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In living zooxanthellate corals, photosymbiosis explains increased metabolism and accelerated skeletal growth, accounting for the success of these corals in shallow-water tropical reefs. Mesozoic corals of the order Scleractinia appeared in the geologic record during the Middle Triassic, but it was not until the Late Triassic that these corals became prominent reef builders—a change hypothesized to coincide with the advent of photosymbiosis. There is considerable discussion, however, concerning algal symbiosis and the timing of their co-evolution with corals. Thus, the beginning of photosymbiosis in the earliest corals of the Middle Triassic has not been established, nor whether their paleoecology was similar to that of modern corals. Many massive colonial reef-building corals lay down thick, discrete bands in their skeletons that record annual growth. We discovered and illustrate here growth bands in Middle Triassic corals from central Nevada, in particular Ceriostella variabilis, whose skeletal structure and bands are well-preserved in Middle Triassic biostromes of central Nevada. To test the photosymbiosis hypothesis we studied colony growth forms in these fossil corals and performed a quantitative analysis of the bands, both in C. variabilis and in a morphologically similar living zooxanthellate reef coral, Montastraea faveolata. Results of these analyses revealed growth bands and colony shapes almost identical in both living and fossil corals. These findings suggest that photosymbiosis was present in Middle Triassic corals at a very early stage in their Mesozoic history. Scleractinians were also likely zooxanthellate from the onset of their Middle Triassic occurrence but for unexplained reasons were not as efficient as modern corals in building reefs.
New echinoderm holdfast discoveries from Gondwana demonstrate that pelmatozoans have been cementers able to attach to carbonate firmgrounds since the basal middle Cambrian. Echinoderms were thus colonizing shallow, high-energy environments well before the appearance of the first true carbonate hardgrounds in the Furongian. Morphological innovations and adaptations to firmground media ( = substrates) were first developed in softground, clayey, offshore environments where echinoderms cemented to shell fragments. This preadaption allowed echinoderms to quickly and effectively exploit the newly emerging hardground habitats in the Furongian to Early Ordovician.
Fossiliferous shallowing-upward parasequences in the early Oligocene Byram Formation of Mississippi provide an ideal opportunity to explore the effects of differences in data preparation, format, and analysis in testing for faunal persistence. Ten replicate bulk samples collected from each of three lithologically similar horizons in successive parasequences yielded >13,000 molluscan individuals. Assemblages were first compared using more traditional descriptive statistics that examined aspects of taxonomy, abundance, and guild structure, and were then compared using multivariate techniques with similarity coefficients that employed both compositional and abundance data. Different procedures for identifying and excluding rare taxa prior to analysis were also investigated. Simple numerical methods emphasized similarities among assemblages; hierarchical clustering, nonmetric multi-dimensional scaling (NMDS), and analysis of similarities (ANOSIM) revealed differences among the faunal horizons. Multivariate tests indicated stasis only when using presence-absence data with a large number of rare taxa excluded. When abundance data are utilized, however, faunal change through time is suggested, regardless of how rare taxa are treated. The choice of data format, rare taxon exclusion policy, and analytical approach all affect the interpretation of results and the outcome of hypothesis tests for stability, suggesting that differences in methodology partly contribute to disagreement among studies examining faunal persistence. Furthermore, various properties related to ecosystem structure are ascribed different degrees of importance depending on the study. Thus, comparisons of patterns should be made on equal footing to ensure that differences in methodology are not contributing to differences in interpretation.
The Sun River Bonebed is a monodominant assemblage of late juvenile lambeosaurine elements from the Upper Cretaceous Two Medicine Formation of north-central Montana, United States. Detailed excavation revealed an unusual paleobiologic and depositional signature. Although the bonebed occurs in a succession of beds representing anastomosing stream deposits in a seasonal paleoenvironment, the assemblage consists of a conglomerate of bone and calcareous clasts in a matrix of silty mud and free-floating sand grains. Internally, the bed exhibits normal grading of bone and calcareous clasts, poor sediment sorting, and preferred orientation of elongate elements, all characteristics common to debris flow deposits. The mud-rich matrix, poor sorting, and graded clasts of the bonebed suggest the assemblage was entrained and deposited by a cohesive debris flow, perhaps initiated through entrainment of fine overbank sediment by a seasonal flood. Nearly complete skeletal disarticulation and weathering of some bones indicate a brief period of postmortem exposure prior to debris flow entrainment. Fracture styles suggesting fresh breaks and frequent abrasion may reflect pre-flow trampling or chaotic flow transport. A significant number of elements also exhibit wet rot. The uniformity in taphonomic effects among elements suggests a mass mortality event, a rarity for debris-flow-hosted bonebeds, though the specific cause of death is uncertain. The age class dominance is interpreted to reflect original paleobiology, rather than abiotic postmortem selection, and establishes the Sun River Bonebed as the first bonebed of predominantly late juvenile material, with no adult material, in the Two Medicine Formation.
