Marshall (1995) used the distribution of the endpoints of 50% range extensions added to the stratigraphic ranges of individual taxa to bracket the position of an extinction boundary. Here we describe two improvements to Marshall's method. First, we show that more precise estimates of the position of such a boundary may be obtained using range extensions with confidence levels of less than 50% (e.g., 20%). Second, we introduce a new method of calculating confidence intervals that explicitly takes into account the position of the highest fossil find. Incorporating these improvements leads to confidence intervals for simulated data sets that are approximately four times more precise than those obtained by using Marshall's (1995) original method and approximately twice as precise as those using other published methods. We provide a look-up table that shows for different numbers of taxa the confidence level that should be used to maximize the precision of the estimated position of the extinction boundary, while ensuring that the boundary still lies within the stratigraphic interval bounded by at least one range extension. Unlike some other methods, our method is nonparametric and does not make the restrictive assumption of uniform preservation and recovery potential. We apply the method to Macellari's (1986) ammonite data from the late Cretaceous of Seymour Island, Antarctica.

Tracking the geometry of all 597 ammonoid genera from the Lower Devonian into the Lower Triassic, a 145-Myr period that spans three mass extinctions, shows that Paleozoic ammonoid shell geometries were strongly biased for a few combinations of whorl expansion (W), whorl overlap (D), and whorl shape (S). Just three modal combinations accommodated approximately 432 genera (72% of total) and just one combination accommodated 239 genera (40%). All three primary modal forms have similar low expansion rates (W ≈ 1.75) and differ only in coiling tightness (D). These geometries resulted in long body chambers (≈400°) with Nautilus-like static in-life aperture orientations (≈30°) for the great majority (>80%) of Paleozoic ammonoids. The ancestral clade Agoniatitida included a unique spectrum of openly coiled geometries that went extinct at the Frasnian/Famennian boundary (and were not seen again until the Triassic). The Devonian/Mississippian extinction terminated the brief, explosive radiation of the Clymeniida (64 genera). The dominant Paleozoic clade, the Goniatitida (ca. 130 Myr, 374 genera [64% of total]), survived both the F/F and D/M extinctions, but began declining well before the Permian/Triassic crisis. The long-lived Prolecanitida (40 genera [7%]) appeared shortly after the D/M extinction, persisted as a low-diversity clade through the Carboniferous, and gave rise to the Ceratitida in the mid-Permian, from which were derived all Mesozoic ammonoids. After each major extinction event the phylogenetic composition of ammonoid stocks was fundamentally reordered and geometries were recanalized. Without external disturbances, as the relatively uninterrupted Mississippian through Permian record shows, the history of ammonoid shell geometry would probably have been a record of much greater constancy, perhaps tied much more closely to the Lower Devonian geometric landscape.

High-resolution (annual to decadal) paleoecological records of community composition can contribute a long-term perspective to conservation biology on baseline ecological variability and the response of communities to environmental change. We present here a detailed comparison of species assemblage characteristics (species richness, abundance, composition, and occurrence frequency) in live, dead, and recent fossil ostracode samples from Lake Tanganyika, East Africa. This study calibrates the fidelity of paleoecological samples (i.e., both death and fossil assemblages) to live diversity patterns for the purpose of reconstructing community dynamics through time.

