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 firstname.lastname@example.org with any questions.
Understanding the relationships between morphological disparity and environment, geography, and scale require examination at the local level. Even with disparity metrics that are inherently sample size independent, the nature of rare species and the segregation of common and rare species within morphospace can create substantial sampling issues. Eight well-sampled, Late Ordovician crinoid assemblages were examined for potential biases in the study of local disparity. Disparity is based on the ordination of discrete characters. The rare and common species within these assemblages contributed equally to disparity. In spite of this pattern, rare species in some localities occupy a different area of morphospace, causing disparity to vary greatly with sampling intensity. Morphological rarefaction based on the number of specimens shows that disparity weighted by abundance is constant past a sample size of approximately 30 individuals. This metric is dependent on the evenness within an assemblage as well as the abundance within subgroups in morphospace. Disparity weighted according to abundance gives a view of the functional disparity of an assemblage, which is more applicable in studies of local disparity, though unweighted disparity is still preferred in regional-scale studies and in investigations of morphospace filling through a clade's history.
Estimates of taxonomic richness and abundance are complicated by sampling biases. The failure to sample rare taxa is most often attributed to inadequate sampling and to removal during the process of sample-size standardization. Here I present two methods for unveiling rare diversity by integrating species presence/absence data from museum collections and the literature with quantitative estimates of species richness and abundance gathered from field-based bulk samples. Combining museum, literature, and field data can provide a more comprehensive estimate of taxonomic richness and abundance without substantial increase in current sampling effort. First, in a given bulk sample, the lowest proportional abundance value observed can be used to estimate the maximum abundance of rare species known to have occurred at the locality at least once but not recorded in the current sample. Second, a model-selection approach can be used, in which a set of relative abundance distribution models are fit to the bulk-sample abundance data and the parameter estimates for the best model used to calculate the abundance distribution for all species known from the locality. The Paleogene marine fossil record of the U.S. Gulf Coastal Plain is suitable for applying these methods, because (1) the molluscan fauna is well represented in museum collections and the literature, (2) the molluscan fauna has been taxonomically standardized, and (3) many classic localities remain accessible for standardized bulk sampling. I introduce these methods by applying them to a single locality and then, using the faunas of the Gosport, Moodys Branch, and Red Bluff Formations, I demonstrate how the model-fitting approach can be used to compare taxonomic richness among multiple localities. A substantial fraction of the molluscan richness known from each locality is not captured in bulk samples and much of this unobserved richness may be attributed to the rarity of species. The multiple-locality comparison suggests that the greatest Paleogene decline in standing richness occurred in the middle Eocene and that the recovery of richness following the Eocene-Oligocene extinction was quite rapid despite substantial loss of taxa. These analyses underscore the magnitude of veiled diversity in marine fossil assemblages and the potential of existing sources of data to unveil rare taxa, allowing them to be incorporated into quantitative diversity studies.
Recent analytical advances have permitted quantitative evaluations of evolutionary mode in populations of fossil organisms by providing tests of the null hypothesis that patterns of stratigraphic character variation do not differ from the expectations of a random walk. If the hypothesis can be rejected, then stasis and anagenesis represent alternative evolutionary modes discernable using values of the Hurst estimate. We used this approach to evaluate evolutionary mode in the bryozoan genus Peronopora across 34 characters in eight unbranched, cladistically defined, evolutionary sequences. Eight monophyletic crown species and eight paraphyletic (phenetically distinct) metaspecies constitute 16 species-rank taxa within the genus.
In seven of 15 species-rank transitions that had adequate sample sizes, significant character state changes—both phyletic gradualism and punctuated equilibrium—coincided with speciation events 11% of the time and were limited to more derived, crownward, ancestor-descendant pairs. Each of the 34 measured characters exhibited instances of transpecific stasis or anagenesis. Anagenesis of some characters persisted across unbranched lineages over 13 species (i.e., across 12 speciation events), whereas character stasis continued through unbranched lineages in up to 16 species (i.e., persisted unchanged across all 15 speciation events). Transpecific stasis and anagenesis were recognizable in over one-half of the data set, with stasis being approximately twice as common as anagenesis.
Across all character state transitions, approximately one-half reflect stasis, 30% anagenesis, and 20% could not be differentiated from a random walk. Similarly, across species and metaspecies characterized by a single intraspecific mode, stasis was twice as common as anagenesis and three times more common than undifferentiated random walks. The remaining instances of multiple intraspecific evolutionary modes occurred more commonly within metaspecies than within species. This difference might reflect the more frequent presence of unrecognized cryptic species or subspecies within metaspecies of Peronopora. Instantaneous rates of evolution can be estimated both within and between species of Peronopora for characters displaying anagenesis, potentially providing quantitative insights into evolutionary changes within the lineage.
