Energetics in the global marinefauna: A connection between terrestrial diversification and change in the marine biosphere

Abstract

The invasion of the land started in the Ordovician and continued into the Carboniferous. The peak ofdiversification rate and increase in biomass of terrestrial vegetation occurred during the Devonian. However, no permanent increase in marine diversity occurred at that time. Instead, a plateau of fluctuating but non-trending diversity extended for 200 million years from the later Ordovician to the beginning of the Late Permian. A second major increase in terrestrial productivity occurred with the diversification of the angiosperms. This began in the Cretaceous and continued into the Cenozoic. Although marine diversity also increased during the Mesozoic and Cenozoic, no permanent offset in diversification rate occurred during this interval. Instead, marine diversity increased nearly continuously through the Jurassic, Cretaceous, and Cenozoic at the same exponential rate of growth established early in the Mesozoic. Despite the lack of a clear signal of change in marine diversity associated with either interval of terrestrial plant diversification, patterns of turnover in the marine fauna during the Devonian and the Cretaceous suggest that these were times of increasing productivity in the oceans. In the Devonian, replacement in dominant orders of many taxa are consonant with the idea of increased food supplies and greater energetics in the fauna (energetics refers to the combination of biomass, general physical activity, metabolic rates, and the concomitant need for a level of food consumption sufficient for the support of metabolic needs). The possible linkage of increased marine productivity and increased energetics in the marine fauna is particularly clear in the turnover in dominance at the top of the food chain. During the Devonian, relatively sluggish, low metabolic rate predators were replaced by generally more active taxa with greater overall energy needs. Nautiloids, eurypterids, and asteroids decreased in diversity as ammonoids, malacostracans, and jawed fishes increased. Likewise, many changes that can be ascribed to increasing availability of food supplies in the oceans began in the Cretaceous. Not only was there a major increase in the diversity of phytoplankton, but the teleost fish and brachyuran crabs both exploded in diversity, predatory boring by gastropods increased, and deposit feeding echinoids become important. These changes during the Cretaceous fit a model of changing energetics in which (a) there was more primary productivity in the oceans, (b) sufficient biomass of prey became available to support a variety of specialized predators, and (c) enough organic detritus regularly accumulated in the sediment to support an increase in deposit feeding organisms. Because of the coincidence in timing of these changes with intervals of terrestrial diversification, it is possible that they were triggered (or fueled) by increased nutrient input from increasing productivity on land. Although increase in marine diversity is one possible consequence of intervals of terrestrial diversification, this did not seem to take place, either in the Devonian or the Cretaceous. A second logical consequence, however, increase in the energetics of the marine fauna, does match the timing of terrestrial diversification. Increases in terrestrial productivity and biomass would have increased the supply of organic detritus and dissolved organic materials to the oceans, as well as possibly increasing the supply of inorganic nutrients because of increased weathering rates as a result of increased organic acids and organic materials in soils. Both the Devonian “precursor to the Mesozoic marine revolution” and the concentration in the Cretaceous of the start of many of the changes associated with the Mesozoic marine revolution itself may have been supported by increases in diversity and biomass on land.