Both life and death assemblages were collected from rocky sites in a mixed substrate habitat (total of ten sampling visits over 22-month period) over spatial scales of less than a meter to about 3–12 meters. Fossil assemblages were derived from sediment cores collected in sandy substrates adjacent to the rocky sites. Species richness in paleoecological assemblages is comparable to that in a year's accumulation of life assemblages sampled approximately monthly. The temporal resolution of the fossil samples in Lake Tanganyika could thus be as short as one year. Species abundance distributions were statistically indistinguishable among data sets. Rank abundance tests demonstrated that death and fossil assemblages were quite similar, although life assemblages differed substantially in the composition of their dominant species. Species composition differences between life and paleoecological assemblages appear to reflect the area of spatial integration represented by an assemblage—i.e., death and fossil assemblages are integrated over multiple habitat types, whereas life assemblages dominantly represent the rocky habitats where they were collected. Species occurrence frequencies in paleoecological data identified ecologically persistent species and may be useful for delimiting local species pools. Analysis of sampling efficiency indicates that approximately 28% of species in each paleoecological assemblage are “unique”; i.e., they are not likely to be present in an additional subsample from the same sample. Ordination reveals that life assemblages of ostracodes are characterized by high spatiotemporal heterogeneity. Variability in species composition was lower in paleoecological assemblages, presumably as a result of spatial and temporal averaging.

Death and fossil assemblages of Lake Tanganyika appear to preserve many characteristics of living benthic ostracode assemblages with high fidelity. Spatiotemporal averaging allows paleoecological assemblages to render information about the average composition of ostracode communities over short timescales, at spatial scales of several meters, and across habitat types. Sampling shell assemblages in surficial sediments thus represents a more efficient way of assessing the average ecological conditions at a locality than repeated live sampling. Furthermore, paleoecological analyses can generate novel insights into long-term community variability and membership with direct relevance to conservation.

Better understanding of the functional biology of early angiosperms may clarify ecological factors surrounding their origin and early radiation. Phylogenetic studies identify Amborella, Nymphaeales (water lilies), Austrobaileyales, and Chloranthaceae as extant lineages that branched before the radiation of core angiosperms. Among living plants, these lineages may represent the best models for the ecology and physiology of early angiosperms. Here we combine phylogenetic reconstruction with new data on the morphology and ecophysiology of these plants to infer early angiosperm function. With few exceptions, Amborella, Austrobaileyales, and Chloranthaceae share ecophysiological traits associated with shady, disturbed, and wet habitats. These features include low and easily light-saturated photosynthetic rates, leaf anatomy related to the capture of understory light, small seed size, and clonal reproduction. Some Chloranthaceae, however, possess higher photosynthetic capacities and seedlings that recruit in canopy gaps and other sunny, disturbed habitats, which may have allowed Cretaceous Chloranthaceae to expand into more diverse environments. In contrast, water lilies possess ecophysiological features linked to aquatic, sunny habitats, such as absence of a vascular cambium, ventilating stems and roots, and floating leaves tuned for high photosynthetic rates in full sun. Nymphaeales may represent an early radiation into such aquatic environments. We hypothesize that the earliest angiosperms were woody plants that grew in dimly lit, disturbed forest understory habitats and/or shady streamside settings. This ecology may have restricted the diversity of pre-Aptian angiosperms and living basal lineages. The vegetative flexibility that evolved in the understory, however, may have been a key factor in their diversification in other habitats. Our inferences based on living plants are consistent with many aspects of the Early Cretaceous fossil record and can be tested with further study of the anatomy, chemistry, and sedimentological context of Early Cretaceous angiosperm fossils.

The effects of specialization on subsequent morphological evolution are poorly understood. Specialization has been implicated in both adaptive radiations that result from key innovations and evolutionary “dead ends,” where specialized characteristics appear to limit subsequent evolutionary options. Despite much theoretical debate, however, empirical studies remain infrequent. In this paper, we use sister-group comparisons to evaluate the effect of morphological specialization to a particular ecological niche, hypercarnivory, on subsequent taxonomic and morphological diversity. Six sets of sister groups are identified in which one clade exhibits hypercarnivorous characteristics and the sister clade does not. Comparison results are summed across the categories “hypercarnivore” and “sister group.” We also evaluate whether increasing degrees of specialization are correlated with decreasing phenotypic variation. Results presented here indicate that specialization to hypercarnivory has no effect on taxonomic diversity, but a strong effect on subsequent morphological diversity related to the jaws and dentition, and that increasing specialization does not correlate with morphological diversity except in the most specialized saber-toothed taxa, which exhibit higher variance than less specialized morphs, possibly due to selection on other characteristics.