Several lines of theoretical and empirical evidence suggest that there can be a positive correlation between alpha diversity (genus richness) of marine communities (D) and average longevity of marine genera included in these communities (L). One possible reason for such a correlation is that diversity can be expected to give rise to ecosystem stability, which, in turn, may slow down the extinction of taxa. However, this hypothesis has not been verified on the global scale. The analysis of two large data sets (Sepkoski's compendium of fossil marine genera and the Paleobiology Database) shows that the correlation (1) actually exists and (2) is robust to some possible sources of errors in L and D estimation. Further analysis reveals that the correlation is not a secondary pattern caused by any of the following factors: (1) encounter probability of taxa, which is greatly influenced by differential incompleteness of the fossil record; (2) degree of sediment lithification, which is one of the major factors affecting the preservation of fossils; (3) onshore-offshore gradient; (4) parallel growth of both L and D through the Phanerozoic; (5) paleolatitudinal gradient. Although there may be other factors that influence both L and D in a similar way, the results generally confirm the hypothesis that higher alpha diversity enhances longevity of genera.
Biomechanical models illustrate how the principles of physics and physiology determine function in organisms, allowing ecological inferences and functional predictions to be based on morphology. Dynamic lever and linkage models of the mechanisms of the jaw and skull during feeding in fishes predict function from morphology and have been used to compare the feeding biomechanics of diverse fish groups, including fossil taxa, and to test ideas in ecological morphology. Here we perform detailed computational modeling of the four-bar linkage mechanism in the skull and jaw systems of Dunkleosteus terrelli, using software that accepts landmark morphological data to simulate the movements and mechanics of the skull and jaws during prey capture. The linkage system is based on the quadrate and cranio-thoracic joints: Cranial elevation around the cranio-thoracic joint forces the quadrate joint forward, which, coupled with a jaw depressor muscle connecting the jaw to the thoracic shield, causes the jaw to rotate downward during skull expansion. Results show a high speed transmission for jaw opening, producing a rapid expansion phase similar to that in modern fishes that use suction during prey capture. During jaw closing, the model computes jaw and skull rotation and a series of mechanical metrics including effective mechanical advantage of the jaw lever and kinematic transmission of the skull linkage system. Estimates of muscle cross-sectional area based on the largest of five specimens analyzed allow the bite force and strike speed to be estimated. Jaw-closing muscles of Dunkleosteus powered an extraordinarily strong bite, with an estimated maximal bite force of over 6000 N at the jaw tip and more than 7400 N at the rear dental plates, for a large individual (10 m total length). This bite force capability is among the most powerful bites in animals. The combination of rapid gape expansion and powerful bite meant that Dunkleosteus terrelli could both catch elusive prey and penetrate protective armor, allowing this apex predator to potentially eat anything in its ecosystem, including other placoderms.
Stable carbon and oxygen isotope ratios were measured for carbonate in samples of hadrosaurid tooth enamel and dentine, and gar scale ganoine and dentine from five geologically “contemporaneous” (two-million-year resolution) and geographically distant late Campanian formations (Two Medicine, Dinosaur Park, Judith River, Kaiparowits, and Fruitland) in the Western Interior Basin. In all cases, isotopic offsets were observed between enamel and dentine from the same teeth, with dentine being characterized by higher and more variable carbon and oxygen isotope ratios. Isotopic offsets were also observed between gar ganoine and hadrosaur enamel in all sites analyzed. Both of these observations indicate that diagenetic overprinting of enamel isotope ratios did not entirely obfuscate primary signals. Decreases in carbon and oxygen isotope ratios were observed in hadrosaur enamel from east to west, and overlap in isotope ratios occurred only between two of the sampled sites (Dinosaur Park and Judith River Formations).
The lack of isotopic overlap for enamel among localities could be due to diagenetic resetting of isotope ratios such that they reflect local groundwater effects rather than primary biogenic inputs. However, the large range in carbon isotope ratios, the consistent taxonomic offsets for enamel/ ganoine data, and comparisons of enamel-dentine data from the same teeth all suggest that diagenesis is not the lone driver of the signal. In the absence of major alteration, the mostly likely explanation for the isotopic patterns observed is that hadrosaurids from the targeted formations were eating plants and drinking waters with distinct isotopic ratios. One implication of this reconstruction is that hadrosaurids in the Late Cretaceous of the Western Interior did not migrate to an extent that would obscure local isotopic signatures.
The significant impact of extant carnivores, particularly spotted hyenas, on the depositional history and physical characteristics of archaeofaunal and paleontological assemblages is well recognized. We focus on the behavioral ecology of extant spotted hyenas (Crocuta crocuta) in relation to bone accumulations produced by one East African clan at communal dens. Limbs and skulls of prey animals more frequently appear at dens than do other carcass portions. These items reflect the relative abundance of prey species near dens; carnivore remains are poorly represented. Comparative analysis reveals that bones are deposited far more slowly (<7 carcass portions per month) and accumulations tend to be smaller at Crocuta dens than at dens of either brown (Parahyaena brunnea) or striped (Hyaena hyaena) hyenas. We propose that extant Crocuta bone accumulation rates and sizes are likely affected by prey species abundance, clan size, social interactions within the clan, and the type and availability of den sites. We also suggest that the potential for intraspecific behavioral variability in bone accumulation patterns is important when comparisons are made among spotted hyena populations and across hyena species. For example, accumulation patterns may be dramatically influenced by the temporal span, potentially ranging from days to hundreds or thousands of years, in which bones are collected, depending on the species-specific history of occupation at a given site. Understanding the behavioral and ecological variability likely to influence bone accumulation patterns at dens used by different hyaenids will allow taphonomists and zooarchaeologists to refine their knowledge of mechanisms underlying site formation processes and potential causes of variability in deeper-time den assemblages.