High productivity on the Florida Platform during the Pliocene has been ascribed to upwelling and to freshwater input of nutrients. To test these hypotheses, high-resolution stable isotopic and Sr/Ca analyses have been performed on 14 Conus and Turritella gastropod shells collected from the middle Pliocene Pinecrest Beds (Units 7 and 4) and the Plio-Pleistocene Caloosahatchee Formation. Assuming a published Pliocene seawater δ18O of 1.02‰ derived from a coupled ocean-atmosphere general circulation model (OAGCM), reconstructed paleotemperatures of Units 7 and 4, and the Caloosahatchee are respectively 25.1 ± 1.4 °C, 16.1 ± 0.6 °C, and 22.4 ± 0.5 °C. Unit 7 paleotemperatures are similar to, and Caloosahatchee paleotemperatures slightly lower than, modern sea surface temperatures (SSTs) in the Sarasota Bay (24.5 ± 0.4 °C). In contrast, Unit 4 paleotemperatures are unrealistically low. Sr/Ca ratios, however, suggest no significant paleotemperature difference between Pinecrest Units 7 and 4 and the Caloosahatchee Formation, indicating seawater δ18O variations, rather than temperature differences, are responsible for δ18O differences. High δ18O and low δ13C values of these samples likely reflect highly evaporated freshwater input combined with oxidation of terrestrial debris, as a brackish environment is indicated by marine and freshwater fauna in Unit 4. This isotopic pattern is similar to that for modern Florida Bay mollusks, which are influenced by discharge of Everglades waters. Furthermore, episodic enrichments in Fe, U, and P in some shells suggest nutrient input from submarine groundwater discharge. The data, therefore, support the contention that the dominant cause of high productivity in this region was enhanced nutrient input from freshwater influx.
The distribution and significance of lingulide brachiopods (Lingularia selwyni Whiteaves) and the trace fossil Lingulichnus in storm-generated, subtidal sandstone beds in the upper Toad and Liard formations of northeastern British Columbia is summarized. Storm-generated sandstone beds from two depositional environments (proximal offshore to offshore transition and lower shoreface) were analyzed. Lingulide material is present in only a small proportion of the storm-generated sandstone beds in the study interval. This distribution is interpreted to reflect the inherent patchiness of infaunal communities both in pre- and post-event communities. Middle Triassic lingulides, like their extant relatives, were capable of (1) surviving storm-induced burial by extending their burrows upward and reconnecting with the sediment-water interface; (2) surviving storm-induced exhumation and transport; and (3) reinhabiting the sediment and reestablishing themselves in new settings after relocation by storms. Depositional environment played a pivotal role in the likelihood that individual lingulides survived storm events. Lingulide faunas deposited in the proximal offshore to offshore transition experienced up to 15%–25% mortality during storm events with most storm survivors (90%–95%) able to reestablish themselves in feeding position at the sediment-water interface. Lingulide faunas deposited in lower shoreface settings experienced considerably higher mortality, with ∼60%–75% of the animals dying prior to the end of the storm and a further ∼45% surviving the storm events but failing to reestablish themselves at the sediment-water interface in life position. Escape burrowing and reburrowing following exhumation, entrainment, and transport are fundamental adaptations for infauna occupying such dynamic depositional settings as the subtidal littoral zone (neritic zone). This ability was likely integral in the long-term success of lingulides in Phanerozoic marine successions.
