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The most abundant and diverse graptolite assemblages are found in offshore, deep-water black shales—the classical “graptolite facies” (deep-water or isograptid biofacies). The mean duration of Ordovician graptolite species confined to the deep-water facies (here referred to as “group 1” species) is 2.19 Myr, significantly shorter than the mean duration of species in the deep-water facies that are also known in sediments of the shallow-water shelf or platform (“group 2” species) −4.42 Myr, indicating a significantly higher extinction probability (p = <0.001). These figures are based on the precise age ranges of species derived from the time-calibrated composite sequence of 1446 Ordovician to early Devonian graptolites, built by the constrained optimization procedure (CONOP) from 256 measured sections worldwide, and exclude the effects of the Hirnantian mass extinction. The difference between groups cuts across families, morphological types, and pandemic/endemic distributions. An environmental influence is strongly suggested, and although both groups were planktonic, they were unlikely to have shared the same habitat in the water column. The new duration measurements therefore are interpreted as favoring a depth-stratification of graptolite habitats in the water column.
In a seminal paper in 1975, Gould proposed that postcanine occlusal area (PCOA) should scale metabolically (0.75) with body mass across mammals. By regressing PCOA against skull length in a small sample of large-bodied herbivorous mammals, Gould provided some marginal support for this hypothesis, which he then extrapolated as a universal scaling law for Mammalia. Since then, many studies have sought to confirm this scaling relationship within a single order and have found equivocal support for Gould's assertion. In part, this may be related to the use of proxies for both PCOA and body mass, small sample sizes, or the influence of a “taxon-level effect,” rendering Gould's scaling “universal” problematic.
Our goal was to test the universality of Gould's prediction and the impact of the taxon-level effect on regressions of tooth size on body mass in a large extant mammalian sample (683 species spanning 14 orders). We tested for the presence of two types of taxon-level effect that may influence the acceptance or rejection of hypothesized scaling coefficients. The hypotheses of both metabolic and isometric scaling can be rejected in Mammalia, but not in all sub-groups therein. The level of data aggregation also influences the interpretation of the scaling relationship. Because the scaling relationship of tooth size to body mass is highly dependent on both the taxonomic level of analysis and the mathematical methods used to organize the data, paleontologists attempting to retrodict body mass from fossilized dental remains must be aware of the effect that sample composition may have on their results.
We analyze relationships among a range of ecological and biological traits—geographic range size, body size, life mode, larval type, and feeding type—in order to identify those traits that are associated significantly with species duration in New Zealand Cenozoic marine molluscs, during a time of background extinction. Using log-linear modeling, we find that bivalves have only a small number of simple, two-way associations between the studied traits and duration. In contrast, gastropods display more complex interactions involving three-way associations between traits, a pattern that suggests greater macroecological complexity of gastropods. This is not an artifact caused by the larger number of gastropods than bivalves in our data set. We used stratified randomized resampling of families to test for associations between traits that might result from shared inheritance rather than ecological trait interactions; we found no evidence of phylogenetic effects in any associations examined. The relationships revealed by our study should serve to constrain the range of possible biological mechanisms that underlie these relationships. As previously observed, two-way associations are present between large geographic range and increased duration, and between large geographic range and large body size, in both bivalves and gastropods. In gastropods, planktotrophic larval type is associated with large range size through a three-way interaction that also involves duration; there is no direct association of larval type and geographic range. Gastropods also display two-way associations between duration and life mode, and duration and feeding type. We note that in gastropods, an infaunal life mode is associated with large range size, whereas in bivalves infaunality is associated with reduced range size.
