Ten planktonic foraminiferal zones are identified and six stratigraphic sequences are interpreted in the Turonian–Maastrichtian interval of the Michu-1 well in the Upper Magdalena Basin. Anoxic bottom-water conditions prevailed during the Turonian and Coniacian followed by dysoxic conditions in the Santonian and Campanian. Oxic conditions were established during the Maastrichtian age. Upwelling occurred for most of the Late Cretaceous. Coccolithophorids, planktonic foraminifera, radiolaria, ammonites, clupeoid fish, and mosasaurs dominated the food-web structure of the Turonian–Coniacian interval. Following a major turnover during the Coniacian–Santonian boundary interval, dinoflagellates were the primary producers that sustained radiolarians, a sparse population of planktonic foraminifera, clupeoid fish, and ammonites. Dinoflagellate blooms (peridinoids) were frequent during the Campanian interval and sustained a sparse population of planktonic foraminifera and abundant clupeoid fish whose feces (phosphatic pellets) were grazed extensively by a specialized population of buliminids dominated by the genus Siphogenerinoides.

The Tres Esquinas Member of the La Luna Formation is a glauconite-rich phosphorite unit associated with changes that took place in depositional environments in the Maracaibo Basin at the end of the Late Cretaceous. The unit marks the end of La Luna Formation sedimentary cycle. This paper presents results of petrographic studies of samples from two sections, one outcrop (Río Guaruries), and one well core (Perijá), and from seismic profiles across the Perijá and Colón areas in the Maracaibo Basin.

The Tres Esquinas Member is three meters thick in both locations. In the Río Guaruries outcrop, it contains an abundant foraminiferal fauna in addition to common allochemical material. Perijá core samples contain voluminous carbonate matrix, scarce allochemical materials, rare foraminifera, and poor definition of mineral facies, which may reflect higher-energy conditions on the sea floor.

The deposition of the Tres Esquinas Member resulted from altered sea-floor topography during an episode of intense tectonic activity in the eastern part of the Maracaibo Basin. The modified shelf configuration ended anoxic conditions on the sea floor and led to increased erosion characteristic of the Tres Esquinas Member.

The Late Cretaceous seas of Venezuela were characterized by deposition of organic carbon-rich limestones, cherts, and shales in the Maracaibo and Barinas/Apure Basins of western Venezuela (La Luna and Navay Formations) and in the Maturín and Guarico Sub-basins of eastern Venezuela (Querecual, Tigre, and San Antonio Formations). Deposition of these units typically occurred under low-energy, low-oxygen conditions that were punctuated by the episodic activity of bottom currents, debris flows, turbidites, syndepositional faulting, and by intermittent oxygenation. These events produced a characteristic set of sedimentary structures that can be used to provide more detailed interpretations of each depositional system. Four main types of sedimentary structures noted in this study are: (1) hydrodynamic—primarily produced through the actions of currents; (2) rheotropic—structures produced during loading and dewatering of unconsolidated sediments; (3) biogenic—formed through the actions of living organisms, such as trace fossils; and (4) diagenetic—structures and textures formed through near surface and burial diagenetic processes.

Identification of these structures can provide new insights on the hydrodynamic and biologic settings of Venezuelan Cretaceous strata. Utilizing observations made in this study along with previously published works, bottom-water conditions in the La Luna depositional system are interpreted to have been maintained consistently in a state of anoxia or very low dissolved oxygen content. This contrasts with bottom waters in the Querecual depositional system, during which strong periods of anoxia or low dissolved oxygen content were punctuated by periods of intermittent oxygenation.

The biostratigraphy of the La Luna Formation has long been in dispute, despite the importance of this unit as the most important source rock for hydrocarbons in Venezuela. In this paper, calcareous microfossil biostratigraphy combined with carbon-isotope stratigraphy provides a stratigraphic framework for the formation, permitting revision of temporal and spatial patterns of deposition of organic-rich sediments. Detailed studies were conducted on a cored borehole and five outcrop sections distributed across the Maracaibo Basin of western Venezuela.

