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Alternating light-dark laminae within stromatolites have been attributed to a phototactic response of the constituent microbial communities, whereby filaments orient vertically during the day and recline at night. This study examines the orientation of cyanobacterial filaments within a laminated siliceous stromatolite from a Yellowstone National Park hot spring to identify the controls on microfabric development and whether phototaxis plays a role. Results indicate that filament orientation is predominantly perpendicular or parallel to lamination, even when laminae are steeply inclined. Thus, phototaxis is not a significant control of microfabric development in these stromatolites. Vertical aspects of the fabric are dominated by hourglass-shaped filament bundles (hourglass structures) adjacent to rounded pores, rather than being defined by individual filaments. The rounded pores likely represent oxygen-rich bubbles generated during photosynthesis. Upon stabilization by filaments, upward buoyancy of the bubbles rotated the bundles toward a vertical orientation. Thus, vertical aspects of the fabric in this stromatolite result from buoyancy forces rather than phototaxis. Examples from the Neoproterozoic Beck Spring Dolomite reveal that similar subvertical hourglass structures are present in the rock record and may be better preserved than individual filaments. The presence of rounded pores (fenestrae) and hourglass structures in ancient microbialites, here termed the hourglass-associated fenestral fabric, can serve as an indication of biogenic influence in stromatolites, especially in the absence of preserved filaments, and may be an indication of oxygenic photosynthesis.
The assumption that particular depositional environments are associated with certain taphonomic patterns has given rise to the concepts of taphofacies and taphonomic mode. This association has been well tested in the marine invertebrate realm—albeit often with restricted geographic and stratigraphic sampling—but less so among terrestrial vertebrates. Here I characterize the overall taphonomic patterns of 36 published terrestrial vertebrate fossil assemblages based on a suite of 12 well-reported taphonomic characteristics, and assess which factors could influence the observed modification patterns. Very little systematic pattern is observable in the distribution of taphonomic characteristics among any of the published data with respect to sedimentary environment of preservation, preserved taxa or average taxon size, inferred mode of mortality, time, or place of deposition. This suggests that the factors controlling the taphonomic modification of terrestrial vertebrate assemblages are extremely complex and emphasizes that most taphonomic histories involve a multiplicity of contingent, interacting processes. The increased sedimentological heterogeneity of terrestrial environments likely plays an important part in producing this complexity, along with regional changes in climate and accommodation. Given the lack of consistent taphonomic patterns, isotaphonomy should not be assumed for comparisons of vertebrate fossil assemblages, even from the same facies, without strong supporting evidence. The only robustly supported consistent taphonomic pattern is the similarity among assemblages from the Dinosaur Park Formation (p = 0.039). This is attributed partly to the unique genetic processes of the Dinosaur Park Formation assemblages and partly to the relatively short time span of their deposition, hence less variance in the factors that could control taphonomic pattern (e.g., climate) than found in formations deposited over longer periods.
Large benthic foraminifera (LBF) were major contributors to many Paleogene carbonate platforms around the world. These photosymbiotic foraminifera lived in warm, oligotrophic, shallow waters within the photic zone. Such Paleogene families as the nummulitids, alveolinids, and orthophragminids rose to prominence in the late Paleocene, thrived in the early and middle Eocene, and declined in the late Eocene and Oligocene. Diversity data from these three families were studied to understand better the controls on the rise of Paleogene LBFs. Analyzed data included total diversity (total number of species per biozone), number of first occurrences per biozone, and number of last occurrences per biozone. Results indicate that there were four intervals of increased total diversity, increased first occurrence, and increased last occurrence for all three families studied. These four intervals follow closely after important climatic events within the Paleogene: the mid-Paleocene biotic event (MPBE), the Paleocene–Eocene thermal maximum (PETM, a hyperthermal event), the early Eocene Climatic Optimum (EECO) and the middle Eocene Climatic Optimum (MECO). The shallow marine biotic community, on a global scale, reacted to such climatic warming events as the MPBE, PETM, EECO, and MECO, based on these diversity trends. Our data also show a pattern of an increase in the number of last occurrences followed by an increase in the number of first occurrences, which suggests that the overall increase in species diversity is due to faunal turnover, as has been interpreted for the large benthic foraminiferal turnover that occurred at the PETM.
Valley-filling deposits of the Nama Group, southern Namibia, record two episodes of erosional downcutting and backfill, developed close together in time near the Ediacaran-Cambrian boundary. Geochronological constraints indicate that the older valley fill began 539.4 ± 1 Ma or later; the younger of these deposits contains unusually well-preserved populations of the basal Cambrian trace fossil Treptichnus pedum. Facies analysis shows that T. pedum is closely linked to a nearshore sandstone deposit, indicating a close environmental or taphonomic connection to very shallow, mud-draped sandy seafloor swept by tidal currents. Facies restriction may limit the biostratigraphic potential of T. pedum in Namibia and elsewhere, but it also illuminates functional and ecological interpretation. The T. pedum tracemaker was a motile bilaterian animal that lived below the sediment-water interface—propelling itself forward in upward-curving projections that breached the sediment surface. The T. pedum animal, therefore, lived infaunally, perhaps to avoid predation, surfacing regularly to feed and take in oxygen. Alternatively, the T. pedum animal may have been a deposit feeder that surfaced largely for purposes of gas exchange, an interpretation that has some support in the observed association of T. pedum with mud drapes. Treptichnus pedum provides our oldest record of animals that combined anatomical and behavioral complexity. Insights from comparative biology suggest that basal Cambrian T. pedum animals already possessed the anatomical, neurological, and genetic complexity needed to enable the body plan and behavioral diversification recorded by younger Cambrian fossils.