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Early Neoproterozoic reefs of the Little Dal Group, Northwest Territories, are built by stromatolites and thrombolites containing calcified filamentous cyanobacteria and interstitial cement. Micritic and microcrystalline carbonate grew in or on extracellular cyanobacterial sheaths, preserving filaments when mineralization was early relative to sheath degradation, or grumeaux when mineralization was later. Filamentous microstructure is volumetrically predominant in the reefs; less common are micritic and grumelous microstructures already known from late Proterozoic stromatolites and Phanerozoic thrombolites. Textural intergradation of filamentous-calcimicrobial microstructure with these non-filamentous microstructures reflects microstructural variation developed through differential preservation at the scale of individual filaments and laminae. Textural gradients from filaments to grumeaux, and from calcimicrobial to stromatolitic and thrombolitic microstructure types, imply that a wide variety of microbialite microstructure types can be derived from a single progenitor community. This suggests that taphonomic variables may be as important in the development of microbialite microstructure as the biology of the microbial mat community. It also challenges recent suggestions that the Neoproterozoic increase in thromboids was related to the rise of multicellular organisms. These conclusions have broad implications for the interpretation of fossil microbialites, many of which might have been more closely related in origin than hitherto suspected.
Brachiopods were one of the most diverse groups of reef-dwellers during the Paleozoic, and their degree of specialization for reef habitats provides an important way of assessing the ecologic complexity in reef communities. Silurian (Wenlockian) reef brachiopods in Gotland and Wisconsin are compared to level-bottom brachiopods in Gotland, Wisconsin, and the Welsh Borderland. The reef—level-bottom comparisons are made at the level of single bulk samples, as combined groups of samples from single reefs and from single level-bottom communities, and as combined groups of samples from several reefs that occupy the same stratigraphic horizon. Species diversity of reef brachiopods is higher than that of level-bottom brachiopods, but the amount of this difference decreases from the scale of entire reefs to the scale of single samples, where Shannon indices (H′) and rarefaction curves of reef and level-bottom samples commonly overlap. In terms of species richness, maximum reef values reach 44 species (n = 3452 individuals), while maximum level-bottom values reach 32 species (n = 7732 individuals). Cluster analysis separates associations of reef and level-bottom brachiopods, but there is also some compositional overlap of reef and level-bottom samples. Although the reefs contain more pedunculate brachiopods and fewer strophic, free-lying brachiopods than level bottoms, the reef brachiopods include no morphotypes or genera that are not also known from the level bottom. Wenlockian reef brachiopods in Gotland and Wisconsin were open surface dwellers with very close taxonomic, functional, and ecologic ties to level-bottom communities. As such, they indicate very similar levels of ecologic complexity between reef and high-diversity level-bottom communities. Cryptic reef brachiopods are known from the Silurian, but appear to have been rare. Brachiopods did not include common reef specialists until the Late Paleozoic. The slow pace of brachiopod specialization for reefs and the very close resemblance of Silurian reef and level-bottom brachiopods reflect the ecologic simplicity and long-term diversity plateau of the Paleozoic evolutionary fauna.
Previous multivariate statistical analyses identified at least seven associations of sporomorphs within a major vegetational group of late Triassic-earliest Jurassic times identified as corresponding to arctic or cool-temperate climatic conditions. Detailed examination of how these associations responded to the climatic changes near the Triassic-Jurassic boundary allows them to be classified in terms of their thermophily and drought tolerance. These deductions also give rise to a fairly detailed reconstruction of climatic changes at the time. Consideration of the presumed family affinities of the various sporomorphs gives some support to the climatic interpretations; and allows tentative reconstructions of some of the vegetation.
Tertiary Crassostrea oysters grew large and thick shells, whereas their descendants, living Crassostrea, grow comparatively smaller and thinner shells. To test for ecological differences between fossil and living Crassostrea that may account for differences in body size, the stable isotope sclerochronology was examined of two North American species, late Oligocene C. gigantissima (Belgrade Formation, North Carolina) and Quaternary C. virginica (Chesapeake Bay and Mississippi Delta), both purported to share an ancestor-descendent relationship. δ18O and δ13C profiles across skeletal growth increments in two well-preserved C. gigantissima shells show significant differences with profiles from Pleistocene and Recent C. virginica. Significantly higher δ18O and δ13C values and smaller seasonal isotopic ranges with less variability show that C. gigantissima lived in a more fully marine environment than C. virginica. Consequently, C. gigantissima probably was exposed to more predation, competition, and bioerosion than C. virginica. Isotopic profiles also show that C. gigantissima formed skeletal growth increments annually from seasonally varying growth rates, which permits estimation of life span and growth rate. Thicker shells and faster growth rates in C. gigantissima may reflect the greater exposure to fully marine predation and bioerosion, as well as to the conditions associated with higher salinity. Associations between C. gigantissima and phosphatic sediments, intense bioerosion, and other large suspension feeders in the Belgrade Formation are suggestive of an elevated planktonic food supply that may have permitted C. gigantissima to grow thicker shells. If estuaries and lagoons have served as refugia from marine predation since the Mesozoic, as these environments do today, then C. gigantissima may represent a lineage that had left its brackish refuge for shallow-marine environments in the Eocene. The replacement of C. gigantissima by C. virginica in the early Miocene may not represent an ancestor-descendent relationship, but an extinction of C. gigantissima and continued survival of thin-shelled Crassostrea in brackish environments.
