Temperature and Evolution: Southern Ocean Cooling and the Antarctic Marine Fauna

Abstract

Temperature can influence the physiology of marine organisms on a variety of time scales ranging from short-term fluctuations (tidal, vertical migration) to long-term climatic change. During the past 60 Ma shallow water marine organisms living at high southern latitudes around the margins of the continental fragments of Gondwana have experienced a decrease in mean seawater temperature from about 15°C in the early Tertiary to the present range of roughly + 2° to − 1.8°C. The early Cretaceous fauna around Gondwana was relatively rich and diverse. Despite the influence of glaciation the present fauna is rich in biomass and can show a very high within-site diversity. Some groups, however, notably fish and groups with calcareous skeletons such as bivalves and gastropods, are low in species richness. Evidence from physiology suggests that adaptation to low temperature is not a particularly severe evolutionary problem. The concept that the tropics are more equable than the polar regions is purely anthropocentric and entirely inappropriate for marine organisms. Polar organisms generally have a low cost of maintenance allowing higher growth efficiencies and thus affording a distinct energetic advantage over warmer water forms. Relating periods of extinction to a lowering of seawater temperature leads to a paradox in that the rates of cooling are so much slower (by several orders of magnitude) than those with which living marine organisms can cope, that it is difficult to see why previous marine communities could not adapt to track the change in temperature. One explanation is that altough a long-term change in mean temperature is often slow, this may be accompanied by severe short-term changes with which the fauna cannot cope. Also, it is unlikely that temperature change alone causes widespread extinction, but temperature varying with other ecological factors. If temperature change is indeed a problem then the direction is immaterial, climatic ‘amelioration’ is just as much a problem as climatic ‘deterioration’. Clearly physiology, ecology and palaeontology have much to teach each other.