AZC 46 Supplement (2003)
Issue:
VOL NA
Keywords:
Paleoclimate, paleoecology, fossil insects, seasonality, equability, Eocene
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Wheeler's Dilemma
Abstract:
We identify certain biota as “tropical” and others as “temperate” in the modern world, yet this distinction was not as clear in the early Tertiary. Indeed, many authors have commented upon the well-documented co-occurrence of tropical and temperate biotas in early Tertiary deposits; REID and CHANDLER (1933) called this “one of the most vexed of climatological problems.” Almost a century ago, William Morton WHEELER (1910) noted the surprising mix of warm and cool-adapted ant genera found together in Baltic amber. We review some other warm/cool assemblages of insects, plants and vertebrates, which occur at Eocene extra-tropical localities worldwide. Explanations of time averaging, transitional assemblages between climatic regimes and mixing of biocoenoses are unsatisfying. Instead, these seemingly anomalous mixtures are consistent with hypothesized low temperature seasonality in early Tertiary temperate regions. The sorting of biota into temperate and tropical-identified may be a later Cenozoic phenomenon that reflects our specific modern climatic regime of high seasonality in extra-tropical regions, rather than an innate and historical biogeographic association of these organisms with their present latitudinal ranges. Thus, the presence of clearly thermophilic organisms outside of low latitudes in early Tertiary assemblages may indicate milder winters, not necessarily tropical or subtropical climate.
