Microfossil bonebeds are multi-individual accumulations of disarticulated and dissociated vertebrate hardparts dominated by elements in the millimeter to centimeter size range (≤75% of bioclasts ≤5 cm maximum dimension). Modes of accumulation are often difficult to decipher from reports in the literature, although predatory (scatological) and fluvial/hydraulic origins are typically proposed. We studied the sedimentology and taphonomy of 27 microfossil bonebeds in the Campanian Judith River Formation of Montana in order to reconstruct formative histories. Sixteen of the bonebeds examined are hosted by fine-grained facies that accumulated in low-energy aquatic settings (pond/lake microfossil bonebeds). Eleven of the bonebeds are embedded in sandstones that accumulated in ancient fluvial settings (channel-hosted microfossil bonebeds). In lieu of invoking separate pathways to accumulation based on facies distinctions, we present a model that links the accumulation of bioclasts in the two facies. We propose that vertebrate material initially accumulates to fossiliferous levels in ponds/lakes and is later reworked and redeposited as channel-hosted assemblages. This interpretation is grounded in reasonable expectations of lacustrine and fluvial depositional systems and supported by taphonomic data. Moreover, it is consistent with faunal data that indicate that channel-hosted assemblages and pond/lake assemblages are similar with regard to presence/absence and rank-order abundance of taxa.

This revised model of bonebed formation has significant implications for studies of vertebrate paleoecology that hinge on analyses of faunal data recovered from vertebrate microfossil assemblages. Pond/lake microfossil bonebeds in the Judith River record are preserved in situ at the scale of the local paleoenvironment, with no indication of postmortem transport into or out of the life habitat. Moreover, they are time-averaged samples of their source communities, which increases the likelihood of capturing both ecologically abundant species and more rare or transient members of the paleocommunity. These attributes make pond/lake microfossil bonebeds excellent targets for paleoecological studies that seek to reconstruct overall community membership and structure. In contrast, channel-hosted microfossil bonebeds in the Judith River record are out of place from a paleoenvironmental perspective because they are reworked from preexisting pond/lake assemblages and redeposited in younger channel facies. However, despite a history of exhumation and redeposition, channel-hosted microfossil bonebeds are preserved in relatively close spatial proximity to original source beds. This taphonomic reconstruction is counter to the commonly held view that microfossil bonebeds are biased samples that have experienced long-distance transport and significant hydrodynamic sorting.

All else being equal, species with short life spans are expected to be overrepresented in time-averaged death assemblages relative to their standing abundance in the living community, but the magnitude of the distortion of proportional abundance and assemblage evenness has received little attention. Here, information from 30 data sets on the living and dead abundances of marine bivalves in local habitats is combined with a global compilation of bivalve life spans to determine whether bias from mortality rate can explain observed differences in species proportional abundances. Although bivalve maximum life spans range from one to 75 years in these data sets, indicating annual mortality rates of 0.97 to 0.09, the “life span bias” (LB) of a species–the difference between its proportional abundance expected dead and that observed alive–is consistently small in magnitude (average change <2%, maximum about 20%) and random in sign relative to observed discordance (OD = difference between that species' proportional abundance observed dead and that observed alive). The aggregate result for 413 living species occurrences is a significantly positive but weak correlation of OD to LB, with only 10% of variation in OD explained. The model performs better among longer-lived species than among shorter-lived species, probably because longer-lived species conform better to the model assumption that species maintain a constant proportional abundance in the living assemblage over time. Among individual data sets, only seven exhibit significant positive correlations between OD and LB. The model also under-predicts the cases where a death assemblage is dominated by a species that is shorter lived than the dominant species in the living assemblage, indicating that some factor(s) other than or in addition to mortality rate is responsible for OD. We can find no evidence of preservational bias linked to life span, for example through body size. This negative outcome reflects a weak biological relationship between life span and living abundance among bivalves in local habitats, contrary to the terrestrial paradigm, and points toward a simpler model of time-averaged death assemblage formation where higher abundances reflect (under-sampled) past populations. Contrary to long-held expectations, variation in population turnover among species is not a major source of taphonomic bias in time-averaged death assemblages among bivalves and perhaps among other marine groups: bias must arise largely from other factors.