Planktic foraminifera have fair to good preservation, and nannofossils are poorly preserved. Many of the Cenomanian to Campanian planktic foraminiferal marker species are present, permitting the application of a traditional zonal scheme. An informal nannofossil biostratigraphic zonal scheme, based primarily on dissolution-resistant species, has been developed. Integration of these zonal schemes has enabled the correlation of changes in carbon-isotope ratios to the global C-isotope stratigraphy. The results have been used to estimate temporal variation in sedimentation rates as well as to reconstruct depositional patterns across the Maracaibo Basin. Deposition began in the eastern and northwestern part of the Maracaibo Basin in the middle Cenomanian and progressed towards the south and west, reaching the southwestern corner by the middle Coniacian. Although the uppermost part of the formation could not be dated, deposition in the eastern basin continued at least into the Coniacian and in the western basin at least until the middle–late Santonian. Sedimentation rates were highly variable with a period of condensed sedimentation at the Cenomanian–Turonian boundary.

Depositional paleoenvironments of the La Luna Formation were established based on investigation of sedimentology, foraminiferal assemblages, and stable isotope composition of two sections in the Venezuelan Andes. Planktonic foraminiferal biostratigraphy indicates that the age of the San Miguel section spans from the late Turonian to the early Campanian and the Las Hernández section ranges from the early Coniacian to the early Campanian. The base of the La Luna Formation becomes progressively younger towards the south. Early-diagenetic processes have altered stable isotopic composition of carbonates, however stratigraphic changes in carbon isotope values can be used to correlate between the sections.

The environment of deposition of the La Luna Formation changed during the Late Cretaceous. Interval I, from the early Coniacian to mid-Santonian, was characterized by anoxic conditions. Alternating anoxic-dysoxic environments in Interval II lasted from the mid-Santonian to the base of the Campanian. Interval III, in the early Campanian, was marked by more oxygenated conditions. These trends can be partially observed in other ocean basins.

A previously undescribed carbonate-shrub facies has been discovered in the lower member of the Noonday Dolomite—a Neoproterozoic post-glacial cap carbonate. This unusual facies consists of centimeter-scale structures composed of micrite and encased in early-marine cement, and exhibits an overall clotted, mottled appearance. Shrub architecture is characterized by occurrence of a central stalk with diverging branches that are composed of micrite leaves. A combination of biological, environmental, and diagenetic influences contributed to the growth and present appearance of Noonday Dolomite carbonate shrubs. Comparison with modern and ancient known abiogenic and biogenic shrub-like structures indicates that microbial communities were most likely responsible for at least localizing and initiating calcium carbonate growth in the Noonday Dolomite shrubs, although no undisputable microbial fossils have yet been discovered. Diagenetic processes may have obliterated Noonday Dolomite shrub microstructure and obscured any former fossil evidence. Unusual seawater conditions (high alkalinity/extreme calcium carbonate supersaturation) were vital for shrub growth. This research highlights a possible biologic component in Neoproterozoic post-glacial cap carbonates where most studies have focused on abiogenic processes.

The end-Permian extinction reflects one of the greatest biotic crises in earth history. Evidence of the event and its aftermath is well preserved in Permian/Triassic transitional strata of Sichuan Province in South China, originally deposited in the western part of the Yangtze epicontinental sea. Lowest Triassic strata (the Hindeodus parvus conodont Zone) lie with apparent conformity on uppermost Permian skeletal packstone and wackestone, and are overlain by deeper subtidal thinly bedded argillaceous lime mudstone. These beds are characterized by thrombolitic mesostructures that exhibit macrostructures of highly variable columnar shapes, showing a distinctive stratigraphic succession in bed form from planar to domed and subspherical forms. Spheroidal and ellipsoidal micritic bodies (30 µm in average diameter) are preserved in clusters and are interpreted as calcified coccoidal microbes. The earliest Triassic thrombolitic columns and masses are considered to have been constructed by vertical and lateral accretion of mesoclots of microbial origin. Even after the end-Permian extinction, microbial formation of carbonates apparently predominated in Sichuan in localized subtidal to intertidal environments.

Microbialites that formed immediately after the end-Permian extinction represent not only disaster-related forms in stressed environments, but also space- and time-specific, environmentally induced carbonates that may be related in part to the causal mechanism of extinction and delay of biotic recovery. The microbialite sequences in Sichuan Province, South China provide a glimpse of the pre-existing “background” and following “foreground” microbial world, and hence a window that was open temporarily in the post-extinction interval.

