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.
The Upper Ordovician Viola Group of south-central Oklahoma records deposition on a carbonate ramp that extended from platformal settings to a basin within the Southern Oklahoma aulacogen. Lithofacies analysis allows identification of inner, mid- and outer-ramp environments in the Viola Springs Formation, each of which hosted a distinct trilobite biofacies. There is a sharp break in both composition and species diversity of biofacies between the outer ramp within the aulacogen and shallower environments. The outer-ramp Cryptolithine Biofacies is characterized by low species diversity, whereas both the midramp Thaleops Biofacies and the inner-ramp Bumastoides Biofacies contain up to four times as many trilobite species and rare cryptolithines. Comparisons with Cambrian and older Ordovician shelf-to-basin trilobite distributions suggest that the pattern recorded in the Viola Springs is depth related. The overlying Welling Formation includes two biofacies in inner- to midramp environments, with the faunas of the aulacogen being less diverse than those of the platform. The high trilobite species diversity in shallow water environments of the Viola Group supports a dilution model for community reorganization during the Ordovician.
The Upper Ordovician (Caradoc) Collingwood Member of the Lindsay Formation, southern Ontario, Canada, is a strikingly cyclic package of clastics and carbonates. Cycles are 50–150 cm thick and comprise four major components: (1) dark gray to black, organic-rich, laminated shales that grade upsection into (2) dark to light gray calcareous shales or mudstones, (3) lenticular to tabular concretionary argillaceous limestones, and typically (4) light gray calcareous, fossiliferous mudstones or shales and marls. Black shale units have a characteristically sharp basal contact and overlie a condensed shelly pavement. Fossil in shales are preserved as pavements or stringers of trilobite, ostracode, and brachiopod debris, with strong taphonomic bias as a result of prolonged exposure at the sediment-water interface. Gray mudstones and marls are bioturbated and contain numerous low-diversity orthid brachiopod pavements. Persistent tabular concretionary limestone bands were formed by early diagenetic cementation. These concretionary units include shelly beds that alternate with less fossiliferous calcareous mudstones containing noncompressed, spar-filled burrows and articulated, sometimes in situ fossils. Variation in fossil abundance is the result of cyclic variation in sedimentation, ranging from periods of condensation to rapid burial.
Collingwood cycles involve upsection changes including: (1) benthic oxygenation from lower dysoxic to fully oxic biofacies, (2) increased frequency and episodicity of sedimentation, (3) higher net sedimentation rate within gray mudstone to carbonate intervals, (4) increased environmental energy level, and (5) diagenetic cementation of muds a few centimeters below cycle tops. Consistency of these variations suggests an allocyclic mechanism for the Collingwood cycles related to short-term fluctuations in eustatic sea level or climate.
Fossil wood fragments and an associated species-rich invertebrate assemblage, analogous to those found on wood falls in the deep sea today, were found in late Eocene deep-water sediments of the Lincoln Creek Formation in Washington State, United States. This assemblage is the earliest known complex deep-sea biologic community based on decaying wood as its primary source of nutrients. The 495 recovered fossils (exclusive of foraminiferans) belong to 21 species; 7 species relied directly on the wood, either by ingesting it or by feeding on xylophagous microbes; these species are also the most abundant. Seven species were predators or scavengers that were most likely attracted by the wood-dependent species. The remaining seven species represent predators, detritus feeders, and suspension feeders that may or may not have had a relation to the wood fall or its fauna. All species had a benthic mode of life, and pseudoplanktonic taxa are absent, indicating that the colonization of the wood began only once it had arrived on the deep-sea floor. The wood-dependent species belong to taxa that fill the same ecologic niche in the deep sea today, indicating that the modern wood-fall ecosystem had evolved at least by late Eocene time. There is no uniformity or specialization of dispersal strategies among the recovered taxa; they rather reflect those of the phylogenetic group to which they belong. The wood-fall assemblage described here shares several families with fossil whale falls and cold seeps but very few species, a condition that can also be observed at modern examples of these ecosystems.
The Middle Turonian Kaskapau Formation in the Rocky Mountain Foothills of northeastern British Columbia comprises up to 950 m of shallow marine sandstones cyclically interstratified with marine mudstones and minor lagoonal deposits. Transgressive surfaces are commonly mantled by thin pebble lags. A transgressive pebble lag exposed in Quality Creek near Tumbler Ridge town site includes rare, well-rounded quartzite cobbles and a quartzite boulder, which were deposited at a paleolatitude of about 67°N. Encrusting organisms are unevenly distributed over five of the largest clasts. Disciniscid brachiopods are abundant on one cobble but are otherwise rare. A conical-shelled lingulate brachiopod with concentric growth lamellae occurs sparsely, and a weakly ribbed lingulate brachiopod is represented by one shell. Serpulid worm tubes, encrusting bryozoans, and possible basal attachments of corals are also present. Attached foraminifera are abundant and distributed over several clasts. Generic diversity and taxic dominance values are moderate. The fauna includes many elements typical of Cretaceous lower-latitude hard substrates but is unusual in lacking bivalves; their absence might be due to a combination of abundant clastic supply, high turbidity, possibly lowered salinity in the nearshore zone, and relatively low water temperature (∼10°C).