To understand how well fossil assemblages represent original communities, paleoecologists seek comparisons between death assemblages and their source communities. These comparisons have traditionally used nearshore, marine invertebrate assemblages for their logistical ease, high abundance, and comparable census data from living communities. For large marine vertebrates, like cetaceans, measuring their diversity in ocean ecosystems is difficult and expensive. Cetaceans, however, often beach or strand themselves along the coast, and archived data on stranded cetaceans have been recorded, in some areas, over several decades. If the stranding record is interpreted as a death assemblage, then the stranding record may represent a viable alternative for measuring diversity in living communities on directly adjacent coastlines. This study assessed the fidelity of the cetacean stranding record in the eastern North Pacific Ocean. The living community in this region has been studied for over 100 years and, recently, extensive and systematic live transect surveys using ship-based observing platforms have produced a valuable source of live diversity data. Over this same period, the U.S. Marine Mammal Stranding Program has collected and archived a record of cetacean strandings along the U.S. Pacific coastline, providing an ideal death assemblage for comparison. Using fidelity metrics commonly used in marine invertebrate taphonomy, I determined that the stranding record samples the living cetacean community with high fidelity, across fine and coarse taxonomic ranks, and at large geographic scales (>1000 km of coastline). The stranding record is also richer than the live surveys, with live-dead ratios between 1.1 and 1.3. The stranding record recovers similar rank-order relative abundances as live surveys, with statistical significance. Also, I applied sample-based rarefaction methods to generate collector's curves for strandings along the U.S. Pacific Coast to better evaluate the spatiotemporal characteristics of the stranding record. Results indicate that saturation (i.e., sampling >95% assemblage) at species, genus, and family levels occurs in less than five years of sampling, with families accumulating faster than species, and larger geographic regions (i.e., longer coastlines) accumulating taxa the most rapidly. The high fidelity of the stranding record, measured both in richness and by ranked relative abundance, implies that ecological structure from living cetacean communities is recorded in the death assemblage, a finding that parallels marine invertebrate assemblages, though at far larger spatial scales. These results have implications for studying cetacean ecology in both modern and ancient environments: first, these results imply that the stranding record, over sufficiently long time intervals, yields a richer assemblage than using line-transect methods, and faithfully records aspects of community structure; and second, these results imply that geochronologically well-constrained fossil cetacean assemblages might preserve ecologically relevant features of community structure, depending on depositional and taphonomic conditions.

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 progresses in our knowledge of mouse odontogenesis have enhanced rodent tooth morphology as a model for Evo-Devo studies. Deciphering the connection between macroevolution and microevolution, however, especially in the case of mammalian teeth, requires examples to illustrate how morphological differences among species, or higher taxa, can stem from population-level processes. In this paper we use paleontological material to study intraspecific variation of tooth morphology in the late Miocene species Progonomys clauzoni, over a short span of geological time in a restricted area. Progonomys is of particular interest as a stem genus of all murine rodents (Old World rats and mice). We use morphometrical and statistical methods to illustrate how change in the amplitude in variation at the population level through geological time is associated with the emergence of new characters. Some of these new characters, including functional ones, become fixed in parallel in distinct murine lineages. Nine million years ago, Progonomys clauzoni displayed variational properties of the developmental system shared by the Murinae, which can also explain some singular tooth characteristics that now are scattered among the diverse lineages. Further morphometric studies, however, are necessary to explain how the variety of cusp patterns observed in Progonomys clauzoni can be explained by developmental properties.

