Linda C. Ivany

Phanerozoic trends in the global diversity of marine invertebrates.

It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing 3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.

Effects of sampling standardization on estimates of Phanerozoic marine diversification.

Global diversity curves reflect more than just the number of taxa that have existed through time: they also mirror variation in the nature of the fossil record and the way the record is reported. These sampling effects are best quantified by assembling and analyzing large numbers of locality-specific biotic inventories. Here, we introduce a new database of this kind for the Phanerozoic fossil record of marine invertebrates. We apply four substantially distinct analytical methods that estimate taxonomic diversity by quantifying and correcting for variation through time in the number and nature of inventories. Variation introduced by the use of two dramatically different counting protocols also is explored. We present sampling-standardized diversity estimates for two long intervals that sum to 300 Myr (Middle Ordovician-Carboniferous; Late Jurassic-Paleogene). Our new curves differ considerably from traditional, synoptic curves. For example, some of them imply unexpectedly low late Cretaceous and early Tertiary diversity levels. However, such factors as the current emphasis in the database on North America and Europe still obscure our view of the global history of marine biodiversity. These limitations will be addressed as the database and methods are refined.

Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary.

The Eocene/Oligocene boundary, at about 33.7 Myr ago, marks one of the largest extinctions of marine invertebrates in the Cenozoic period. For example, turnover of mollusc species in the US Gulf coastal plain was over 90% at this time. A temperature change across this boundary—from warm Eocene climates to cooler conditions in the Oligocene—has been suggested as a cause of this extinction event, but climate reconstructions have not provided support for this hypothesis. Here we report stable oxygen isotope measurements of aragonite in fish otoliths—ear stones—collected across the Eocene/Oligocene boundary. Palaeotemperatures reconstructed from mean otolith oxygen isotope values show little change through this interval, in agreement with previous studies. From incremental microsampling of otoliths, however, we can resolve the seasonal variation in temperature, recorded as the otoliths continue to accrete new material over the life of the fish. These seasonal data suggest that winters became about 4 °C colder across the Eocene/Oligocene boundary. We suggest that temperature variability, rather than change in mean annual temperature, helped to cause faunal turnover during this transition.

Coordinated stasis: An overview

Abstract Coordinated stasis, as defined herein, represents an empirical pattern, common in the fossil record, wherein groups of coexisting species lineages display concurrent stability over extended intervals of geologic time separated by episodes of relatively abrupt change. In marine benthic fossil assemblages, where the pattern was first recognized, the majority of species lineages (60 to more than 80%) are present in their respective biofacies throughout timespans of 3–7 million years. Most lineages display morphological stasis or only very minor, typically non-directional, anagenetic change in a few characters throughout a prolonged time interval; evidence for successful speciation (cladogenesis) is rare, few lineages ( Causes of coordinated stasis and of regional ecological crisis/reorganization remain poorly understood. Tracking of spatially shifting environments appears to be the rule, rather than adaptation to local change. Incumbent species appear to have a very strong advantage and may excluded potential immigrants, as evidenced by temporary incursions of exotic taxa (“incursion epiboles”); this suggests a role for ecological and biogeographic factors in maintaining paleoecological stability. Stabilizing selection may be critical for producing morphological stability in individual lineages. Episodic crises appear to involve environmental perturbations that were too pervasive and/or abrupt to permit local tracking of environment to continue. Some faunal turnovers associated with unconformities may be partially an artifact of stratigraphic incompleteness. Others, however, seem to occur within conformable successions and were evidently rapid. Widespread anoxia, changes in current patterns, and/or climatic change associated with major marine transgression are common correlates of faunal turnovers in marine habitats in the Appalachian Basin. The phenomenon of coordinated stasis has been noted, albeit not fully documented, in a number of ancient marine and terrestrial ecosystems. An important goal for evolutionary paleoecology should be to document the patterns of stability and change in common and rare members of fossil assemblages in order to discern the relative frequency of coordinated stasis in the rock record, to evaluate the mechanisms by which such apparent evolutionary and ecological stability might be produced, and to seek clues (e.g., paleobiological and stratigraphic patterns, geochemical anomalies) as to causes of abrupt pulses of faunal change.

Eocene climate record of a high southern latitude continental shelf: Seymour Island, Antarctica

