yielded the bulk of fossil specimens from this locality. A minimum of 518 taxa have been identified to date including 79 algae (all but one of which are diatoms), 72 vascular plants, 9 nonmarine mollusks (13 taxa of marine mollusks were carried to the site by early Indians), 135 arthropods, 27 lower vertebrates, 138 birds, and 58 mammals. A detailed list of the biota, too lengthy for inclusion here, is available from the authors on request.

An analysis of the structure and kinematics of the forelimbs and hindlimbs of pterosaurs, and functional analogy with recent and fossil vertebrates, supports a reappraisal of the locomotory abilities of pterosaurs. A hypothesis of structural, aerodynamic, and evolutionary differences distinguishing vertebrate gliders from fliers is proposed; pterosaurs fit all the criteria of fliers but none pertaining to gliders. The kinematics of the reconstructed pterosaur flight stroke reveal a down-and-forward component found also in birds and bats; structural features of the shoulder girdle and sternum unique to pterosaurs may be explained in light of this motion. The recovery stroke of flight was accomplished, in birdlike fashion, by a functional reversal of the action of the M. supracoracoideus by the pronounced enlargement of the acrocoracoid process, which acted as a pulley. The wing membrane was supported and controlled through a system of stiffened, intercalated fibers, which were oriented like the main structural elements in the wings of birds and bats.

The hindlimbs of pterosaurs were independent of the wing membrane, and articulated like those of other advanced archosaurs and birds, not like those of bats. The gait was parasagittal and the stance digitigrade. Because of limitations on the motion of the forelimb at the shoulder, pterosaurs could not have walked quadrupedally. However, bipedal locomotion appears to have been normal and quite sufficient in all pterosaurs. There is nothing batlike about pterosaur anatomy; on the other hand, pterosaurs bear close structural resemblances to birds and dinosaurs, to which they are most closely related phylogenetically.

Many cheilostome bryozoans of diverse phylogenetic origin grow as erect, arborescent colonies with branches of modified planar form composed of two layers of zooids back to back. Regular branching enables a growing colony to expand in surface area, and hence in the number of zooids that feed, reproduce, and perform other vital functions, at an accelerating rate. During growth, branches first all diverge, then increasingly converge, and in late stages of growth begin to interfere with each other's growth and function. Interference can set limits to the width and thickness of branches and hence to the number and size of zooids.

Simulation of growth using a 3–dimensional mathematical model shows that a narrow range of possible values of branching angles minimizes branch interference in late growth stages. These values are prevalent in fossil and modern species. Branch spacing at later growth stages is correlated with the distance between branches at first crossing, providing room for feeding organs of the two facing layers of zooids to protrude and function. Interbranch distances dwindle as branches increasingly converge, so emphasis on minimizing interference at a late stage sets a practical limit to growth beyond that stage. To gain this long-term benefit requires adhering to a regular pattern throughout growth. The considerable variation in branching properties in fossil and modern species, and a variability in spacing inherent in the growth pattern itself, limit the amount of usable interbranch space. Despite a higher intraspecific variability, branching properties are as distinctive interspecifically as zooidal properties, and variability is randomly distributed through the colony. A small reduction in variability between fossil and modern species suggests that increasing regularity may provide a selective advantage in the utilization of interbranch space.

Discriminant function analysis is used to examine the role of within-species differentiation in macroevolution of four Miocene bivalve genera. According to the punctuational model, macroevolution is decoupled from microevolution; within-species trends may be unrelated to between-species differences. The reductionist alternative suggests that intraspecific (primarily geographic) differentiation is the basis of macroevolution. If macroevolution depends on within-species differentiation, discriminant function analysis should separate interspecific and intraspecific populations of a genus using the same set of characters.

Chesapecten macroevolution apparently involved within-species differentiation. Shell length, height, and posterior auricle length differentiate geographic populations within species as well as interspecific populations. Results for Astarte, Anadara, and Eucrassatella are more consistent with the punctuational model. Characters which readily separate species of a genus are much less successful at differentiating intraspecific populations. Rank correlation techniques indicate that, for all three genera, the order of importance of characters in interspecific discrimination is not significantly related to their order in intraspecific discrimination. Decoupling of macroevolution from microevolution is implied for these taxa.

