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 email@example.com with any questions.
Details of the three-dimensional morphology of the ichnofossil Phycosiphon incertum collected from deposits on submarine slopes are reconstructed by processing a series of images obtained from polished sections of the samples. Samples were collected from the mudstone around a slump scar in the Paleocene Shiomi Formation, northern Japan, which is characterized by the occurrence of slump scars. The reconstructed morphology of Phycosiphon incertum is a meandering tube with a flattened ellipse cross section. The tubes are flattened in a plane oblique to the bedding surfaces and aligned along the same direction at both the interior and exterior of the slump scar. Flattening of the tubes was likely caused by sediment compaction, and the tube flattens toward the horizontal plane that is oblique to the bedding plane because of the paleoslope inclination. The difference between the bedding and flattening planes of the tubes of Phycosiphon incertum may imply paleoslope inclination. When the inclination of the bedding plane of the Shiomi Formation is corrected using the flattened surfaces, the bedding plane dips by 9° toward the southeast, which conforms to the paleocurrent direction of the turbidites. The morphology of Phycosiphon incertum can, therefore, be used as a paleoslope indicator.
A total of 17 mammalian communities from south-central Spain to northern Germany spanning the middle Miocene Climatic Optimum (ca. 17–14 Ma) are analyzed. Mammalian body-weight structures (cenograms) are constructed and discussed in terms of paleoenvironmental affinities. They indicate an arid climate and open environments in south-central Spain and closed and densely forested regions with very humid conditions to the north. The data reveal the presence of a strong latitudinal environmental gradient during this period. Other studies based on the fossil records of plants or ectothermic vertebrates showed that virtually no temperature gradient was present in Europe at that time and that mean annual temperatures were very high, probably around 20° C. Mammalian communities, therefore, show the presence of a southwest–northeast aridity-humidity gradient and confirm recent findings based on air-breathing fish distribution and pollen spectra. Atmospheric circulations could be responsible for this meridional gradient, but other regional causes cannot be ruled out. This study indicates that mammal body weight distributions are excellent proxies to investigate the environmental and climatic evolution and compare well with other paleoenvironmental proxies.
Although microvertebrate accumulations are commonly used for paleoecological reconstructions, taphonomic processes affecting the final taxonomic composition of an accumulation are often ignored. This research explores the effects of abiotic taphonomic processes on the taxonomic composition of terrestrial microvertebrate accumulations by comparing a floodplain and a channel lag deposit from the Maastrichtian Hell Creek Formation in eastern Montana. Distribution of skeletal elements with specific physical attributes and relative abundance of taxa correlate with the hydraulic indicators (i.e., grain size, sedimentary structures) of the depositional facies. Transport distances, hydraulic equivalencies of dominant skeletal elements, amount of hydraulic sorting and reworking, and degree of time averaging vary between deposits and significantly affect taxonomic distributions. Relative abundance data, in conjunction with chi-square test results and rank-order analysis, show that size, shape, abrasion, and taxonomic compositions vary significantly between assemblages. The fine-grained assemblage is dominated by tabular, low-density elements, such as cycloid scales and fish vertebrae. Dense, equidimensional elements, such as teeth and ganoid fish scales, dominate the sandstone assemblage. Rank-order analysis results demonstrate that relative abundance of hydraulically equivalent skeletal elements from morphologically similar organisms can be compared regardless of accumulation in nonisotaphonomic deposits. Statistical comparisons were made among osteichthyans using ganoid scales, caudates using vertebrae, ornithischians using teeth, and testudinates using shell fragments. Results show that portions of the assemblage analyzed using hydrodynamically equivalent elements are not significantly different, despite different depositional environments.
The Castlecomer Fauna, a Konservat-Lagerstätte from the Upper Carboniferous of Ireland, is dominated by two species of exceptionally preserved spinicaudatan conchostracan branchiopod crustaceans; over 300 articulated specimens are known from short lengths of core from two boreholes. The spinicaudatans were entombed at various angles to bedding in thin event beds deposited in a paralic environment. Cuticle is the only tissue represented. The fossils are essentially two-dimensional as specimens collapsed following decay-induced loss of the structural strength of the cuticle. Some three-dimensionality was retained, however, as clay-grade material, probably sediment, infiltrated the carapace during or after burial, preventing the valves from being compacted to the same plane as the body of the specimen. Cuticle-lined voids created by decay of the labile tissues (notably within the tubelike antennal rami) were lined or infilled by authigenic clay minerals that were subsequently metamorphosed. The antennae, mandibles, distal parts of the claspers, and the telson are preserved in excellent detail, often retaining all or most of their original relief. Other anatomical features (antennal protopods, head, proximal part of claspers, trunk appendages) are usually preserved in two dimensions, either as a film of carbon or a dark-colored stain. The antennules, maxillules, and maxillae are rarely preserved. Decay experiments confirm that this variation reflects differences in the recalcitrance (thickness and degree of sclerotization) of the cuticle that covered different parts of the body. Complete decay of certain appendages precedes appreciable deterioration in the morphology of others. Some of the disparity exhibited by exceptionally preserved fossil arthropods is likely to be taphonomic rather than biological in origin.
