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Mechanisms governing taxon replacement and subsequent radiation remain little understood. We examine possible forcing factors in a turnover of subfamily dominance seen within the fossil record of green lacewings (Neuroptera: Chrysopidae), a common, cosmopolitan, nocturnally active insect family. Analyses indicate that Nothochrysinae dominated the family in the Eocene, while today they are relictual and the cosmopolitan Chrysopinae dominates with > 97% of its > 1200 species. Our findings suggest that this turnover is consistent with two key adaptations in the Chrysopinae: a tympanum that detects echolocation sounds of bats, which appeared in the fossil record and rapidly radiated during this time (a Red Queen interaction), and increased climatic tolerance coincident with the onset of post-Eocene global icehouse world climate (a Court Jester effect).
Microbialites provide some of the oldest direct evidence of life on Earth. They reached their peak during the Proterozoic and declined afterward. Their decline has been attributed to grazing and/or burrowing by metazoans, to changes in ocean chemistry, or to competition with other calcifying organisms.
The freshwater microbialites at Laguna Bacalar (Mexico) provide an opportunity to better understand microbialite growth in terms of interaction between grazing organisms versus calcium carbonate precipitation. The Laguna Bacalar microbialites are described in terms of their distinct mesostructures. Stromatolites display internal lamination, attributed to the precipitation of calcite and the upward migration of cyanobacteria during periods of low sedimentation. Thrombolitic stromatolites show internal lamination in addition to internal clotting. The clotting is seen as a result of binding and/or trapping of micritic peloids by cyanobacteria and attributed to periods of high sedimentation. The carbonates in both microbialites had similar C- and O-stable–isotopic signatures, both enriched in 13C relative to bivalves, suggesting photosynthetic CO2 uptake was the trigger for carbonate precipitation. This implies that the rate of microbialite growth is largely a function of ambient carbonate saturation state, while the texture is especially dependent on accretion rates and sediment deposition on their surface. Importantly, the coexistence with grazing animals suggests no significant inhibition on microbialite growth, thereby calling into question the decline of microbialite as a result of metazoan evolution. Varying sedimentation rates are likely important in controlling the distribution of thrombolite–stromatolite packages in the geological record, given the importance of this factor at Bacalar.
The potential of ichnofabrics to yield information on endobenthic communities, organism behaviors, and paleoenvironmental conditions is widely recognized among sedimentary geologists. However, the extreme range in both ichnologic fidelity and temporal resolution of ichnofabrics is commonly not fully appreciated. Ichnofabrics developed in slowly deposited pelagic sediments, such as the Cretaceous Demopolis Chalk (U.S. Gulf coastal plain), represent one extreme. Ichnofabrics in the Demopolis Chalk, preeminently expressed only at transitions between marls and chalks, are time-averaged composites that formed over periods exceeding 8 kyr. Ichnologic fidelity is low; distinct biogenic structures reflect only the work of elite deep-tier tracemakers. Moreover, modeling that employs densities of elite burrows and burrow systems, sedimentation rates, and conservative estimates of trace-maker life spans indicates that preserved ichnofossils likely represent less than 10% of the time recorded in associated host sediments. Limited temporal resolution and completeness of these and comparable ichnofabrics call for caution in paleoenvironmental and paleoecologic interpretations and have implications for understanding behaviors, spatial distributions (patchiness), and vertical segregation (tiering) of tracemakers in marine pelagic substrates.
By comparing censuses of living, shallow marine molluscan communities with their associated time-averaged dead remains, we gain an understanding of compositional changes that have taken place through time. Our ability to diagnose anthropogenic or natural environmental changes based on dissimilarities between life and death assemblages, however, is contingent on the existence of a time lag between changes in the life assemblage and their manifestations in the time-averaged death assemblage (taphonomic inertia). Here, we utilized a unique set of multiyear census records of live and dead marine mollusks from Smuggler's Cove, St. Croix, U.S. Virgin Islands to determine the magnitude of taphonomic inertia in this setting. We found that over a more than three-decade interval from 1980 to 2012, both the life and death assemblage composition varied significantly. Most of this variation was driven by shifting rank orders of several key species, including: Cerithium litteratum, Modulus modulus, and lucinid bivalves. Decadal-scale variations in the life assemblage for the most abundant species, C. litteratum, are mirrored in death assemblages collected throughout the entire study interval, suggesting that the upper limit of taphonomic inertia is a decade or less. On the other hand, notable differences in composition between life assemblages, but not death assemblages, among samples collected 1.5 years apart suggest a minimum degree of inertia that is greater than seasonal. Precipitation and temperature, investigated as possible environmental drivers of change, vary significantly over the period in question, and likely play some role in regulating the abundances of these key species, although this remains speculative.