Sara B. Pruss

Proliferation of Early Triassic wrinkle structures: Implications for environmental stress following the end-Permian mass extinction

Lower Triassic strata in the western United States and northern Italy have yielded the first reported occurrence of microbially mediated wrinkle structures in shallow subtidal siliciclastic paleoenvironments since the Cambrian. We hypothesize that wrinkle structures formed under reduced infaunalization conditions during the aftermath of the end-Permian mass extinction. These wrinkle structures occur in siliciclastic sedimentary rocks in association with hummocky cross-stratification and, in some cases, trace fossils such as Asteriacites , Rhizocorallium , Planolites , and Gyrochorte that indicate a subtidal paleoenvironment. Wrinkle structures were found in the Campil Member in the Werfen Formation of northern Italy and in the Virgin Limestone Member of the Moenkopi Formation and the upper member of the Thaynes Formation exposed in the western United States. The presence of wrinkle structures is likely related to a unique taphonomic window that allowed for the preservation of these delicate features on storm-dominated siliciclastic shelves. Wrinkle structures commonly formed in a variety of subtidal paleoenvironments during the Proterozoic–Cambrian, but thereafter became restricted to intertidal-supratidal and deep-sea environments. This restriction has been attributed to the increase in infaunalization of metazoans following the Cambrian radiation. The proliferation of wrinkle structures in subtidal settings during the Early Triassic suggests that infaunal bioturbation was reduced after the end-Permian mass extinction and that this reduction lasted for millions of years.

Early Triassic Trace Fossils of the Western United States and their Implications for Prolonged Environmental Stress from the End-Permian Mass Extinction

Abstract The end-Permian mass extinction left an indelible mark on trace-fossil assemblages. This is evident during the lag phase of the biotic-recovery interval that occurred in the Early Triassic. Research on the Spathian Virgin Limestone Member of the Moenkopi Formation, western United States, has yielded a mixed carbonate–siliciclastic trace fossil assemblage. The presence of such traces as Thalassinoides, Laevicyclus, and Gyrochorte indicate that previously unreported metazoan behaviors had reappeared in equatorial regions by the close of the Early Triassic. Whereas diversity of trace-fossil assemblages increased from earliest to late Early Triassic time, persistent small size, low average ichnofabric index, low bedding-plane coverage, and reduced tiering point to prolonged stressful environmental conditions following the end-Permian mass extinction—conditions for which there is an abundance of global sedimentological evidence. This trace-fossil assemblage provides a record of soft-bodied organisms that might not otherwise be detected from the study of body fossils alone; it therefore acts as a constraint on the timing of the biotic recovery. This assemblage from the Virgin Limestone Member also serves as an indicator of environmental conditions that might not otherwise be gleaned, illustrating the utility of using trace fossils as environmental proxies during the lag phase of a biotic recovery. The presence of some of these traces (i.e., Thalassinoides) in lowermost Triassic strata of western Canada suggests that the recovery of trace-makers after the end-Permian mass extinction was asynchronous, and that northerly latitudes may have experienced a less-protracted biotic recovery than in equatorial regions.

CARBONATES IN SKELETON-POOR SEAS: NEW INSIGHTS FROM CAMBRIAN AND ORDOVICIAN STRATA OF LAURENTIA

Abstract Calcareous skeletons evolved as part of the greater Ediacaran–Cambrian diversification of marine animals. Skeletons did not become permanent, globally important sources of carbonate sediment, however, until the Ordovician radiation. Representative carbonate facies in a Series 3 (510–501 Ma) Cambrian to Tremadocian succession from western Newfoundland, Canada, and Ordovician successions from the Ibex area, Utah, USA, show that, on average, Cambrian and Tremadocian carbonates contain much less skeletal material than do post-Tremadocian sediments. Petrographic point counts of skeletal abundance within facies and proportional facies abundance in measured sections suggest that later Cambrian successions contain on average <5% skeletal material by volume, whereas the skeletal content of post-Tremadocian Ordovician sections is closer to ∼15%. A compilation of carbonate stratigraphic sections from across Laurentia confirms that post-Tremadocian increase in skeletal content is a general pattern and not unique to the two basins studied. The long interval (∼40 myr) between the initial Cambrian appearance of carbonate skeletons and the subsequent Ordovician diversification of heavily skeletonized organisms provides an important perspective on the Ordovician radiation. Geochemical data increasingly support the hypothesis that later Cambrian oceans were warm and, in subsurface water masses, commonly dysoxic to anoxic. We suggest that surface waters in such oceans would have been characterized by relatively low saturation states for calcite and aragonite. Mid-Ordovician cooling would have raised oxygen concentrations in subsurface water masses, establishing more highly oversaturated surface waters. If correct, these links could provide a proximal trigger for the renewed radiation of heavily skeletonized invertebrates and algae.