The gradient of rivers that supplied gravel to the Kaskapau shoreline was estimated from clast size (typically <30 mm) and bed thickness in coeval fluvial conglomerate. Results suggest river gradients of about 0.3–0.4 m/km close to the shoreline. Calculations based on the encrusted cobbles suggest an unreasonably steep river gradient of about 1.9 m/km. The most likely alternative transport mechanism is rafting in the roots of trees, although the high paleolatitude might not preclude rafting by winter river ice.
Age-dependent extinction is an observation with important biological implications. Van Valen's Red Queen hypothesis triggered three decades of research testing its primary implication: that age is independent of extinction. In contrast to this, later studies with species-level data have indicated the possible presence of age dependence. Since the formulation of the Red Queen hypothesis, more powerful tests of survivorship models have been developed. This is the first report of the application of the Cox Proportional Hazards model to paleontological data. Planktonic foraminiferal morphospecies allow the taxonomic and precise stratigraphic resolution necessary for the Cox model. As a whole, planktonic foraminiferal morphospecies clearly show age-dependent extinction. In particular, the effect is attributable to the presence of shorter-ranged species (range < 4 myr) following extinction events. These shorter-ranged species also possess tests with unique morphological architecture. The morphological differences are probably epiphenomena of underlying developmental and heterochronic processes of shorter-ranged species that survived various extinction events. Extinction survivors carry developmental and morphological characteristics into postextinction recovery times, and this sets them apart from species populations established independently of extinction events.
The causes of global biological diversification and the nature of ecological change during taxonomic radiations are central questions in paleobiology. It has long been recognized, however, that apparent increases in global taxonomic richness need not be mirrored in local communities. Here, using field data and literature sources from Laurentia, it is shown that genus richness in well-preserved subtidal macrobenthic marine communities increased by as much as a factor of two from the Middle Cambrian to the Late Ordovician. Several potential sources of bias have been addressed in this study, including taphonomic effects, water depth, lithology, sample size, and differences in the relative numerical abundance of taxa (evenness). Excluding samples from environments that may have been oxygen stressed and omitting samples that closely follow Cambrian biomere extinction intervals result in much less complete temporal sampling and significantly influence apparent short-term patterns of biodiversity, but do not change fundamentally the overall richness and evenness trajectories. The apparent magnitude and timing of the increases in local richness and evenness, however, is sensitive to such factors. The overall richness trajectory is also sensitive to how evenness is treated quantitatively. If evenness is assumed to represent a biological positive correlate of richness, then rarefaction is most appropriate and suggests an Early Ordovician richness increase. If evenness is treated strictly as a bias in richness estimates, then most of the richness increase appears to have occurred by the late Cambrian (Marjuman), with only a modest increase after the Arenigian.
How does the choice of size metric, specimen selection, and taxonomic level affect the results of macroevolutionary or ecological analyses? Four molluscan data sets are used to address this question as follows. First, the relationships among various size metrics are examined using a morphometric data set of Late Cretaceous–Oligocene veneroid bivalves. Second, the relationship between the size of bulk-sampled specimens and the size of species' type specimens is examined using bulk-sampled bivalves and gastropods from the Coffee Sand (Upper Cretaceous, Mississippi). Third, the same relationship is examined using mollusk-dominated field censuses from the type Cincinnatian (Upper Ordovician, Ohio, Indiana, and Kentucky). Fourth, the relationship between the size of the type species of a genus and median species size is examined using literature-derived measurements of bivalve type specimens from the recent eastern Pacific continental shelf. Together these data sets provide estimates of the biases imposed by measuring different kinds of specimens and using different methods of estimating body size. The geometric mean of length and height is highly correlated with more complex morphometrically based metrics and is our preferred bivalve size metric. Bulk or randomly sampled specimens are significantly smaller than species' type specimens for the Cretaceous dataset but significantly larger for the Ordovician dataset. Genus' type-species size is an unbiased estimate of median species size. These results suggest that large-scale studies can use the size of the type species of a genus as an unbiased proxy for a type-specimen size of a genus' median species, but that species' type-specimen size is a biased proxy for bulk or randomly sampled specimens. In addition, this study emphasizes the importance of using a single type of measurement within studies and suggests that combining multiple types of specimens (e.g., type specimens and bulk-sampled specimens) could lead to substantive errors.