Many late Pleistocene fossil localities contain the remains of multiple mammoths. Some of these sites have been interpreted as representing the mass death of an entire herd, or family group, of mammoths. These assemblages have been cited as evidence of intense human predation and used to reconstruct mammoth population dynamics. However, these interpretations remain controversial because the taphonomic settings of many sites are still debated. To reconstruct the taphonomic setting of each site and the movement patterns of mammoths among sites, I used analyses of carbon, oxygen, and strontium isotope ratios in mammoth tooth enamel. The carbon isotopes of fossils vary with diet and local vegetation, oxygen isotopes vary with local climate, and strontium isotopes vary with local soil chemistry. If Pleistocene mammoths traveled together in small family groups, then mammoths from sites that represent family groups should have lower isotopic variability than mammoths from sites containing unrelated individuals. I tested this conjecture by comparing the isotopic variability among mammoths from two sites—one that represents the mass death of a single herd (Waco, Texas) and one representing a time-averaged accumulation (Friesenhahn Cave, Texas)—and then used these analyses to examine mammoths from three Clovis sites: Blackwater Draw, New Mexico; Dent, Colorado; and Miami, Texas. Low levels of carbon isotope variability were found to be the most diagnostic signal of herd/family group association. Although the variability of oxygen and strontium isotope ratios proved less useful for identifying family group assemblages, these signals did provide information about the movement patterns of individuals among different sites. High levels of variability in each of the isotope systems at Clovis sites suggest that all of the sites examined represent time-averaged accumulations of unrelated individuals, rather than the mass deaths of family groups.

In addition, analyses of the mean isotope values of Clovis mammoths show that although most mammoths from Blackwater and Miami had similar values, the values of Dent mammoths were significantly different. This demonstrates that the Dent mammoths belonged to a separate population and suggests that Clovis mammoths did not routinely undertake long distance (≥600 km) migrations.

To reconstruct the paleodiet and habitat preference of gomphotheres, we measured the carbon and oxygen isotope composition of 68 bone and tooth samples for three species of Gomphotheriidae from 24 different localities (Argentina, Bolivia, Ecuador, Chile, and Brazil). Additionally, we measured the isotopic oxygen in the phosphate of 30 samples to control diagenetic alteration. We calculated the correlation between pairs of δ¹⁸O_p–δ¹⁸O_c values for enamel, dentine, and bone, taken from the same individual in order to verify whether the oxygen of structural apatite carbonate was in equilibrium with body water. Because of the good correlation obtained among pairs of the three skeletal components, we considered the δ¹³C results of all components to be equally representative of both gomphothere groups, and we used them collectively in the analysis of the data.

To compare the different groups of specimens, we divided the samples into six groups, taking into account their taxonomy as well as their geographic and stratigraphic distribution. Cuvieronius specimens from Chile were exclusively C₃ plants eaters, whereas specimens from Bolivia and Ecuador had a mixed C₃-C₄ diet. Stegomastodon showed a wider range of dietary adaptations. Specimens from Quequén Salado in Buenos Aires Province were entirely C₃ feeders, whereas the diet of specimens from La Carolina Peninsula (Ecuador) was exclusively C₄. The remaining South American groups analyzed were C₃-C₄ mixed-feeders. Carbon isotope composition of bone and teeth decreased as latitude increased. We found evidence of an exclusively C₃-dominated diet at approximately 35–41°S. This result confirms that ancient feeding ecology cannot always be inferred from dental morphology or extant relatives. Data from middle and late Pleistocene indicated that, over time, there was an adaptive change in paleodiet from predominantly mixed-feeders to more specialized feeders. We propose that this dietary evolution was one of the causes that forced gomphotheres to extinction in South America. In addition, the data presented in this paper suggest that because of the different feeding preferences among mastodons, mammoths, and gomphotheres, only the bunodont gomphotheres reached South America.