Possible early foraminiferans in post-Sturtian (716–635 Ma) cap carbonates

Foraminifera are an ecologically important group of modern heterotrophic amoeboid eukaryotes whose naked and testate ancestors are thought to have evolved ∼1 Ga ago. However, the single-chambered agglutinated tests of these protists appear in the fossil record only after ca. 580 Ma, coinciding with the appearance of macroscopic and mineralized animals. Here we report the discovery of small, slender tubular microfossils in the Sturtian (ca. 716–635 Ma) cap carbonate of the Rasthof Formation in Namibia. The tubes are 200–1300 μm long and 20–70 μm wide, and preserve apertures and variably wide lumens, folds, constrictions, and ridges. Their sometimes flexible walls are composed of carbonaceous material and detrital minerals. This combination of morphologic and compositional characters is also present in some species of modern single-chambered agglutinated tubular foraminiferans, and is not found in other agglutinated eukaryotes. The preservation of possible early Foraminifera in the carbonate rocks deposited in the immediate aftermath of Sturtian low-latitude glaciation indicates that various morphologically modern protists thrived in microbially dominated ecosystems, and contributed to the cycling of carbon in Neoproterozoic oceans much before the rise of complex animals.

ASSESSING THE ECOLOGICAL DOMINANCE OF PHANEROZOIC MARINE INVERTEBRATES

Abstract Ecological studies have revealed that the functional roles of dominant species in modern communities are often more important than overall diversity in governing community composition and functioning. Despite this recognition that abundance and diversity data are both required for a complete understanding of ecological processes, many paleoecological studies focus on presence-absence data, possibly because of concerns regarding the taphonomic fidelity of time-averaged fossil accumulations. However, the abundance of organisms in shell beds has been shown to provide a fairly accurate record of the living community, suggesting that the benefits of relative-abundance data should be reconsidered. Recognition of ecologically dominant species in local fossil assemblages should be based on counts of relative abundance and assessment of ecological role. Ecological dominance at larger spatial or temporal scales can be quantified using the mean rank order of a clade and the proportion of assemblages where the clade is present, providing unbiased, quantitative values for measuring the ecological importance of a clade. Their utility has been tested with three case studies encompassing a range of geographic and taxonomic scales, using a database of 1221 Ordovician–Paleogene quantitative fossil collections. The dominance metrics for rhynchonelliform brachiopods, bivalves, and gastropods broadly parallel anecdotal trends, even including some more detailed patterns documented by regional studies. An examination of substrate preferences for brachiopod and bivalve orders confirms the abundance of infaunal bivalves in siliciclastics and epifaunal bivalves in carbonates, but it also reveals intriguing patterns regarding substrate preferences among rhynchonelliform brachiopod orders. The final case study analyzed changes in dominance between early Mesozoic fossil assemblages from Tethys and Panthalassa, documenting significant geographic differences in the ecological importance of rhynchonelliform brachiopods and bivalves. These large-scale dominance patterns often approximately matched those inferred from diversity trends; however, there are also times when dominance was decoupled from diversity, indicating that further investigation of ecological dominance will provide additional insights into ecological influences on the Phanerozoic history of life. “Are most species simply passengers in ecosystems that are run basically by a few dominants?” (Worm and Duffy, 2003, p. 631)

Lipid biomarkers in ooids from different locations and ages: evidence for a common bacterial flora

