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Limiting factors affecting faunal distribution in the Ordovician rocks of the Precordillera basin, western Argentina, are analyzed and tentatively established. Eight biotic intervals ranging from the Tremadoc to the Ashgill are defined based upon the distribution in time and space of articulate brachiopod, trilobite, sponge, and bryozoan genera. High magnitudes of turnover are recognized on the basis of vertical ranges of taxa; this observation also is supported by low indices of carryover and holdover. Times of high faunal change define boundaries between the biotic intervals. Abiotic constraints largely prevail over ecological factors in controlling most substantial shifts in faunal composition and distribution at the interval boundaries. Taxonomic composition through different intervals is consistent with a change in paleobiogeographic affinities throughout the Ordovician, from markedly Laurentian affinities in the lowermost Ordovician to increasing Gondwanic affinities towards the Middle and Late Ordovician. At a smaller scale, the development of successive intervals is overprinted strongly by changes in facies and tectonic-induced setting. The composition and structure of assemblages were modified dramatically in the late Arenig-early Llanvirn with the changeover from sponge and brachiopod-dominated assemblages to brachiopod and trilobite-dominated assemblages with abundant bryozoans. This change occurs in consort with a shift from carbonate facies to mixed carbonate-siliciclastic and exclusively siliciclastic facies, and is considered to be the result of sea-level rise and the development of subsiding depocenters in the northern and southern parts of the Precordillera basin. On paleoecological grounds, this episode has allowed the biotic intervals to be grouped in two distinct stages. The lower stage includes intervals 1 to 3, whereas the upper stage includes intervals 6 to 8; intervals 4 and 5 represent the diachronous arrangement of both types of assemblages due to the gradual southward flooding of the basin. Abiotic factors acted at different temporal and spatial scales. Hence, their effect on faunal composition is explained best by distinguishing hierarchical levels of incidence. On this basis, three major hierarchical levels of factors are proposed. The general context in which faunas developed is provided by the shifting position of the Precordillera terrane throughout the Ordovician; accordingly, a first-order hierarchical level is represented by paleogeographic position. The second-order level is represented by the combined effects of rapid sea-level rise and basin deepening, with a high paleoecological impact on the fauna. The third-order level includes single factors producing minor changes in otherwise stable community structures (e.g., minor sea-level changes, fluctuations in water temperature, different facies types).
Dinosaur track beds occur at several localities in the uppermost Cretaceous (Maastrichtian) North Horn Formation in the Wasatch Plateau, central Utah. The track bed localities, separated by up to 80 km, also contain dinosaur body fossils. At the type locality at North Horn Mountain in Emery County, more than 100 individual exposures and/or stratigraphic levels within a 1.2-km2 study area exhibit tracks in vertical cross-sectional view. These biogenic structures are similar to others that have been interpreted elsewhere as deep dinosaur tracks. At the type section, track beds vertically span at least 183 m from the base of the formation up to a few meters below the highest dinosaur eggshells, which are interpreted to occur immediately below the Cretaceous-Tertiary (K-T) boundary interval. Track occurrence in the North Horn Formation demonstrates that large dinosaurs were present in central Utah until very shortly before the K-T boundary.
The track structures feature deformation, overprinting, and slip striae that are interpreted to exhibit individual dinosaur behavior. Some striae exhibit repeating patterns that suggest tubercle configuration or scale patterns on dinosaur feet. Track groupings at any individual level suggest that many animals repeatedly congregated in topographic lows of the floodplain or near shifting anastomosing river systems, and track-size distribution largely suggests the presence of different sizes or age groups of herbivores. The conditions produced by anastomosed fluvial environments provided the depositional setting responsible for formation and preservation of most of the North Horn tracks.
The potential of fossil wood and charcoal from browncoal deposits as sources of reliable paleoenvironmental information is explored with material from the Lower Rhine Embayment (Germany). The presence of charcoalified material demonstrates proof of natural wildfires in Tertiary mire environments, most probably during, or after periods of increased drainage and drying of surface vegetation and litter. The results presented suggest that sampling from charcoal layers may provide a more statistically reliable data set for study of such environments. Inclusion of taxa recovered from charcoal layers might compensate for the taphonomic and preservational bias of Tertiary lignitic floras based solely on the collection of lignitic wood. These data confirm the hypothesis that, during certain intervals of the Miocene, both monocotyledonous and dicotyledonous angiosperms might have dominated or represented significant portions of the peat-forming vegetation. The importance of palms and shrubby woody taxa, such as ‘Cyrilla’, is particularly striking from new evidence of charcoalified remains. In addition, certain wood anatomical features observed from well preserved lignitic wood and charcoal may be used as indicators of environmentally modulated growth: (1) clear growth rings testify to the existence of a seasonal climate; (2) wide variations in growth ring characters indicate variable environmental conditions; and (3) high incidence of dicotyledonous taxa, with abundant small vessels and scalariform perforation plates, is interpreted as evidence of a mesic environment.