Species arise and establish themselves over the geologic time scale. This process is manifested as a change in the relative frequency of occurrences of a given species in the global pool of species. Our main goal here is to model this rise and the eventual decline of microfossil species using a mixed-effects model where groups each have a characteristic occurrence trajectory (main effects) and each species belonging to those groups is allowed to deviate from the given group trajectory (random effects). Our model can be described as a “hat” with logistic forms in the periods of increase and decline. Using the estimated timings of rises and falls, we find that the lengths of the periods of rise are about as long as the lengths of the periods when species are above 50% of their estimated maximal occurrences. These latter periods are here termed periods of dominance, which are in turn about the same length as the species' periods of fall. The peak rates of the rises of microfossils are in general faster than their peak rates of falls. These quantified observations may have broad macroevolutionary and macroecological implications. Further, we hypothesize that species that have experienced and survived high levels of environmental volatility (specifically, periods of greater than average variation in temperature and productivity) during their formative periods should have longer periods of dominance. This is because subsequent environmental variations should not drive them to decline with ease. We find that higher estimated environmental volatility early in the life of a species positively correlates with lengths of periods of dominance, given that a species survives the initial stress of the environmental fluctuations. However, we find no evidence that the steepness of the rise of a species is affected by environmental volatility in the early phases of its life.
Both the body fossils and trackways of sauropod dinosaurs indicate that they inhabited a range of inland and coastal environments during their 160-Myr evolutionary history. Quantitative paleoecological analyses of a large data set of sauropod occurrences reveal a statistically significant positive association between non-titanosaurs and coastal environments, and between titanosaurs and inland environments. Similarly, “narrow-gauge” trackways are positively associated with coastal environments and “wide-gauge” trackways are associated with inland environments. The statistical support for these associations suggests that this is a genuine ecological signal: non-titanosaur sauropods preferred coastal environments such as carbonate platforms, whereas titanosaurs preferred inland environments such as fluvio-lacustrine systems. These results remain robust when the data set is time sliced and jackknifed in various ways. When the analyses are repeated using the more inclusive groupings of titanosauriforms and Macronaria, the signal is weakened or lost. These results reinforce the hypothesis that “wide-gauge” trackways were produced by titanosaurs. It is commonly assumed that the trackway and body fossil records will give different results, with the former providing a more reliable guide to the habitats occupied by extinct organisms because footprints are produced during life, whereas carcasses can be transported to different environments prior to burial. However, this view is challenged by our observation that separate body fossil and trackway data sets independently support the same conclusions regarding environmental preferences in sauropod dinosaurs. Similarly, analyzing localities and individuals independently results in the same environmental associations. We demonstrate that conclusions about environmental patterns among fossil taxa can be highly sensitive to an investigator's choices regarding analytical protocols. In particular, decisions regarding the taxonomic groupings used for comparison, the time range represented by the data set, and the criteria used to identify the number of localities can all have a marked effect on conclusions regarding the existence and nature of putative environmental associations. We recommend that large data sets be explored for such associations at a variety of different taxonomic and temporal scales.
Despite increasing concerns about the effect of sampling biases on our reading of the fossil record, few studies have considered the completeness of the fossil remains themselves, and those that have tend to apply non-quantitative measures of preservation quality. Here we outline two new types of metric for quantifying the completeness of the fossil remains of taxa through time, using sauropodomorph dinosaurs as a case study. The “Skeletal Completeness Metric” divides the skeleton up into percentages based on the amount of bone for each region, whereas the “Character Completeness Metric” is based on the number of characters that can be scored for each skeletal element in phylogenetic analyses. For both metrics we calculated the completeness of the most complete individual and of the type specimen. We also calculated how well the taxon as a whole is known from its remains. We then plotted these results against both geological and historical time, and compared curves of the former with fluctuations in sauropodomorph diversity, sea level, and sedimentary rock outcrop area. Completeness through the Mesozoic shows a number of peaks and troughs; the Early Jurassic (Hettangian–Sinemurian) is the interval with highest completeness, whereas the mid-to-Late Cretaceous has completeness levels that are consistently lower than the rest of the Mesozoic. Completeness shows no relationship to rock outcrop area, but it is negatively correlated with sea level during the Jurassic–Early Cretaceous and correlated with diversity in the Cretaceous. Completeness of sauropodomorph type specimens has improved from 1830 to the present, supporting the conclusions of other recent studies. However, when this time interval is partitioned, we find no trend for an increase in completeness from the 1990s onward. Moreover, the 2000s represent one of the poorest decades in terms of average type specimen completeness. These results highlight the need for quantitative methods when assessing fossil record quality through geological time or when drawing conclusions about historical trends in the completeness of taxa. The new metrics may also prove useful as sampling proxies in diversity studies.