Two types of ichnofossils from Pleistocene outcrop at Willapa Bay are described. Because both trace fossils are characterized by an inclined to horizontal tunnel, are unlined, have an exaggerated J-shaped morphology, rarely branch, and have an unconstricted apertural opening, they have been classified as Psilonichnus upsilo. Type A and B.
Psilonichnus upsilo. Type A is generally 1 to 3 cm in diameter and is infilled with laminated sediment. In general, P. upsilo. Type A is observed in ancient point-bar deposits. It has an extremely simple architecture that is almost identical to that produced by the crab Hemigrapsus oregonensi. in modern tidal flats at Willapa Bay. Psilonichnus upsilo. Type B normally exceeds 10 cm diameter and is infilled with laminated sediment. The passive infill commonly is deposited in couplets and may be delivered to the burrow network by tide-generated currents. Psilonichnus upsilo. Type B is observed in intertidal flat deposits. The overall morphology of this trace fossil is most similar to burrows generated by large crustaceans such as crabs, stomatopods, and lobsters.
The occurrence of these traces leads to four findings: (1) Psilonichnus upsilo. has a more variable architecture than discussed in the literature. The size and angle of the tunnel are variable, and Psilonichnu. may aggrade, forming Teichichnu.-like structures. (2) In the modern bay, burrowing shrimp dominate subtidal, point-bar, and intertidal deposits. The Pleistocene record indicates that burrowing crabs sometimes occupied similar niches in the ancient bay. (3) Laminated, heterolithic burrow fills provide evidence of rhythmic sedimentation. These laminae represent tidal or episodic sedimentation and provide the only evidence of such processes in otherwise muddy deposits. (4) A large burrowing crab that might make P. upsilo. Type B may not be present in the modern bay. However, such a trace maker was present when these Pleistocene deposits accumulated.
A comparison of the taxonomic composition, relative abundance, diet, feeding habitat, and size of live and dead insects was conducted in Willcox Playa, an ephemeral lake in southeastern Arizona. Death assemblages of beetles, the most abundant group of insects in this study, are preserved in the shallow, subsurface sediments along the shoreline of the lake. Fifty-six percent of the living beetle families and twenty-eight percent of the living beetle genera were found in the sediments; one-hundred percent of the families and ninety-one percent of the genera found dead were also present in the live fauna. Relative abundances of beetle families in the live assemblage are significantly different than relative abundances in the death assemblage. Among diet and habitat groups, necrophagous and ground-dwelling beetles are over-represented in the death assemblage, while wood-inhabitants and aquatics are under-represented. The death assemblage contains a greater proportion of smaller, more robust beetles. Such biases also may occur in fossil assemblages and should be considered in paleoecological and paleoenvironmental reconstructions.
The fossil record of the Cretaceous is critical for understanding the evolution of modern tetrapods. Using a measure of relative completeness of the fossil record—the Simple Completeness Metric (SCM)—quality of the fossil record and diversity during the Cretaceous appear to be closely related, suggesting an artifactual component. The SCM calculations also show that knowledge of the fossil record has improved in the last ten years. Recent proposals that modern orders of birds and mammals originated early in the Cretaceous are rendered unlikely by four arguments: (1) the SCM calculations indicate that the fossil record of Cretaceous birds and mammals is relatively good; (2) it is unlikely that all modern orders, independently, would have remained cryptic throughout the Cretaceous; (3) control samples of exquisitely preserved tiny Cretaceous tetrapods lack any specimens of modern groups of birds and mammals; and (4) the suggestion that the undiscovered ancestors of modern groups are to be found in unsampled parts of the Earth is not supported by cladistic evidence.
The designation “thrombolite” was proposed by Aitken for “cryptalgal structures related to stromatolites, but lacking lamination and characterized by a macroscopic clotted fabric.” In that same paper, he referred to the characteristic mesostructural elements as clots. Since publication of that paper, there have been several attempts at redefinition that have further complicated our understanding of, and communication about, thrombolites.
A distinction must be made between the thrombolite column and the mesostructural clots (mesoclots) that comprise it. Poorly preserved thrombolite columns have only vague mesoclots recognizable, thus leading to confusion. This is particularly true if the columns, themselves, are complex and comprise larger thrombolite bioherms or biostromes. Mesoclots are generally polymorphic mm- to cm-sized objects whereas columns, which may also be polymorphic, are one to several orders of magnitude larger. In order for there to be clear communication among microbialite paleontologists and sedimentologists, a standard guideline for mega-, macro-, meso-, and microstructural studies of thrombolites should be followed.