A high-resolution record of Eocene paleotemperature variation is preserved within the high southern latitude, marine shelf succession of the La Meseta Formation on Seymour Island, off the NE side of the Antarctic Peninsula. 87 Sr/ 86 Sr ratios of bivalve shell carbonate indicate that the La Meseta Formation spans virtually the entire Eocene, and suggest the presence of an early middle Eocene unconformity. Paleoclimatic and paleoceanographic inferences are based on the stable oxygen and carbon isotope values of two genera of bivalves collected with a high degree of stratigraphic resolution within the formation, and with multiple replicate samples from each horizon. δ 18 O data indicate roughly 10 °C of cooling from the early Eocene climatic optimum (~15 °C) through the end of the Eocene (minimum ~5 °C), much of which took place in two comparatively short intervals at ca. 52 and ca. 41 Ma. Many features of the isotope curves generated from this Eocene shelf section are apparent in δ 18 O and δ 13 C data from the Southern and global oceans, including warm intervals that likely correspond to the early and middle Eocene climatic optima (EECO and MECO). A rapid middle Eocene shift to much more positive values is the most signifi cant in the section and refl ects a drop to universally cooler temperatures in the late middle and late Eocene that might also be associated with a short-lived glacial advance. However, even using a somewhat depleted value for δ 18 O of seawater in the Antarctic peninsular region, average Seymour Island shelf-water paleotemperatures did not reach freezing before the end of the Eocene. δ 13 C data similarly refl ect the documented middle Eocene surface ocean enrichment followed by more negative values, but depletion is much more pronounced on Seymour Island and persists for the remainder of the Eocene, suggesting a combination of upwelling, metabolic effects, and/or atypical carbon cycling on the shelf in this region. Isotope data capture information about changes in the paleoenvironment that also had consequences for the biota, as published paleontological records document marked change in the nature of terrestrial and marine biota at this time. The fact that middle Eocene cooling and biotic turnover in the Peninsular region correspond well in time to the proposed initial opening of Drake Passage suggests that the formation of gateways, in addition to changes in pCO 2 , had signifi cant consequences for the Earth’s climate system during the Paleogene.

Evidence for an earliest Oligocene ice sheet on the Antarctic Peninsula

There is growing consensus that development of a semipermanent ice sheet on Antarctica began at or near the Eocene-Oligocene (E-O) boundary. Beyond ice-rafted debris in oceanic settings, however, direct evidence for a substantial ice sheet at this time has been limited and thus far restricted to East Antarctica. It is unclear where glacier ice first accumulated and how extensive it was on the Antarctic continent in the earliest Oligocene. Sediments at the top of the Eocene marine shelf section on Seymour Island, Antarctic Peninsula, include glacial marine deposits and a lodgment till with clasts derived from a variety of rock units on the peninsula. Dinoflagellate biostratigraphy and strontium isotope stratigraphy indicate an age at or very close to the E-O boundary. Glacier ice extending to sea level in the northern peninsula at this time suggests the presence of a regionally extensive West Antarctica ice sheet, and thus an even more dramatic response to the forcing factors that facilitated high-latitude ice expansion in the earliest Oligocene.

Continental Drift and Phanerozoic Carbonate Accumulation in Shallow-Shelf and Deep-Marine Settings

Knowledge of past rates of transfer of rock‐forming materials among the principal geologic reservoirs is central to understanding causes and magnitudes of change in earth surface processes over Phanerozoic time. To determine typical rates of global sediment cycling, we compiled information on area, volume, and lithology of shallow‐water sediments by epoch for both terrigenous clastics and marine carbonates. Data on amounts of surviving continental terrigenous rock (as opposed to deep oceanic, and including “terrestrial,” “marginal marine,” and “marine” deposits) exhibit positive age/area trends wherein greatest areas and volumes of conglomerate, sandstone, and shale are represented by younger sequences. Global volumes of terrigenous‐clastic sediment yield a mean cycling rate of 0.00124/m.yr., similar to that determined for Eurasian (0.00127), North American (0.00058 [A. B. Ronov], 0.00352 [T. D. Cook and A. W. Bally]), African (0.0017), and South American (0.0021) clastic sequences. Surficial erosion results in the mean destruction of ∼0.124% of terrigenous rock volume per million years of reservoir age. In contrast, surviving epicratonic and shelf‐margin carbonate sequences yield negative cycling rates of about −0.164%/m.yr. Surviving areas and volumes increase with sequence age; that is, the amount of limestone and dolostone preserved in shallow‐water settings increases back in time to maximal areal extents in the Middle and Upper Cambrian. Mass/age data on terrigenous‐clastic successions indicate generally constant rates of crustal erosion over Phanerozoic time. Decrease in size of the shallow‐marine carbonate reservoir forward in time therefore suggests generally invariant rates of global limestone accumulation and a shift in sites of accumulation from shallow‐cratonic to deep‐oceanic settings over much of the past 540 m.yr. Causes of this eon‐scale depositional translocation of carbonate sediment from shallow‐ to deep‐marine settings cannot be satisfactorily linked to either changes in global sea level or the evolution of carbonate‐producing plankton because neither exhibits a pattern of unidirectional change in position or abundance since the Early Phanerozoic. However, tabulation of long‐term variation in areas of continental shelves from paleogeographic maps reveals a generally uniform decrease in low‐latitude (<30°) platform area with decreasing age over most of the Phanerozoic. Moreover, rate of change of low‐latitude shelf area (∼ \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Revisiting Raup: Exploring the influence of outcrop area on diversity in light of modern sample-standardization techniques

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