This study seeks to test appropriate sampling and statistical regimes for comparing biofacies compositions through the use of several statistical methods developed for analyzing community structure. Bulk sampled data from the Ambocoelia-chonetid biofacies within the Middle Devonian Hamilton Group of New York are used as a test data set. Samples were collected from a single fourth-order regression within this dysoxic facies across a paleoenvironmental basinal gradient. Within locality, lateral replicates and subsampling are used to gauge the degree of faunal patchiness at outcrop scale and to assess the impact of patchiness within this biofacies on the effectiveness of statistical methods for detecting biofacies gradient boundaries. This study demonstrates that a sampling effort that accounts for within-outcrop patchiness (scale of 5–15 m) is adequate to test for differences in faunal composition in this biofacies across the sampled basinal gradient. Furthermore, our results show that this biofacies exhibits a gradient in within-locality patchiness, which increases as the assemblage is sampled more proximal to the sediment source. Tests of faunal patchiness and geographic differences in biofacies composition obtained with nonmetric multidimensional scaling, analysis of similarities (ANOSIM), and multinomial models using maximum likelihood and information theory (MNM/AIC) produced similar results. MNM/AIC is recommended as a compliment to ANOSIM and ordination techniques for analyzing bulk sample data because MNM/AIC allows for comparison of competing hypotheses about population distributions based on the relative likelihood of each hypothesis and the information required to construct each hypothesis. ANOSIM alone can only determine the significance of a particular clustering against a null hypothesis of no structure and not relative to alternative structures.
Exceptionally large teeth attributable to the theropod genus Troodon are abundant in Upper Cretaceous rocks (Campanian–Maastrichtian) of northern Alaska. The dominance of low-angle light in this Cretaceous high-latitude environment seems to have selected for an abundance of Troodon. The population of these Alaskan specimens— morphologically the same as teeth attributable to this taxon in other regions—show a mean approximately twice the size of those found in such southern latitudes as southern Alberta and Montana. Microwear patterns on the teeth of this taxon in Alaska and Montana suggest that these animals were eating similar food items. There appears to be good correlation between tooth size and body size in theropods, thereby allowing the inference that the increase in the size of Alaskan Troodon teeth equates to an increase in body size. The increased body size for Alaskan Troodon is likely related to increased availability of food resources as a result of decreased numbers of other predatory dinosaurs, particularly tyrannosaurids, in the ecosystem.
The occurrence of the Rosselia ichnofabric in the basal sandstone beds of the late Miocene Pullen Formation (basal Wildcat Group) provides strong evidence that supports our interpretation of a littoral-to-inner-shelf depositional setting for the lowest division of this formation. These sediments were deposited on a wave-cut platform developed along a steep coastline. Our study reconfirms the utility of the Rosselia ichnofabric in the identification of inner-shelf depositional settings having high net deposition rates, represented by the basal Pullen sandstone beds. We demonstrate how recognition of this unique ichnofabric can inform larger-scale tectonic interpretations. The correct interpretation of paleoenvironment in the case of the lowest Pullen beds confirms the view that rapid regional subsidence occurred shortly after their deposition. This allows us to eliminate a previously published alternate hypothesis that the initial accumulation of Pullen sediments occurred at bathyal depths on an inner trench slope.
Grazing trails of the Valviferan isopod Chiridotea coeca are examined to test the relationships between trail morphology and the distribution of food (organic carbon). These isopods burrow up to 1 cm beneath the surface within ripple troughs and planar-bedded sand in the upper intertidal zone. The burrows are grouped into three forms based on the tortuosity of their course and degree of looping and trail crossover in plan view. Sediment samples taken directly from the trail furrows are used to establish the total organic-carbon content associated with each burrow morphology. There is an increase in organic-carbon content from burrows of low tortuosity (linear burrows) to burrows of higher tortuosity (convolute burrows with many crossovers), suggesting that benthic food content directly influences the behavior of C. coeca. Detailed study of trace emplacement further reveals a relationship between C. coeca and food content leading to the recognition of three grazing styles. These are directly related to the plan-view morphology of the trail and reflect the depth at which the isopod tunnels in the sediment. Graphical analysis of weight percent organic carbon against grazing style shows a positive correlation between deep burrowing and high benthic food content. The morphology of C. coeca trails differs from deep-sea turbidite and flysch deposits. In the deep-sea environment resources are replaced slowly, and food distribution is comparatively low and uniform, leading to regular meanders and complex traces that do not crossover existing trails. In the intertidal zone, C. coeca encounters sporadically distributed resources that are replenished semidiurnally. The resultant burrows exhibit increasing crossovers with increasing food content, representing a once-over feeding strategy designed to rapidly harvest a high-value, renewable resource.