The Lower Permian (Wolfcampian) Hueco Formation in the Dona Ana Mountains of south-central New Mexico contains a 30-m-thick, mixed siliciclastic-carbonate succession exposed for up to 1 km perpendicular to regional paleoslope. The succession was deposited in shallow-marine, wave- and tide-dominated estuarine, and fluvial environments, and is arranged into three fourth-order sequences. The lower two sequence boundaries incise marine fossiliferous packstone and offshore to lower-shoreface mudstone-siltstone and are overlain by fluvial sediment comprising the lowstand systems tract. Fluvial-channel sedimentation above the lower two sequence boundaries changed from vertical to lateral accretion in response to decreasing gradient in the incised valley.

The transgressive systems tract of the lower two sequences begins with a tidal ravinement surface, which is mantled locally by a pebble–cobble lag of rip-up clasts and fossils. The tidal ravinement surface cuts channels up to 4 m deep, locally removing transgressive-estuarine and some lowstand-fluvial sediment. Symmetrically rippled sandstone-siltstone deposited near the mouth of the estuary overlies the tidal ravinement surface and is overlain and locally truncated by a wave ravinement surface. In sequence 2, the highstand systems tract consists of a progradational package of heterolithic mudstone–siltstone–sandstone and bivalve packstone deposited in the central basin of the estuary; and sandstone with wood and bivalves, and carbonaceous mudstone and sandstone deposited in bayhead deltas and marshes. Sequence boundary 3 is characterized by a restricted-marine ostracode packstone sharply overlying a marsh mudstone with a vertic Calcisol and burrows of the Glossifungites Ichnofacies, and was produced on an interfluve. Sequence 3 is not present in the northern exposures, having been removed by erosion associated with sequence boundary 4. The temporal scale (10⁵ yrs) of the fourth-order sequences implies a glacial-eustatic origin. Interbeds of shallow-marine fossiliferous packstone and mudstone–siltstone in the lower part of the succession resemble fifth-order sequences in the Midcontinent, but also may represent parasequences or autocycles.

A taphonomic investigation of a fossil assemblage near Coleman, Texas, U.S.A., has revealed a rarely seen part of the cephalopod taphonomic pathway. The cephalopods occur in a brown shale with a low-diversity, gastropod-dominated fauna that is mixed together with a conifer-seed fern-dominated terrestrial flora. All of the cephalopod shells (n=2000 ) are damaged. The brown shale is interpreted as representing a brackish-water or quiet, muddy beach environment, based in part on the delicate three-dimensional preservation of the terrestrial plant debris and the absence of stenohaline benthic marine fauna.

Most upper Paleozoic cephalopods (especially ammonoids) are thought to have lived in offshore (middle to outer shelf) environments. The environmental interpretation and the massive damage observed on the cephalopod shells support the interpretation that this cephalopod fauna has been transported from one or more distant offshore biotopes. These cephalopods had shells with positively buoyant conchs that floated after death onto the mud beach, where they were trapped in a Konzentrat fossil-Lagerstätte with the indigenous gastropod fauna and a well-preserved terrestrial flora.

The significance of this taphonomic analysis is the documentation of a transported cephalopod fauna from an offshore biotope to a paleoshoreline environment. By studying the Coleman cephalopods, a specific criterion for assessing the type and degree of shell damage for beached fossil assemblages has been developed; e.g., predominance of fragmentary shells (Type D). The criterion can be used to evaluate the taphonomy of other cephalopod faunas around the world.

Thousands of aggregating Archaeocidaris echinoids, many with Crurithyris planoconvexa brachiopods and fenestellid and encrusting bryozoan epibionts on their spines, occur in a Pennsylvanian (Missourian) echinoderm Lagerstätte in the Winchell Formation of north-central Texas. This occurrence documents the earliest recorded case of epibionts on echinoids, including usually unpreserved epibionts repeatedly attached to echinoid hosts. Additionally, this occurrence provides further insight into deep-time biotic interactions between mobile hosts and their attaching organisms.

All epibionts are preferentially distributed on the test-proximal ends of the spines. In a contemporaneous sample, no significant size differences are noted between echinoids with and those without epibionts. Epibiont distribution on the spines may occur from one or a combination of causes: antifouling behavior of the echinoid, accidental mechanical removal of epibionts via abrasion, or larval preference in settling. Crurithyris planoconvexa size ranges agree with previous studies of other attached brachiopod size distributions.