A large sample of the Pikermi and Samos ungulates was examined by microwear analysis using a light stereomicroscope (561 extinct and 809 extant comparative specimens). The results were used to infer the dietary adaptations of individual species and to evaluate the Pikermian Biome ungulate fauna. Many of the bovids have wear consistent with mixed feeding, although a few mesodont taxa apparently enjoyed an exclusive browsing and or grazing diet. The giraffids spanned the entire dietary spectrum of browsing, mixed feeding, and grazing, but most of the three-toed horses (Hippotherium) were hypsodont grazers. The colobine monkey Mesopithecus pentelici displays microwear consistent with a mixed fruit and leaf diet most likely including some hard objects. Similar results were obtained from prior scanning electron microscopy microwear studies at 500 times magnification and from the light microscope method at 35 times magnification for the same species. Results show that diet can differ between species that have very similar gross tooth morphology. Our results also suggest that the Pikermian Biome was most likely a woodland mosaic that provided a diversity of opportunities for species that depended on browsing as well as species that ate grass. The grasses were most likely C3 grasses that would grow in shaded areas of the woodland, glades, and margins of water. The ungulate component of the Pikermi and Samos fauna was more species-rich and more diverse in diet than the ungulates observed in modern African forests, woodlands, or savannas, yet dietarily most similar to the ungulates found in woodland elements of India and to some extent of Africa. It is unlikely that the Pikermi and Samos ungulates inhabited dense forests because we find no evidence for heavy fruit browsing. Conversely, a pure savanna is unlikely because many mixed feeders are present as well as browsers. Extant woodland African species are morphologically and trophically very similar to the African savanna species. Therefore the evolution of grazing and of hypsodont morphology for Africa may have evolved within the Plio-Pleistocene woodlands of Africa. Our results show that major dietary and morphologic ungulate evolution may take place within woodlands rather than as a consequence of species moving into savannas both during the late Miocene of Pikermi and Samos and during the Pleistocene–Recent of Central Africa.

All evolution attributable to natural selection, at any level, is due to a causal covariance between fitness and phenotype. Over macroevolutionary time scales, species selection is one of many possible mechanisms for generating large-scale morphological trends. For species selection to sort morphology, a correlation between morphology and taxonomic diversification rate must be present. Other trend mechanisms (driven mechanisms, e.g., a bias in the direction of speciation) produce a systematic change in the mean phenotype over time. All mechanisms can co-occur. Here I demonstrate (1) an inverse correlation between diversification rate and calyx complexity that demonstrates the effect of species selection on morphology. Genera with simple calyces tend to increase in diversity, whereas genera with complex calyces have a net decrease in diversity; and (2) the presence of a driven trend mechanism in monobathrid crinoids where descendant genera tend to be simpler than their ancestors. The separate effects of these two classes of trend mechanisms can be combined by using the Price's Theorem, which partitions the contribution to the overall change in calyx complexity over time accurately among selection and driven mechanisms. Price's Theorem provides significant conceptual and methodological clarification of the contribution of multiple and interacting hierarchical mechanisms in generating large-scale trends.

Constrained seriation of a species-locality matrix of the Australian Cenozoic mammal record resolves a preliminary sixfold succession of land mammal ages apparently spanning the late Oligocene to the present. The applied conditions of local chronostratigraphic succession and inferences of relative stage-of-evolution biochronology lead to the expression of a continental geological timescale consisting of, from the base, the Etadunnan, Wipajirian, Camfieldian, Waitean, Tirarian, and Naracoortean land mammal ages. Approximately 99% of the 360 fossil assemblages analyzed are classifiable using this method. Each is characterized by a diagnostic suite of species. An interval of age magnitude may eventually be shown to lie between the Camfieldian and Waitean, but is currently insufficiently represented by fossils to diagnose. Development of a land mammal age framework marks a progressive step in Australian vertebrate biochronology, previously expressed only in terms of local faunas. Overall, however, the record remains poorly calibrated to the Standard Chronostratigraphic Scale. Codifying the empirical record as a land mammal age sequence provides an objective basis for expressing faunal succession without resort to standard chronostratigraphic terms with the attendant (and hitherto commonly taken) risks of miscorrelating poorly dated Australian events to well-dated global events.

Natural accumulations of skeletal remains represent a valuable source of ecological data for paleontologists and neontologists alike. Use of these records requires a quantitative assessment of the degree to which potential biasing factors affect how accurately ecological information from the living community is recorded in the sedimentary record. This has been a major focus in recent years for taphonomists working with marine records, yet terrestrial systems have remained virtually unstudied—particularly communities of small-bodied taxa. Our ability to assess the potential origins and effects of postmortem bias in terrestrial skeletal assemblages (both modern and fossil) has therefore been limited. Predation is a common mechanism by which small-mammal skeletal remains are concentrated; raptors regurgitate the remains of their small-mammal prey in pellets rich in skeletal material, which accumulate below long-term roosting sites, especially in protected areas such as caves and rock shelters. Here I compare small-mammal death assemblages concentrated via owl predation at Two Ledges Chamber, a long-term owl cave roost in northwestern Nevada, with data from modern trapping surveys to evaluate (1) their ecological fidelity to the modern small-mammal community, (2) the effects of temporal variation and time-averaging (over months to centuries) on live-dead agreement, and (3) how spatial averaging affects the landscape-scale picture of the small-mammal community as reconstructed from dead remains. Despite potential obstacles to the recovery of ecological information from skeletal deposits generated via predation, I find high live-dead agreement across all ecological metrics and all temporal comparisons. I also find that the effects of time-averaging (specifically increased species richness of the death assemblage) become significant only at the century scale. Finally, I combine a mixing model approach with a principal coordinates analysis to show that the owls at Two Ledges Chamber sample from all habitats present in the immediate vicinity of the cave, producing a high-fidelity snapshot of the community that is spatially integrated at the local landscape scale.

