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McNamara, K.J. 1994 10 15: Diversity of Cenozoic marsupiate echinoids as an environmental indicator.

Marsupiate echinoids are today largely confined to the seas around Antarctica. Consequently, it has often been inferred that the presence of marsupiate echinoids in the fossil record is indicative of the former existence of low oceanic temperatures. In this study the distribution of marsupiate echinoids through the Cenozoic succession of southern Australia is compared with palaeo-temperature data to test this assumption. The analysis reveals that there is no positive correlation between high marsupiate echinoid diversity during the Cenozoic and low oceanic temperatures. An alternative hypothesis, based on life-history strategies, is investigated. This reveals that marsupiate echinoids show many characteristics typical of organisms with slow growth, long life spans and production of few, large offspring. It is suggested that the northward migration of Australia during the Cenozoic from an original high-latitude location in the early Cenozoic was accompanied by an increase in environmental instability in the southern Australian region in the late Cenozoic. This led to a consequent decrease in marsupiate echinoid diversity. During the Pliocene these direct brooding echinoids were replaced by non-brooders with pelagic lecithotrophic larvae, which dominate the southern coastal echinoid fauna of Australia today. The environmental stability experienced in southern Australia in the early Cenozoic persisted throughout the Cenozoic in the Antarctic region, particularly with regard to predictability of nutrient supply. The result has been the dominance of marsupiate echinoids in that region today. Temporal changes in the diversity of marsupiate echinoids in southern Australia therefore supports the view that their spatial and temporal distribution may be more closely correlated with aspects of their life-history strategy and environmental stability than with low temperature. Echinoidea, evolution, diversity, life-history strategy, Cenozoic.