The echinoid-epibiont association commensal relationship appears to have benefited the epibionts by means of associational defense, transportation benefits, increased water-current and food-gathering possibilities, open habitat for colonization, and decreased competition with other epibionts. Conversely, costs for the epibionts would have included potential mortality from echinoid predators and transport into unfavorable environments. For the echinoids in this association, costs, such as increased drag on spines and exposure to epibiont grazers, far outweighed the benefits (e.g., potential camouflage effects), although both likely were minimal.

Embedment structures are formed by the growth of a skeleton-producing host organism around a cavity maintained by an infesting organism. Through the process of embedding, the infesting animal passively produces a secure dwelling structure. This embedment ethology is common in modern and ancient marine settings. Previously unrecognized embedment cavities from ancient lacustrine stromatolites are identified and described here for the first time.

Cenozoic lake deposits from the Washakie Basin in Wyoming and Turkana Basin in Kenya contain stromatolites with regularly pitted surfaces, which represent openings to embedment cavities. Stromatolitic laminae deflect downwards adjacent to the cavities. These cavities are circular to oval in cross-section and terminate with a blind base. A lack of branching, basal rootstock, and encrustation by stromatolitic laminae suggest that the cavities were not formed by the overgrowth of plant material, as has been suggested previously. Moreover, the rounded and evenly spaced cavities indicate that these structures are not the spaces between abutted columns of stromatolite material.

These findings suggest that some lacustrine stromatolitic ecosystems are more diverse than the body-fossil record demonstrates. It is probable that more embedment structures will be recognized in lacustrine and marine stromatolitic settings.

Predation as an important driver of evolutionary change long has been assumed, despite difficulties to substantiate it with specific examples of predatory interaction, especially for the early Paleozoic diversification of animal life. This study corroborates the existence of shell-drilling predation in the uppermost Neoproterozoic of China. Nearly one-fifth of almost one hundred tubular shells of one of the earliest mineralized animals, Cloudina, are perforated by undoubted predatory borings 15–85 µm wide. By contrast, no specimens of co-occurring shells belonging to Sinotubulites were affected. The identity of the predator remains elusive, but variation in size of the borings suggests a predatory lifestyle throughout its growth, after it reached a minimum size. The relatively uniform distance of the borings from the shell apertures points to either control by the life orientation of the shells, such as the position of the sediment surface, or, more likely, an avoidance response by the predator to protective measures located near the aperture. Assuming Sinotubulites had similar life habits and was potential prey, the absence of borings in this taxon is evidence that these tubes may have been protected by organic material or toxins that fended off shell-drilling predators. Hence, this earliest example of predation in the fossil record already shows prey selectivity and site-specific behavior, pointing to a level of Precambrian predator-prey interaction that approaches the complexity seen in younger Paleozoic benthic animal communities. This is consistent with the suggestion that predation was indeed an active contributor to the Cambrian radiations.

Bulk samples are among the foremost sources of quantitative data retrieved from the fossil record. However, such samples are not sieved in a uniform way, even among research projects with a very similar research focus. Several studies recently have demonstrated the sensitivity of paleontological patterns to changes in sieve size and underscored the importance of controlling for mesh size in paleontological analyses. Building on previous work, this study exploits a large dataset of Miocene mollusks that is fortuitously suitable for exploring the effect of mesh size: dimensions of each fossil were measured, all samples were acquired with fine screens (≤1 mm mesh), and data for numerous paleoecological and taphonomic variables were obtained for each specimen. This large dataset was sieved artificially (i.e., subsampled in computer simulations) to explore the effects of mesh size. The results show that paleontological variables, from taphonomic and paleoecological parameters to diversity indices, can fluctuate, to various degrees, as a function of mesh size. Some parameters (e.g., evenness indices) appear remarkably invariant to mesh size, while others (e.g., encrustation rate) can vary dramatically with a small change in mesh size. Most importantly, even when the compared datasets are sieved uniformly with the same standard mesh size, outcomes of comparative analyses can lead to disparate conclusions when that standard size is changed. The mesh-size sensitivity observed here for a wide assortment of paleontological patterns points to ubiquitous influence of body size on taphonomic, ecological, and evolutionary patterns and underscores the importance of developing sampling strategies and/or corrective analytical measures for making data more comparable in terms of mesh size across and within studies. Future research also should concentrate on evaluating secular trends in size-filtering aspects of extraction methods used to acquire quantitative samples throughout the Phanerozoic fossil record.