Environmental change, such as variation in upwelling intensity and consequent variation in marine primary productivity, may have profound effects on organisms. In the fossil record, morphological variation within lineages may be explained by environmental changes, although it is often difficult to establish a causal relationship between morphological and environmental change. Size and shape change were quantified in suspension-feeding turritellid gastropods from the Pleistocene and Recent of Chile, wherein both temporal and geographic patterns of variation were found. Upwelling and productivity conditions were reconstructed from a variety of published sources, including information on diatom, foraminiferan, and coccolithophore diversity and abundance, organic carbon burial, percent carbonate, and phosphorite deposition. In addition, carbon and oxygen stable isotope ratios from turritellid shell material were analyzed in order to determine temperature and productivity conditions. Comparison of results from the morphological analysis to upwelling/productivity reconstructions indicates that wider whorl shape and larger shell size are associated with more intense upwelling. Increases in the amount of shell material secreted may be explained by increased availability of food associated with stronger upwelling. This interpretation is consistent with observations of increased shell growth recorded for modern, non-turritellid mollusks. This type of analysis may help to establish causation for certain kinds of morphological evolution observed in the fossil record of turritellids and other suspension-feeding gastropods.
Clypeasteroids can be very common in Recent, shallow water environments in a variety of biogeographic settings and represent important members of benthic invertebrate communities. Mass deposits of fossil clypeasteroids are also common and characteristic of many Cenozoic shallow water deposits. Their distribution and formation, however, has received much less attention than molluscan counterparts, although fossil examples are found within all three of the clypeasteroid suborders.
A comparison of two mass deposits of scutellid clypeasteroids from the Miocene of the Mediterranean (Gebel Gharra section, Eastern Desert, Egypt; Alahan Section, Mut Basin, Turkey) shows common features, but also significant differences. Both were formed in high energy, coarse sandy, shoreface environments. The Gebel Gharra section consists of a thick, multi-event accumulation with numerous sedimentary features dominated by complete and fragmented skeletal remains of a single taxon (Parascutella). The accumulations in Alahan represents a single, thin, multi-taxon (Amphiope, Parascutella) deposit dominated by very well preserved, complete specimens. Both units are interpreted as proximal storm deposits based on the general sedimentary environment, clast relationships, and taphonomic features.
Four factors contributing to mass deposits of clypeasteroid sea urchins in Cenozoic sediments include: (1) their gregarious nature with very high density populations; (2) their relatively robust skeletal morphology; (3) the high transport capacity of their flattened, low density skeletons; and (4) their habitat in shoreface environments which is conducive to physical concentrations of skeletal material. The presence of mass clypeasteroid accumulations is compared to other echinoderm deposits and discussed within the context of their rapid evolution in the Cenozoic.
In south-central Oklahoma, the Pooleville Member of the Bromide Formation is a shallowing upward sequence recorded by distal to more proximal storm-influenced shelf facies. The deepest subtidal facies contains horizons of dense clusters of a single species of isoteline trilobite, “Homotelus” bromidensis Esker. Two horizons were excavated and oriented slabs removed for detailed study. The lower horizon is dominated by exoskeletons that have librigenae and hypostomes in place and are interpreted as carcasses rather than molts. The upper horizon is composed largely of molts, as indicated by the opening of the facial sutures and, in many individuals, separation of the cranidium from the remainder of the exoskeleton. In both horizons, facing directions of the exoskeletons are random, and the majority of specimens are in convex-up orientations. All specimens are holaspids and encompass a narrow size range. The trilobite horizons immediately are overlain by sparsely fossiliferous lime mudstone or thin, barren clay layers. This suggests that catastrophic mud blanketing, probably during major storms, preserved the horizons. Thus, the Ordovician clusters represent snapshots of trilobite behavior. As with younger clusters of phacopid trilobites described from the Devonian of New York State, they most likely record behavioral aggregation of individuals for synchronous molting and reproduction. The occurrence of clustering in species from such phylogenetically distant clades as the Asaphina and Phacopida indicates that the aggregation is likely to have been a behavioral characteristic of trilobites in general. These assemblages also provide insight into the depositional dynamics in a otherwise monotonous interval of lime mudstones. They demonstrate an episodic pattern of sedimentation that would be overlooked without taphonomic analysis.
The diversity and abundance of bird and animal tracks preserved in Eocene strata of the Chuckanut Formation in Washington contrasts to the scarcity of body fossils. These ichnofossils were made by vertebrates that inhabited river margins, the only depositional environment favorable for track preservation. Three of the four localities described herein contain tracks from at least two different types of animals. Site SM-6 contains approximately 200 shallow circular plantigrade footprints, perhaps made by a type of archaic mammal of the Orders Pantodonta or Dinocerata. Site RU-1 yielded footprints from a small shorebird and tracks from an early equid or tapiroid. The same type of perissodactyl tracks were preserved at Site KC-1, along with a single webbed bird track, and trackways from a large heron-like bird and a turtle. Site SM-9.5 contained multiple bird tracks of a type not found at the other localities. The discovery of tracks only at Chuckanut Formation sites that expose large bedding planes indicates the importance of considering outcrop architecture during the search for vertebrate ichnofossils, and inspires the hope that similar fossils may eventually be found in correlative formations in the Pacific Northwest.