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.
Coralomorph-lithistid demosponge-microbial reefs are developed in deep subtidal settings of the lower part of the Zhangxia Formation (Crepicephalina Zone: Drumian of Cambrian Series 3) in the Jiulongshan section of the Laiwu area of Shandong Province, North China. These reefs are several tens of centimeters thick and formed upon stabilized substrates of microbial origin. The coralomorph-bearing reefs are clearly distinguishable from demosponge-calcimicrobial reefs and Epiphyton bioherms by the development of dendritic thrombolites and the immediately overlying columnar-layered stromatolites. The microbial degradation of sponges facilitated the precipitation of micritic, peloidal, and clotted carbonates, leading to the formation of coralomorph-bearing thrombolitic cores. In contrast, Epiphyton preferentially grew upward, and the accumulation of Epiphyton clumps formed the dendritic thrombolites and the overlying columnar frameworks of the stromatolites. Marked changes in the microbialite texture are considered to reflect some kind of ecological succession, possibly changing environmental conditions. Therefore, the reefs originated from both heterotrophic and photoautotrophic microbial activities. Concave-upward infillings and lateral bridges are common in the interthrombolite and intercolumnar spaces, suggesting episodic sedimentation on the lateral outgrowths of biofilms and highlighting the columnar-layered structures. The coralomorph-lithistid demosponge-microbial reefs are characteristic of the pioneering, stabilizing, and climax stages of a microbial succession. They are age-specific features of microbial-dominated reefs from deep subtidal settings in the Drumian Stage, when skeletal-dominated reefs were inhibited globally, long before the regime shift in reef construction around the Early Ordovician.
Fossil assemblages are expected to be time-averaged as a result of biological and physical processes that mix skeletal remains. Our quantitative understanding of time-averaging derives primarily from actualistic studies, in which direct numerical dating of individual specimens is used to assess the scale and structure of age mixing in death assemblages (incipient fossil assemblages). Here we examine the age, and the time-averaging of Mactra shells (Bivalvia: Mollusca) gathered from surface mixed siliciclastic-bioclastic sands at three sites on a passive-margin subtropical shelf (the Southern Brazilian Shelf; ∼ 33°S). Sixty Mactra specimens were individually dated using amino acid racemization (AAR) calibrated using radiocarbon ages (n = 15). The time-averaging and the total age variability was based on a Bayesian approach that integrates the estimation errors and uncertainties derived from the posterior distribution associated with the AAR calibration average model. The 14C-calibrated AAR ages, pooled across all three sites, are strongly right-skewed with 97% of the individual mollusk shell age estimates ranging from 0 to 6 cal kyr BP. The magnitude of time-averaging varied inversely with the water depth, from < 15 yr at the deepest site (21 m) up to 1020–1250 yr at the shallowest site (7 m). The substantial variation in the temporal resolution across nearby sites, which are located in a seemingly homogenous depositional setting, indicates the presence of notable (if cryptic) spatial heterogeneities in local sedimentation, production, and exhumation, all increasing with water depth.
Bryozoans, stromatoporoid sponges, and tabulate corals, all colonial metazoans with lamellar, encrusting growth forms, developed and simultaneously diversified during the Great Ordovician Biodiversification Event (GOBE). After revisiting some classic Lower, Middle, and Upper Ordovician reef localities in Laurentia (Franklin Mountains, west Texas, Mingan Islands in eastern Canada, and Champlain Valley in northeastern United States) and Baltica (northern Estonia) and reviewing the literature, we demonstrate that during the Ordovician a newly emerging consortium of sheet-like bryozoans, stromatoporoid sponges, and tabulate corals locally bound together by microbes, automicrite, and cement and solidly rooted in sediment became the dominant reef-builders globally. The diversification of these sheet-like metazoans (SLM), however, clearly lagged behind the first appearance of their respective skeletal ancestors. Their habitat expansion can be exemplified as a case of simultaneous ecological fitting and ecosystem engineering when the independently evolved shared traits were simultaneously co-opted and became advantageous under globally different environmental conditions. This interaction led to the evolutionary diversification of colonial metazoans during the GOBE and to the expansion of novel reef habitats in previously soft-surface settings; a transformation that forever changed marine reefal ecosystems.
Burgess Shale-type preservation is a predominantly Paleozoic style of exceptional fossilization via the process of kerogenization, through which organic tissues are converted to more geologically stable forms of carbon. It is often associated with two additional modes of mineralization: (1) pyritization, collectively the precipitation of pyrite on, near, or replacing decaying organic matter; and (2) aluminosilicification, the association of clay minerals, frequently templating the fossil material. Further, some organisms preserved through the Burgess Shale-type pathway also show limited phosphatization, the replication of tissues by apatite, which is usually restricted to digestive tracts/organs. Here, sixteen Cambrian vermiform (worm-like) fossils, preserved via Burgess Shale-type preservation in three formations of the Great Basin, western US, were analyzed using scanning electron microscopy-based methodologies. These fossils display a wide range of taphonomic character, with visual differences in kerogenous-, pyrite-, and aluminosilicate-associated preservation, in addition to some preserved medial structures presumed to be gut tracts. Microchemical analyses indicate additional unique mineral associations, including barite and monazite, which can be broadly attributed to later-stage diagenetic alteration of the initial preservational mineralization. A consistent model of the chronology and drivers of mineralization is presented, and may prove useful for considering Burgess Shale-type preservation at other localities.
Whereas soils in semiarid environments have received considerable attention, specific biogenic structures produced by the plants and animals that inhabit them are less well understood. Soils in a field site in the Santa Catalina Critical Zone Observatory in the semiarid Sonoran Desert of southern Arizona were investigated to develop a model to improve the interpretation of analogous paleosols and associated soil ecosystems in the geologic record. A flat plateau of desert scrubland was divided into thirty-six 4 m2 plots for description and study. Field methods included mapping of soil surfaces, imaging with ground-penetrating radar, casting of open burrows, description of soil trenches, and soil coring. Laboratory methods included analysis of soil bulk geochemistry, clay mineralogy, and thin sections as well as detailed descriptions of burrow casts. Soils included 10–40 cm thick Entisols and Aridisols with blocky peds, pervasive, coarse (3–5 mm) to very fine (< 1 mm) roots, and were dominated by the burrows of ground squirrels and ants, as well as those of various lizards, snakes, scorpions, spiders, centipedes, termites, and insect larvae. Burrow morphologies included small (< 1 cm) to large (> 1 cm) diameter simple vertical shafts, isolated ovoid chambers, subhorizontal tunnels, subvertical to subhorizontal networks of branching tunnels, and complex branching galleries. Bioturbation was concentrated in the upper 20 cm of the soil profile, whereas roots often extended to the base of the profile. Results from this study advance our understanding of the traces produced by different soil organisms and their impact on soil development in modern settings and will be valuable for the interpretation of the paleosols from semiarid environments in the rock record.