Under favorable circumstances, biogeographic and biostratigraphic data can be combined to identify accurately the time and place of origin of a given taxon, and to reconstruct the pattern of its subsequent radiation. This study considers the dendrasterid sand dollars, which are abundant today along the Pacific Coast of North America. The Neogene sand dollar record in this region is particularly good; in fact, sand dollars have traditionally been used as provincial index fossils.

The dendrasterids originated in central California at the end of the Miocene; the oldest forms are dated at about 6.0–6.5 Ma. They spread south to Baja California during the Pliocene, and then north to Alaska during the Quaternary. This historical pattern is not an artifact of the record; it is consistent with independent paleogeographic evidence. The dendrasterids supplanted an older Mio-Pliocene sand dollar fauna; they are now completely dominant in the temperate coastal waters of the northeastern Pacific. They have reached this position in less than 7 m.y. since their first local appearance. The rapid rise of dendrasterids could be related to their aberrant morphology and behavior; these adaptations allow dendrasterids to suspension-feed, in a manner unique among living echinoids.

Dendrasterids are characterized by “eccentric” test morphologies. Even the oldest species are highly eccentric; transitional forms are unknown. The first dendrasterids appear suddenly in the provincial “Jacalitos Stage,” above an unconformity which represents no more than about 1 m.y. They do not occur in the underlying units, although other fossil sand dollars are abundant. The dendrasterids may have arisen rapidly, through a heterochronic change in the development of older, noneccentric forms. Recent ontogenetic studies have documented the feasibility of this process.

Convergent evolution of hypercarnivorous adaptations in canids has occurred a number of times in the last 40 m.y. among distantly related taxa. The adaptations include an increase in carnassial blade length, reduction or loss of post-carnassial molars, and transformation of the talonid of the lower first molar from a basinlike depression into a trenchant, bladelike cusp. Although the diversity of these specialized canids is typically low in past and present communities, it was unusually high during the Late Oligocene of North America and the Pleistocene of South America. These two comparable events provide an opportunity for exploring possible causes of the evolution of hypercarnivory in canids. Plots of generic diversity against time for North American predators reveal a roughly inverse relationship between the number of hypercarnivorous canid taxa and the numbers of other hypercarnivores, such as creodonts, nimravids, mustelids, and amphicyonids. Similarly, the radiation of hypercarnivorous canids in South America occurred at a time of relatively low diversity of other hypercarnivores. Analysis of trophic diversity within the North American carnivore paleoguild before, during, and after the Late Oligocene reveals considerable taxonomic turnover among carnivores because of immigration and speciation. Late Oligocene hypercarnivorous canids appear to have been replaced first by amphicyonids and large mustelids, and then by felids.

Despite the repeated tendency of canids to evolve adaptations for hypercarnivory, a canid has yet to appear that is completely catlike, that is, without any post-carnassial molars. This possible constraint on morphological evolution in canids is argued to have resulted, paradoxically, in increased flexibility over evolutionary time and a great potential for rapid diversification and clade survivorship. Finally, it is suggested that the iterative pattern of specialization of the lower molars for meat-slicing that is seen in all families of carnivores, past and present, is probably a result of intraspecific competition for food, perhaps among littermates. This intraspecific selective force is countered by competition among species, since there are limits on the number of sympatric hypercarnivorous species within a single community.

Studies of the fates of clades during mass extinctions at the end of the Cretaceous and near the Cambrian-Ordovician boundary have indicated that geographically widespread groups have a much higher probability of survival than narrowly distributed groups. If biogeographic distribution does play a significant role in influencing the outcome of extinction events, then geographic variability in extinction intensity should be mirrored by patterns of endemism. A comparison of data for the terminal Cambrian extinction indicates that survival of trilobite families in Kazakhstan was significantly greater than in North America. As predicted, the Kazakh sequence was characterized by a significantly larger number of pandemic families. The Baltic Province, which includes Scandinavia, England, and Wales, was composed entirely of pandemic groups and did not suffer any trilobite extinction at the family level. The results are consistent with the suggestion that latitudinal variation in extinction magnitude may be the result of differences in biogeographic structure.

Colony growth form in some Silurian heliolitid corals is analyzed by the measurement of their shape in profile. Data are presented for seven species, Stelliporella parvistella, Heliolites interstinctus, H. megastoma, H. daintreei, H. spongodes, Propora tubulata, and Plasmopora scita from three localities in Gotland, Sweden, and three localities in England. Intraspecific growth-form variation is presented on triangle diagrams. These plots allow variation to be compared between species present at each locality and between localities for each species. Results indicate that the overall potential for growth-form variation is genetically controlled and that levels of response to environmental stimuli may differ markedly between species. Stelliporella parvistella is a very plastic species, the only one developing branching growth in addition to other growth forms. Heliolites interstinctus is much less variable, dominantly tabular, domal, and low bulbous in form, but demonstrates a similar response. Propora tubulata has a tightly constrained bulbous growth form that shows little variation between localities. The other species are represented by few specimens, most of which parallel H. interstinctus. The likely moderating influences of light levels, substrate type, sedimentation rate, energy levels, and other variables on growth-form variation and species range are considered. The main environmental factor including ecophenotypic response is concluded to be sedimentation rate. A close correlation between this factor and growth form in S. parvistella indicates that form in this species is a particularly sensitive indicator of sedimentation rate and substrate conditions. No simple equations can be made between specific environments and one particular growth form in these corals.