Ooids are one of the common constituents of ancient carbonate rocks, yet the role that microbial communities may or may not play in their formation remains unresolved. To search for evidence of microbial activity in modern and Holocene ooids, samples collected from intertidal waters, beaches and outcrops in the Bahamas and in Shark Bay in Western Australia were examined for their contents of lipid biomarkers. Modern samples from Cat and Andros islands in the Bahamas and from Carbla Beach in Hamelin Pool, Western Australia, showed abundant and notably similar distributions of hydrocarbons, fatty acids (FAs) and alcohols. A large fraction of these lipids were bound into the carbonate matrix and only released on acid dissolution, which suggests that these lipids were being incorporated continuously during ooid growth. The distributions of hydrocarbons, and their disparate carbon isotopic signatures, were consistent with mixed input from cyanobacteria together with small and variable amounts of vascular plant leaf wax [C27 -C35 ; δ(13) C -25 to -32‰Vienna Pee Dee Belemnite (VPDB)]. The FAs comprised a complex mixture of C12 -C18 normal and branched short-chain compounds with the predominant straight-chain components attributable to bacteria and/or cyanobacteria. Branched FA, especially 10-MeC16 and 10-MeC17 , together with the prevalence of elemental sulfur in the extracts, indicate an origin from sulfate-reducing bacteria. The iso- and anteiso-FA were quite variable in their (13) C contents suggesting that they come from organisms with diverse physiologies. Hydrogen isotopic compositions provide further insight into this issue. FAs in each sample show disparate δD values consistent with inputs from autotrophs and heterotrophs. The most enigmatic lipid assemblage is an homologous series of long-chain (C24 -C32 ) FA with pronounced even carbon number preference. Typically, such long-chain FA are thought to come from land plant leaf wax, but in this case, their (13) C-enriched isotopic signatures compared to co-occurring n-alkanes (e.g., Hamelin Pool TLE FA C24 -C32 ; δ(13) C -20 to -24.2‰ VPDB; TLE n-alkanes δ(13) C -24.1 to -26.2 -‰VPDB) indicate a microbial origin, possibly sulfate-reducing bacteria. Lastly, we identified homohopanoic acid and bishomohopanol as the primary degradation products of bacterial hopanoids. The distributions of lipids isolated from Holocene oolites from the Rice Bay Formation of Cat Island, Bahamas were very similar to the beach ooids described above and, in total, these modern and fossil biomarker data lead us to hypothesize that ooids are colonized by a defined microbial community and that these microbes possibly mediate calcification.

The Laurentian record of Neoproterozoic glaciation, tectonism, and eukaryotic evolution in Death Valley, California

Neoproterozoic strata in Death Valley, California, contain eukaryotic microfossils and glacial deposits that have been used to assess the severity of putative snowball Earth events and the biological response to extreme environmental change. These successions also contain evidence for synsedimentary faulting that has been related to the rifting of Rodinia, and in turn the tectonic context of the onset of snowball Earth. These interpretations hinge on local geological relationships and both regional and global stratigraphic correlations. Here, we present new geological mapping, measured stratigraphic sections, carbon and strontium isotope chemostratigraphy, and micropaleontology from the Neoproterozoic glacial deposits and bounding strata in Death Valley. These new data enable us to refine regional correlations, both across Death Valley and throughout Laurentia, and construct a new age model for glacigenic strata and microfossil assemblages. Particularly, our remapping of the Kingston Peak Formation in the Saddle Peak Hills and near the type locality shows for the first time that glacial deposits of both the Marinoan and Sturtian glaciations can be distinguished in southeastern Death Valley, and that beds containing vase-shaped microfossils are slump blocks derived from the underlying strata. These slump blocks are associated with multiple overlapping unconformities that developed during synsedimentary faulting, which is a common feature of Cyrogenian strata along the margin of Laurentia from California to Alaska. With these data, we conclude that all of the microfossils that have been described to date in Neoproterozoic strata of Death Valley predate the glaciations and do not bear on the severity, extent, or duration of Neoproterozoic snowball Earth events.

