Registered users receive a variety of benefits including the ability to customize email alerts, create favorite journals list, and save searches.
Please note that a BioOne web account does not automatically grant access to full-text content. An institutional or society member subscription is required to view non-Open Access content.
Contact email@example.com with any questions.
The recognition that past catastrophic events may have caused the simultaneous extinction of many taxa has prompted the development of statistical tests to determine the compatibility of the fossil record with such scenarios. Statistical tests necessitate simplifying assumptions, the most significant of which are continuous (as opposed to discrete) data in the sampling of the fossil record and random distribution of fossil occurrences because constant preservation potential and collecting intensity of fossils are assumed. The assumption of random distribution of fossil occurrences has been addressed quantitatively, but the continuity of data has not. The assumption of continuous data is not trivial, for although collecting may appear continuous, the resulting data are never strictly continuous because the stratigraphic positions of fossils are known with limited precision. For one test of simultaneous extinction, simulations demonstrate that the data behave in a less continuous manner as the occurrence rate increases, decreasing the reliability of the test at higher occurrence rates. Out of mathematical simplicity, nearly all tests use simultaneous extinction as the null hypothesis. However, even under ideal sampling circumstances, gradual extinctions that occurred over a sufficiently short stratigraphic interval are likely to be statistically indistinguishable from extinction occurring at a single horizon. Therefore, a range of gradual extinction scenarios will also be compatible with the fossil record even if the null hypothesis is not rejected. Because direct tests of gradual extinction scenarios have not yet been developed, simulations were used to determine the probability that an extinction occurring over a given stratigraphic distance with a given number of taxa will be statistically indistinguishable from extinction at a single horizon. The simulation results allow the test results to be used to define a range of extinction scenarios compatible with the fossil record. Finally, the simulation results were applied to two Late Cretaceous ammonite data sets. Although both data sets sufficiently approximate continuous sampling for the use of statistical tests, the method shows that the data are not always sufficient to rule out either stratigraphically simultaneous extinction or a literal reading of extinction rate as representing gradual or episodic extinction.
Avian skeletal remains occur in many fossil assemblages, and in spite of small sample sizes and incomplete preservation, they may be a source of valuable paleoecological information. In this paper, we examine the taphonomy of a modern avian bone assemblage and test the relationship between ecological data based on avifaunal skeletal remains and known ecological attributes of a living bird community. A total of 54 modern skeletal occurrences and a sample of 126 identifiable bones from Amboseli Park, Kenya, were analyzed for weathering features and skeletal part preservation in order to characterize preservation features and taphonomic biases. Avian remains, with the exception of ostrich, decay more rapidly than adult mammal bones and rarely reach advanced stages of weathering. Breakage and the percentage of anterior limb elements serve as indicators of taphonomic overprinting that may affect paleoecological signals. Using ecomorphic categories including body weight, diet, and habitat, we compared species in the bone assemblage with the living Amboseli avifauna. The documented bone sample is biased toward large body size, representation of open grassland habitats, and grazing or scavenging diets. In spite of this, multidimensional scaling analysis shows that the small faunal sample (16 out of 364 species) in the pre-fossil bone assemblage accurately represents general features of avian ecospace in Amboseli. This provides a measure of the potential fidelity of paleoecological reconstructions based on small samples of avian remains. In the Cenozoic, the utility of avian fossils is enhanced because bird ecomorphology is relatively well known and conservative through time, allowing back-extrapolations of habitat preferences, diet, etc. based on modern taxa.
One of the central goals in paleoecology is to understand the nature and consequences of biotic interactions. In marine systems, it has been argued that one of the major steps in the escalation of biotic interactions was marked by the origins of grazing fishes in the Cenozoic. Here I investigate the origins of herbivory and grazing in marine fishes using analyses of functional morphospace. Closing and opening lever ratios and relative length of the lower jaw are used to construct a plot of functional morphospace, a quantitative description of the potential feeding modes of fishes. Four fish faunas were examined, spanning the Mesozoic and Cenozoic (Triassic, Jurassic, Eocene and Recent). All three fossil faunas are from conservation Lagerstätten in the central Tethys, in the vicinity of coral reefs or coral-bearing hardgrounds. Changes in functional morphospace occupation reveal a marked shift in the Cenozoic, with the appearance of fishes with relatively small forceful jaws. In Recent faunas, this functional morphospace is occupied almost exclusively by grazing herbivores. This taxon-independent morphological signal of herbivory was lacking in the Mesozoic faunas, was first recorded in the Eocene, and persisted throughout the Cenozoic. This suggests that the Cenozoic did indeed witness the appearance and proliferation of herbivory and grazing by marine fishes. The arrival of these piscine herbivores had the potential to fundamentally alter the dynamics of benthic marine communities.
