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Site Map / Index This page last modified: 7th January, 2008 ŠPaul Markwick 2000-2008 |
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Markwick, P.J. (1996) Crocodilian biodiversity in space and time: a reflection of climate, history or preservation? Palaeontological Association Annual Meeting, Birmingham Abstract Modern biodiversity continues to generate interest amongst ecologists, especially as concerns grow over the rapidity with which habitats and species are disappearing. But a consensus as to the cause(s) of biodiversity remains elusive, although increasingly opinions seem to lean to one of two main views: energy or area/history. The consequences of this debate have important implications beyond ecology or even palaeontology. As a palaeoclimatologist the issue is particularly acute because if biogeography and diversity are not solely due to the environment, of which climate is a dominant factor, then retrodicting palaeoclimate from the pattern of such organisms is more than a little problematic. In order to examine this issue, amongst others, I have compiled a database of over 6000 fossil vertebrate localities covering the globe for the last 100 million years. This is complemented by a dataset of 1060 modern climate stations, for which complete non-avian tetrapod species lists have been compiled for about 500. This dataset provides the opportunity to investigate the relationship of climate, the environment and biota, not only in the recent but also the in the geological record. In the study presented here, I concentrate on the record of crocodilians (limited to members of the extant families Alligatoridae, Crocodylidae and Gavialidae), a group frequently equated with 'warmth.' The palaeolatitudinal history of this group shows a circumstantial link with climatic changes, with latitudinal contractions during intervals of known high-latitude cooling (the Oligocene and Plio-Pleistocene) and poleward expansion during warmer intervals (e.g. Early Eocene and Early Miocene). Concomitant with these latitudinal changes is a restriction of the group to more maritime localities in colder times. These distributional patterns are mirrored in the record of crocodilian diversity. After an initial phase of exponential diversification, crocodilian diversity peaks in the Palaeocene and Eocene. With Late Eocene-Oligocene cooling numbers drop, to rebound again during the Miocene as temperatures increase. A second drop is observed in the Plio-Pleistocene. The latitudinal distribution of diversity shows that the equatorial peak observed in the Recent is traceable back to about the Oligocene at which point it appears to bifurcate: further back, the mid-latitudes in each hemisphere are the sites of peak diversity. No major extinction is seen at the K-T boundary. Biogeographic effects are also apparent: the absence of both gavialids and alligatorids from Africa and Australasia throughout their history. Potential sampling bias is assessed qualitatively, using control groups, and quantitatively, using rarefaction, and is shown to have a negligible influence on overall patterns. The results of this study strongly suggest that environmental factors, specifically climate, play a dominant role in the distribution and diversity of crocodilians. |
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