Trace Fossils with Spreiten from the Late Ediacaran Nama Group, Namibia: Complex Feeding Patterns Five Million Years Before The Precambrian–Cambrian Boundary

Abstract Here we describe large, complex trace fossils in the late Ediacaran Omkyk Member of the Zaris Formation, Nama Group, southern Namibia. The horizontal trace fossils are preserved on a number of talus blocks from a bedding plane of a cm-thick sandstone lens from a single stratigraphic horizon less than 100 m below an ash bed dated at 547.3 ± 0.7 Ma. The forms consist of overlapping U-shaped spreiten elements with parallel limbs surrounded by an outer tube. Individual U-shaped elements are 0.2 to 1 cm in diameter, the outer tube is less than 3 mm in diameter, and the forms as a whole range from 5 to 30 cm long and 3 to 10 cm wide. The specimens commonly show a change in direction and change in diameter. The morphology of these trace fossils is comparable to backfill structures, particularly specimens of Paleozoic Zoophycos from shallow water environments. Here we interpret these horizontal spreiten-burrows to record the grazing of the trace-maker on or below a textured organic surface. The identification of large late Ediacaran trace fossils is consistent with recent reports of backfilled horizontal burrows below the Precambrian–Cambrian boundary and is suggestive of the appearance of complex feeding habits prior to the Cambrian trace fossil explosion.

Preservational and morphological variability of assemblages of agglutinated eukaryotes in Cryogenian cap carbonates of northern Namibia

ABSTRACT Laminated carbonates of the Rasthof Formation, deposited in the aftermath of the early Cryogenian low-latitude glaciation (Sturtian, 717–662 Ma), preserve abundant round tests of agglutinated microscopic eukaryotes. Previously, fossil tests were reported in two localities (Ongongo and Okaaru) from microbially laminated carbonates in the Rasthof Formation, which revealed a previous unexplored Cryogenian taphonomic window. In order to better understand the lateral variability in these microfossil assemblages, this work systematically examines fossil tests from two additional localities, South Ombepera and Ombepera, and compares their preservation in thinly and thickly laminated microbial laminites. Cap carbonates in South Ombepera and Ombepera contain abundant, hollow, spheroidal agglutinated tests (50 to 225 µm in diameter). Some of these tests exhibit slitlike or triangular apertures. In contrast, much larger, oval tests with a tapering end dominate the assemblages at Okaaru, whereas oval, laterally compressed and round structures with slits, visors, or central apertures are found at Ongongo. The thinly laminated microbial laminites from Ombepera, South Ombepera, and Okaaru also preserve rare agglutinated tubes attributed to fossils of early Foraminifera. At all four localities, the thinly laminated microbial laminites preserve more microfossils than thickly laminated microbial laminites although these two facies commonly interfinger and are interbedded. This difference shows that conditions present during formation of the thinly laminated microbial laminites favored the preservation of round agglutinators, perhaps during early burial, lithification, and fossilization of the test walls.

Molecular indicators of microbial diversity in oolitic sands of Highborne Cay, Bahamas

Microbialites (stromatolites and thrombolites) are mineralized mat structures formed via the complex interactions of diverse microbial-mat communities. At Highborne Cay, in the Bahamas, the carbonate component of these features is mostly comprised of ooids. These are small, spherical to ellipsoidal grains characterized by concentric layers of calcium carbonate and organic matter and these sand-sized particles are incorporated with the aid of extra-cellular polymeric substances (EPS), into the matrix of laminated stromatolites and clotted thrombolite mats. Here, we present a comparison of the bacterial diversity within oolitic sand samples and bacterial diversity previously reported in thrombolitic and stromatolitic mats of Highborne Cay based on analysis of clone libraries of small subunit ribosomal RNA gene fragments and lipid biomarkers. The 16S-rRNA data indicate that the overall bacterial diversity within ooids is comparable to that found within thrombolites and stromatolites of Highborne Cay, and this significant overlap in taxonomic groups suggests that ooid sands may be a source for much of the bacterial diversity found in the local microbialites. Cyanobacteria were the most diverse taxonomic group detected, followed by Alphaproteobacteria, Gammaproteobacteria, Planctomyces, Deltaproteobacteria, and several other groups also found in mat structures. The distributions of intact polar lipids, the fatty acids derived from them, and bacteriohopanepolyols provide broad general support for the bacterial diversity identified through analysis of nucleic acid clone libraries.