The directionality of long-term trends can be strongly biased by forces intrinsic to a clade. Trends in body size and skeletal shape may be dictated more by variations in survivorship that reflect differences in ecology than by long-term directional changes in the environment. Hence, mass extinctions can help drive evolutionary trends by selectively eliminating some morphologies and permitting the survivors to found the next radiation.

Examples include repeated trends toward larger maximum body size and the evolution of keeled species from those with globose tests in planktonic foraminifera. Both the trends in size and shape develop because small species with globose tests are significantly more resistant to extinction than species that are large or have peripheral keels. Hence, the survivors of both the Cretaceous-Tertiary and Eocene-Oligocene extinction episodes are small, unkeeled taxa. Large species and species with keels evolved convergently after both mass extinctions as the founders radiated anew.

Comparison of three radiations of planktonic foraminifera suggest that the convergent evolution of similar test shapes and sizes is not due to synchronous changes in oceanography that track evolutionary trends. Instead, the reestablishment of habitat heterogeneity is needed to permit the ensuing radiation to unfold rather than to closely guide its progress. Similar evolutionary trends will develop in each radiation as long as the founders have similar morphology and the evolution of variants present in the previous radiation is not precluded by the environment.

Crinoid genera of the subclass Camerata generally dominated Late Silurian through Middle Mississippian pelmatozoan echinoderm assemblages. This dominance reached a peak during the early and middle Mississippian (Kinderhookian-Meramecian), but abruptly ended at the close of the Genevievian Stage (=lowermost Chesterian) in eastern North America. During the Genevievian Stage, crinoids of the subclass Inadunata became taxonomically more diverse but a few camerates, especially Platycrinites and Batocrinus, continued to be numerically dominant in many pelmatozoan assemblages. In eastern North America, platycrinids and batocrinids were reduced drastically near the Genevievian-Gasperian boundary. So obvious is this faunal change that, until recently, the Meramecian-Chesterian Series boundary was recognized as the last occurrence of Platycrinites penicillus. The sudden and drastic decline of numerically dominant platycrinids and batocrinids in eastern North America suggests a mass extinction, but is better interpreted as a range contraction and loss of dominance. Platycrinids, in particular, continued to be significant components of pelmatozoan assemblages in Europe and Asia long after the end of the Genevievian Stage. We infer that this reorganization of pelmatozoan assemblages in eastern North America was a product of predation, siliciclastic tolerance, and current-energy preference, with predation playing a major role. Reorganization resulted in post-Genevievian dominance by (1) cladid crinoids, (2) camerate crinoids that were cladid homeomorphs, and (3) the blastoid Pentremites. Foraminifera, conodonts, corals, brachiopods, bryozoans, and other echinoderm groups were affected little, if any, during this same time.

Three major arguments have been raised against the crucial claim, documented by Whittington and colleagues for the Burgess Shale fauna, and so contrary to traditional views, that disparity of anatomical design reached an early maximum in the history of multicellular life: (1) the presence of many early taxa with low membership and high rank is an artifact of naming; (2) cladistic analysis of Burgess arthropods negates the claim for greater early disparity; and (3) Whittington's argument is a retrospective fallacy based on assigning high rank to differentia only by virtue of their later capacity to define major branches. I show that all these arguments are either false or illogical, and that the claim for increased early disparity is justified: (1) Taxonomic rank is an artifact, but no one has ever based a claim for greater disparity on this false criterion. (2) Cladistics can only deal with branching order, whereas disparity is a phenetic issue. These two legitimate aspects of evolutionary “relationship” are logically distinct. The rooting of a cladogram only illustrates monophyletic ancestry (which no one doubts, as we are not creationists), and cannot measure disparity. (3) The active stabilization of the differentia of Baupläne (for genetic and developmental reasons only dimly understood) provides a powerful rationale for weighting these characters in considerations of disparity; nothing had so stabilized in the Burgess fauna. If these differentia were steadily changing contingencies, rather than actively stabilized features with “deep” architectural status, then the retrospective argument would be justified. Although the three arguments are wrong, the claim for greater early disparity cannot be confidently established until we develop quantitative techniques for the characterization of morphospace and its differential filling through time. This is a dauntingly difficult problem, much harder than cladistic ordering, but not intractable.