Mammalian teeth are made of extremely hard and durable calcified tissues, which make them superb candidates for fossilization. The enamel crown, the outermost layer in the erupted tooth, precludes most microorganisms from entering into the underlying dentine and cementum, supporting tissues that are more vulnerable to microbial destructive action. Here we report the discovery of a single tooth of the late Eocene (ca. 35 Ma) North American basal lagomorph Megalagus, clearly containing filamentous and occasionally branching microfossils. The remarkably preserved microfossils are most similar to actinomycetes, gram-positive Eubacteria. We argue that these microorganisms were growing during the animal's life and thus document the first known tooth infection in placental mammals. Our findings provide insight into the initial development of mammal-dental pathogen interactions.
Earth's earliest known metazoan ecosystems are represented by a handful of globally distributed fossil assemblages, collectively referred to as the Ediacara Biota. Although a number of these deposits have been extensively studied, a large proportion of Ediacaran diversity remains uncharacterized. As a result, our understanding of community structure during this important stage of early metazoan evolution is largely incomplete. Moreover, it is only by examining these deposits from a taphonomic perspective that we can hope to decipher these enigmatic forms and fully reconstruct the Ediacaran community. Using this approach, we describe the anomalous preservation of a distinct, prolific, and previously undescribed Ediacaran biogenic sedimentary structure, informally known as “mop,” from the Ediacara Member of the Rawnsley Quartzite in South Australia. Morphological resemblance, spatial association, size distribution, and examination of intermediary forms indicate a shared origin with the holdfast form genus Aspidella and convergence with Pseudorhizostomites. We interpret mop to have been formed by the dragging or uprooting of a Charniodiscus-like frond through a microbially bound substrate by unidirectional currents. Like a freeze frame, mop captures the momentary interaction of organisms and their physical and biotic environment. Detailed characterization of morphological and sedimentological features suggests that variability of mop and associated forms is due largely to taphonomically controlled factors. A better understanding of problematic structures like mop may elucidate the still-enigmatic Ediacaran substrate and the non-actualistic taphonomic processes at work in the preservation of Ediacaran deposits.
The Late Ordovician Taconic orogeny was associated with volcanic eruptions along the subduction zones of the Iapetus Ocean. One of these eruptions, which led to the deposition of the Deicke K-bentonite Bed, is believed to have been larger than the largest recent and subrecent volcanic eruptions (e.g., Toba, Pinatubo). The Deicke eruption has been proposed to have led to a cooling event and associated faunal turnover during the Sandbian–Katian of Laurentia based in part on the observed lowering of global surface temperature after recent mega-eruptions. We tested for a geologically resolvable climatic perturbation associated with the Deicke eruption by estimating changes in ocean temperatures from the oxygen isotope ratios of single-species separates of conodont apatite from a section of the Carimona Member of the Platteville Formation in southeastern Minnesota, United States, that includes the Deicke K-bentonite. In contrast to predictions of models invoking more or less direct volcanic forcing for Ordovician climate trends, we found no obvious or consistent change in temperature at or above the bentonite, but did see evidence of cooling (∼4 °C) among presumed nekto-benthic taxa in the 0.7 meters of the section below the bentonite. Thus, at least for the study area, there is no evidence that the Deicke eruption induced a significant cooling event.
As direct evidence of paleowildfires, fossil charcoal has so far rarely been reported from Triassic rocks around the world. Indeed there seems to be a scarcity of reports of charcoal between the Permian–Triassic boundary (PTB) and the Ladinian (upper Middle Triassic), an interval of ∼16 myr. There are only a few published records in this time period, consisting either of microscopic charcoal in palynological samples or of indirect evidence such as potential fire scars in wood. Macroscopic charcoal has recently been discovered in the early Middle Triassic (early Anisian) Voltzia Sandstone fossil Lagerstätte in southwestern Germany, providing the oldest macroscopic post-Permian evidence of wildfire currently known. Previous authors have suggested a lack of fuel as a reason for the scarcity of charcoal in Lower-Middle Triassic rocks. As the Voltzia Sandstone includes the oldest known, moderately diverse regional paleoflora after the PTB (interpreted by some authors as representing the recovery of the land flora after end-Permian biotic events), a lack of fuel cannot be claimed as a possible reason for the scarceness of charcoal in these rocks. It seems possible in this particular case that previous researchers simply may have overlooked charcoal remains from this formation, either because they were not recognized or were not considered important at the time.