Observations on the morphology of the early whorls of Eutrephoceras dekayi (Morton), a widespread Cretaceous nautilid, are supplemented with oxygen and carbon isotopic analyses (δ18O and δ13C) of the early septa of five well-preserved specimens to help identify the point of hatching on the shell. Septa 4 and 5 are more closely spaced than preceding septa and probably correspond in time of formation with a constriction or first broken aperture on the outer shell one-third whorl forward of the fourth septum. In modern Nautilus, morphologic, isotopic, and observational data suggest that similar features mark hatching. Between the fourth and fifth septa in E. dekayi, δ18O values show a shift of variable magnitude from heavy to lighter values followed by a return to heavier values over the next one to three septa. This isotopic shift is compatible with a hatching interpretation and may be explained as the result of kinetic and equilibrium effects on emergence from an egg capsule.

Eutrephoceras dekayi hatched at about 9 mm in diameter, one-third the hatching size of modern Nautilus. Like Nautilus, E. dekayi probably produced few young, all of which were active swimmers at hatching. In contrast, Mesozoic ammonoids produced numerous offspring ranging from 0.5 to 1.5 mm in diameter which may have spent some time in the plankton. These differences in life history may correlate with differences in ecologic specialization, environmental tolerance, and habitat between ammonoids and nautilids and may have contributed to their disparate rates of evolution during the Mesozoic.

Capture-mark-recapture of Nautilus belauensis in Palau shows that (a) immature animals added an average of 0.1 mm of shell per day at the ventral circumference (projected mean annual increment of 37 mm), but this rate decreases rapidly as maturity is approached; (b) rate of growth line secretion is irregular, ranging from 4 to 8 days per growth line in immature animals; (c) overall weight is an imprecise measure of size and growth in Nautilus due to fluctuations in crop content and chamber liquid; (d) this species is shown to live for at least 4 yr beyond maturity. Age estimates based on the highest growth rate obtained (0.12 mm/day = 43.8 mm/yr) range from 14.5 to 17 yr for seven animals recaptured 45–355 days after release. Combined, these data indicate that the life span of Nautilus may exceed 20 yr.

Small mammal bone assemblages from the scats of three families of mammalian carnivores are described here as a means of assessing paleontological bone assemblages. These are the Viverridae, Canidae, and Mustelidae. Marking behavior by the carnivores or their use of latrine areas can produce concentrations of bone comparable with those produced from owl pellets, but the nature of the bone assemblages is very different. Skeletal element preservation has a characteristic pattern related to the relative strength of the individual elements, so that the strongest elements are the ones that best survive the physical abrasion of the predator's teeth. Bone breakage is greatest in canids and least in some viverrids. Digestion of the bone occurs to a certain extent with all carnivores, with moderate rounding but little corrosion of either bones or teeth in viverrid-derived bone assemblages, very great rounding and severe corrosion of bone and tooth enamel (but little corrosion of tooth dentine) in canids, and moderate rounding and slight to moderate corrosion of bone and tooth dentine (but less of enamel) in mustelid-derived bone assemblages. Tooth marks are generally rare except in canid-derived bone assemblages. These patterns may enable the identification of predators responsible for the accumulation of fossil predator assemblages, and a consideration of the hunting and other behavior of that type of predator then permit the identification of possible biases in the fossil fauna. In the viverrids, for instance, the mongoose selected for large prey size and against cranial elements, while the genet took all prey sizes available to it and all body parts. Larger canids such as foxes and coyotes took both large and small species, but proportionally more of the most common prey species were present. Skulls were underrepresented. The small canid, the bat-eared fox, selected for the smallest prey species available to it but took all body parts. Mustelids took all sizes of prey, although not necessarily in proportion to their abundance in the habitat, and large species were underrepresented. If the predator type is known, therefore, for a fossil assemblage, these biases can be taken into account in assessing the paleoecological significance of the fauna. An example from the Olduvai Pleistocene deposits is given.