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My research in this area originated through my work on climate proxies and their use in interpreting palaeoclimate. Part of the problem I wished to address lay with the data itself and how to qualify it (this issue is included in my thesis, and is now also published in a joint paper with Richard Lupia: (Markwick and Lupia, 2002). Although the presence of a climate proxy provides information about the contemporary environment of that location, the absence of a climate proxy need not be of any use at all. Unless, it is qualified as a 'significant' absence, by which I mean that it's absence can be shown to be due to unsuitable climatic conditions. One solution is to use taphonomic control groups (as used by Jablonski for invertebrates and by myself for vertebrates). But for Palaeogene vertebrates, even control groups are largely absent and the question is why?
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LEFT: Diversity gradients as a means of distinguishing between taphonomic and ecological causes for the Palaeogene distribution of vertebrates in Asia and North America (Markwick, 2000)
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Although the link between climate and diversity has been a common theme in the palaeoecological literature (viz., low diversities being intepreted as indicating stressful -- cold, highly saline etc., -- environmental conditions), it is the apparent relationship between latitude and modern diversity that is most intriguing. This has been used to suggest a link between diversity and climate (although not without considerable controversy: see Markwick, 2002 and the references therein). If these diversity gradients did have an underlying cause I surmised that maybe they could be used to address whether the paucity of Palaeogene fossil vertebrates in the high latitudes of Asia and North America was a taphonomic or ecological effect. The consequence was a detailed study of the modern pattern of biodiversity and it's possible relationship with climate, complimented by further studies of palaeobiodiversity patterns.
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RIGHT: Non-avian tetrapod species diversity as a function of Mean Annual Temperature (MAT). (Markwick, 2002)
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Modern Biodiversity. In order to define the climate space occupied by modern organisms (especially those that may be potentially useful as climate proxies) I have compiled a large dataset of modern climatological (1060 global climate stations with monthly values for about 30 climate variables) and faunal/floral data (vertebrate faunal lists for North America, Europe, Australia, and southern Africa; partial lists for Arabia, Central and South America; floral lists for North America). Research using this dataset has indicated that it also provides a valuable tool for understanding modern biodiversity, ecology and biogeography and the influence of climate. Preliminary results strongly support arguments for a climatic/environmental (ultimately a consequence of the distribution of energy) cause for the observed pattern of biodiversity, which has major implications for not just biogeography and climate, but also evolution and conservation biology. For further information check out my recent paper on the link between modern biodiversity and climate (Markwick, 2002).
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ABOVE: North American reptilian generic diversity. The initial approach (following many oother studies) was to plot diversity (richness) for each latitude-longitude grid cell. This was replaced for the final study (Markwick, 2002)
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Palaeobiodiversity. The comprehensive fossil crocodilian occurrence dataset I have compiled to map palaeoclimates has been used to examine crocodilian diversity and its relation to climate change. An interesting observation from this study has been the recognition that no major extinction occurs amongst fossil "eusuchians" at the Cretaceous-Tertiary boundary. I have used this to suggest that the mass extinctions at this boundary cannot have been solely in response to climate change.(Markwick, 1998). Because the database now also contains data for other vertebrate groups as well as floras, the same sorts of analyses can be made for these too, and this is currently in progress.
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LEFT: Crown group crocodilian generic diversity in palaeolatitude and time. Note that the southern hemisphere records in the Cretaceous are due to a temporally poorly resolved datapoint in India and can be ignored. As a consequence crown group crocodilians do not expand into the southern hemisphere until after the K-T boundary. (Markwick, 1998)
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Additional information on palaeoecology and biodiversity can be found elsewhere on this site.
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