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.

Biometric analyses are useful tools for the study of organisms, their phylogenetic affiliation, and the pattern and rate of their evolution. Various morphometric techniques have been developed to analyze morphological variation, but methodological choices are often made arbitrarily because quantitative comparisons are lacking or inconclusive. Here we address morphometric quantification of taxa with few unambiguously identifiable landmarks (<15), utilizing ornamented and unornamented gastropod shells. Support vector machines were applied to evaluate classification performances of landmark (LMA), elliptic Fourier (EFA), and semi-landmark analysis (SLM). This evaluation is based on the discrimination of between-group differences relative to within-group variation, and thus allows comparing how the techniques treat different types of biological information. The results suggest that EFA performs slightly better than SLM (and certainly LMA) in discerning a priori identified taxa with unornamented shells, but that SLM is significantly superior to other techniques for ornamented shells. Alignment and homology problems may cause the subtle variations in ornamentation to become blurred as noise in EFA, even though EFA is often cited to be able to deal with complex shapes. Performance of LMA depends entirely on how accurately the structure can be covered with landmarks. Guidelines in choosing a morphometric technique in diverse cases are provided.

Paleoecological analyses that test for spatial or temporal variation in diversity must consider not only sampling and preservation bias, but also the effects of temporal scale (i.e., time-averaging). The species-time relationship (STR) describes how species diversity increases with the elapsed time of observation, but its consequences for assessing the effects of time-averaging on diversity of fossil assemblages remain poorly explored. Here, we use a neutral, dispersal-limited model of metacommunity dynamics, with parameters estimated from living assemblages of 31 molluscan data sets, to model the effects of within-habitat time-averaging on the mean composition and multivariate dispersion of assemblages, on diversity at point (single station) and habitat scales (pooled multiple stations), and on beta diversity. We hold sample size constant in STRs to isolate the effects of time-averaging from sampling effects. With increasing within-habitat time-averaging, stochastic switching in the identity of species in living (dispersal-limited) assemblages (1) decreases the proportional abundance of abundant species, reducing the steepness of the rank-abundance distribution, and (2) increases the proportional richness of rare, temporally short-lived species that immigrate from the neutral metacommunity with many rare species. These two effects together (1) can shift the mean composition away from the non-averaged (dispersal-limited) assemblages toward averaged assemblages that are less limited by dispersal, resembling that of the metacommunity; (2) allow the point and habitat diversity to increase toward metacommunity diversity under a given sample size (i.e., the diversity in averaged assemblages is inflated relative to non-averaged assemblages); and (3) reduce beta diversity because species unique to individual stations become shared by other stations when limited by a larger but static species pool. Surprisingly, these scale-dependent changes occur at fixed sample sizes and can become significant after only a few decades or centuries of time-averaging, and are accomplished without invoking ecological succession, environmental changes, or selective postmortem preservation. Time-averaging results in less inflation of diversity at habitat than at point scales; paleoecological studies should thus analyze data at multiple spatial scales, including that of the habitat where multiple bulk samples have been pooled in order to minimize time-averaging effects. The diversity of assemblages that have accumulated over 1000 years at point and habitat scales is expected to be inflated by an average of 2.1 and 1.6, respectively. This degree of inflation is slightly higher than that observed in molluscan death assemblages at these same spatial scales (1.8 and 1.3). Thus, neutral metacommunity models provide useful quantitative constraints on directional but predictable effects of time-averaging. They provide minimal estimates for the rate of increase in diversity with time-averaging because they assume no change in environmental conditions and in the composition of the metacommunity within the window of averaging.

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.

The flux ratio method is a simple method for estimating the rate of synonymy within a group based on variations over time in the status of species names. Here, we correct an error in this method.