The living species Ginkgo biloba is phylogenetically isolated, has a relictual distribution, and is morphologically very similar to Mesozoic and Cenozoic congenerics. To investigate what adaptations may have allowed this lineage to persist with little or no morphological change for over 100 Myr, we analyzed both sedimentological and floral data from 51 latest Cretaceous to middle Miocene Ginkgo-bearing fossil plant sites in North America and northern Europe. The resulting data indicate that throughout the late Cretaceous and Cenozoic Ginkgo was largely confined to disturbed streamside and levee environments, where it occurred with a consistent set of other plants. These inferred habitats are surprising because the life-history traits of Ginkgo (e.g., slow growth rate, late reproductive maturity, extended reproductive cycle, large and complex seeds, large and slowly developing embryos) are counter to those considered advantageous in modern disturbed habitats. Many flowering plant lineages first appeared or became common in disturbed riparian habitats, and are inferred to have had reproductive and growth traits (e.g., rapid reproduction, small easily dispersed seeds, rapid growth) suited to such habitats. Paleoecological inferences based on both morphology and sedimentary environments thus support the idea that Ginkgo was displaced in riparian habitats by angiosperms with better adaptations to frequent disturbance.
Despite numerous studies, the thermal physiology of dinosaurs remains unresolved. Thus, perhaps the commonly asked question whether dinosaurs were ectotherms or endotherms is inappropriate, and it is more constructive to ask which dinosaurs were likely to have been endothermic and which ones ectothermic. Field data from crocodiles over a large size range show that body temperature fluctuations decrease with increasing body mass, and that average daily body temperatures increase with increasing mass. A biophysical model, the biological relevance of which was tested against field data, was used to predict body temperatures of dinosaurs. However, rather than predicting thermal relations of a hypothetical dinosaur, the model considered correct paleogeographical distribution and climate to predict the thermal relations of a large number of dinosaurs known from the fossil record (>700). Many dinosaurs could have had “high” (≥30°C) and stable (daily amplitude ≤2°C) body temperatures without metabolic heat production even in winter, so it is unlikely that selection pressure would have favored the evolution of elevated resting metabolic rates in those species. Recent evidence of ontogenetic growth rates indicates that even the juveniles of large species (3000–4000 kg) could have had biologically functional body temperature ranges during early development. Smaller dinosaurs (<100 kg) at mid to high latitudes (>45°) could not have had high and stable body temperatures without metabolic heat production. However, elevated metabolic rates were unlikely to have provided selective advantage in the absence of some form of insulation, so probably insulation was present before endothermy evolved, or else it coevolved with elevated metabolic rates. Superimposing these findings onto a phylogeny of the Dinosauria suggests that endothermy most likely evolved among the Coelurosauria and, to a lesser extent, among the Hypsilophodontidae, but not among the Stegosauridae, Nodosauridae, Ankylosauridae, Hadrosauridae, Ceratopsidae, Prosauropoda, and Sauropoda.
The euphractine Macroeuphractus outesi, from the late Pliocene Chapadmalalan SALMA of Buenos Aires Province, Argentina, is one of the largest dasypodids known. Its skull preserves features remarkable for an armadillo. Its complete dental arcade and large caniniform teeth have received attention in the literature as indicative of scavenging behavior. This report considers the degree of carnivory within the context of the generally omnivorous feeding behavior of euphractine armadillos through morphological and biomechanical analyses. Morphological analyses reveal that the main differences between M. outesi and other euphractines are the enlargement of the cranium, particularly of the temporal fossa; more prominent muscular scars for origin of the temporalis muscle; a more expanded rostrum, particularly in dorsoventral height; a more powerful anterior dentition, especially in the great enlargement and caniniform modification of M2; and a deeper and more robust zygomatic arch. Biomechanical analyses indicate that the moment arm of the temporalis musculature is greater than that recorded for other armadillos. These analyses indicate that the temporalis was probably larger and played a more important role in Macroeuphractus than in other euphractines, a pattern that is more usual for carnivorous mammals. Combined with the second upper molariform, which is caniniform, the features suggest that Macroeuphractus occupied an extreme position in the carnivorous-omnivorous feeding behavior of euphractines. Its large size indicates that it could have easily preyed on hare-sized vertebrates.
Disparity appears to decrease or remain stable over geological time in numerous groups. This pattern is sometimes explained in terms of developmental constraints, but labile developmental systems might produce the same pattern should novelties interact, counterbalancing their individual effects. We test the hypothesis that counterbalancing can reduce disparity by comparing ontogenies of shape among nine species of piranhas to identify the developmental novelties. All three parameters examined change multiple times, sometimes dramatically. By comparing levels of disparity between species at two developmental phases, at the transition from larval to juvenile phases, and at maximum adult body size, we find that disparity decreases significantly and substantially over ontogeny. That reduction occurs because of, rather than despite, novelties of postlarval morphogenesis. Some interacting novelties are historically independent and affect different developmental phases, others are historically independent and affect the same developmental phase, and still others are historically correlated and affect either the same or different developmental phases. By modeling hypothetical ontogenies, constraining developmental parameters mathematically to one of the observed values, we find that variation in each parameter, taken by itself, and combinations of them taken two at a time, tend to increase disparity. It is the interactions among all three that reduce disparity. In this group divergent ontogenies transform disparate larvae into similar adults.