"We know that Mars once had liquid substances flowing over its surface, but why do we think it was water? Could it not have been something else?"
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Epicontinental Seas Versus Shallow Ocean-Facing Settings: Mass Extinction and Diversification in Two Different Worlds?
PI: Arnold Miller
At a broad scale, shallow-marine environments preserved in the geological record of the Phanerozoic Eon can be grouped into two paleogeographic categories with fundamentally different properties: epicontinental seas, unusually broad, shallow settings that once covered large portions of several continents; and shallow, ocean-facing settings, which were coastal areas marginal to continents and islands that interface directly with the open ocean (e.g., continental shelves). With respect to these settings, the Phanerozoic was marked by a transition in the locus of global marine biodiversity. Whereas preserved biotas of the Paleozoic (the first of the three eras comprising the Phanerozoic) came largely from epicontinental seas, there was a shift towards ocean-facing settings in the post-Paleozoic (the Mesozoic and Cenozoic eras), and, perhaps in parallel, a number of fundamental macroevolutionary differences have now been diagnosed in Paleozoic versus post-Paleozoic marine biotas. Against this backdrop, the purpose of the proposed research is to investigate in detail the comparative macroevolutionary properties of epicontinental seas versus shallow, open-ocean-facing settings throughout the Phanerozoic, particularly for time intervals when both settings are well represented in the geological record. To achieve our objectives, we will be augmenting and analyzing data from the Paleobiology Database (http://paleodb.org), which will enable us to investigate the macroevolutionary properties of both settings in paleogeographic and paleoenvironmental contexts. Our initial comparisons for the Permian through Cretaceous periods suggest that open-ocean-facing settings exhibited significantly higher extinction rates than epicontinental seas during major mass extinctions but not at other times. Coupled with the observation that open-ocean facing settings were growing in relative importance through the Phanerozoic, this suggests that there may have been secular transitions in the response of the global marine biota to potential mass extinction-inducing mechanisms, such as large-body impacts. Our initial analyses also suggest a major, Late Jurassic transition in origination rates exhibited among taxa in open-ocean settings, which may be related to changes in the paleogeographic ranges of taxa and the opening of the Atlantic Ocean. These and other issues will be form the backbone of the proposed investigation, which is relevant to Exobiology/Evolutionary interests in the Evolution of Advanced Life and NASA’s broader strategic interest in the Origin and Evolution of Earth’s Biosphere, particularly as they pertain to diagnosing the signatures and causes of mass extinctions, as well as environmental and paleogeographic conditions that promote the diversification of complex life.
May 16, 2012