Organisms emit, detect, and respond to a huge array of environmental signals. The distribution of a given signal is dependent, first of all, upon the original spatial distribution of signal sources, the source landscape. The signal sources can be fixed or moving and their output can be stable or ephemeral. Different sources can also occupy the same general spatial location, such as insects living on a host plant. The emitted signals are modified by relevant transport processes, which are often strongly scale and environment dependent. Chemical signals, for example, are propagated by diffusion and turbulence. The resulting complex, three-dimensional, and dynamic distribution of signals in the environment is the signal landscape; it is the environment of potentially available information in which sensory systems function and have evolved. Organisms also differ widely in what signals they can actually detect; the distribution of signals that an organism can potentially respond to is its information landscape. Although increasing the kinds and specificity of signals that can be detected and processed can lead to improved decision making, it almost always comes at an increased cost. The greater the spatial and temporal complexity of the environment, the greater are the costs of incomplete information and the more advantageous is the development of improved information-gathering capabilities. Studies with simulation models suggest how variability in the spatial structure of source and signal landscapes may control patterns of animal movement that could be represented in the trace fossil record. Information landscapes and the corresponding sensory systems should have evolved in concert with major transitions in the history of life. The Ediacaran to Cambrian interval is one of the most intensively studied periods in the history of life, characterized by the profound environmental and biological changes associated with the bilaterian radiation. These include the advent of macroscopic predation, an increase in the size and energy content of organisms, and the transition in seafloors from laminated matgrounds to mixgrounds produced by the development of macroscopic infaunal bioturbation. The overall effect of these transitions was to markedly increase the spatial complexity of the marine environment. We suggest that this increased spatial complexity, in turn, drove the evolution of macroscopic sense organs in mobile bilaterians, leading to their first appearance during the Cambrian. The morphology and distribution of these sense organs should reflect the life habits of the animals that possessed them. Our overall hypothesis was that there was a “Cambrian Information Revolution,” a coevolutionary increase in the information content of the marine environment and in the ability of and necessity for organisms to obtain and process this information. A preliminary analysis of the Maotianshan Shale (Chengjiang) biota indicates that the distribution of eyes and antennae in these animals is consistent with predictions based on their life habit.
Recent studies have provided detailed insight into life cycles of early amphibians. These ontogenies were diverse and their evolution involved numerous kinds of change, which can now be addressed by comparison of ontogenetic trajectories. The plesiomorphic trajectory included (1) an early period in which a larval, aquatic predator was established, (2) an intermediate period in which the axial skeleton was strengthened, and (3) a final period during which the jaw joint, braincase, and limbs were ossified, producing an adult capable of terrestrial locomotion if completed. Heterochrony, among other factors, enabled the fine-tuning of the ontogenetic formation of ecologically important features (feeding, respiration, locomotion). Most common was a simple truncation of the trajectory that produced aquatic taxa of various kinds, while changes in the ontogenetic sequence often had a deeper impact on morphology. The most fundamental changes were accompanied by multiple heterochronies, resulting in the condensation or unpacking (stretch-out) of developmental events: metamorphosis evolved by an ever closer packing, whereas a novel larval feeding mechanism was established by a pull-apart of numerous critical events.
We present a morphometric analysis of water transport cells within a physiologically explicit three-dimensional space. Previous work has shown that cell length, diameter, and pit resistance govern the hydraulic resistance of individual conducting cells; thus, we use these three parameters as axes for our morphospace. We compare living and extinct plants within this space to investigate how patterns of plant conductivity have changed over evolutionary time. Extinct coniferophytes fall within the range of living conifers, despite differences in tracheid-level anatomy. Living cycads, Ginkgo biloba, the Miocene fossil Ginkgo beckii, and extinct cycadeoids overlap with both conifers and vesselless angiosperms. Three Paleozoic seed plants, however, occur in a portion of the morphospace that no living seed plant occupies. Lyginopteris, Callistophyton, and, especially, Medullosa evolved tracheids with high conductivities similar to those of some vessel-bearing angiosperms. Such fossils indicate that extinct seed plants evolved a structural and functional diversity of xylem architectures broader, in some ways, than the range observable in living seed plants.