Ecology and Evolution
of Cambrian Plankton
Nicholas J. Butterfield
Probable eukaryotic phytoplankton first appear in the fossil record in the Paleoproterozoic but undergo almost no morphologic change until the Early Cambrian. The radiation of diverse acanthomorphic phytoplankton in exact parallel with the Cambrian explosion of large animals points to an ecologic linkage, probably effected by the introduction of small herbivorous metazoans into the plankton. By establishing the second tier of the Eltonian pyramid in the marine plankton, such mesozooplankton might be considered a proximal and ecologic cause of the Cambrian explosion.
THE PLANKTON COMPRISES the majority of all modern marine biomass and metabolism, is the ultimate source of most exported carbon, and plays an essential role at the base of most marine ecosystems (Nienhuis 1981; Berger et al. 1989). Thus, it is hardly surprising to find it figuring in broad-scale considerations of Early Cambrian ecology (e.g., Burzin 1994; Signor and Vermeij 1994; Butterfield 1997), biogeochemical cycling (e.g., Logan et al. 1995), and evolutionary tempo and mode (e.g., Knoll 1994; Rigby and Milsom 1996). The Cambrian is of course of particular interest in that it constitutes one side of the infamous Precambrian-Cambrian boundary, the preeminent shift in ecosystem structure of the last 4 billion years. The question is, what role, if any (cf. Signor and Vermeij 1994), did the plankton play in the Cambrian explosion of large animals? The answer entails a critical analysis of the fossil record, combined with a consideration of indirect lines of evidence and a general examination of plankton ecology and how it relates to large-animal metabolism. There is in fact a good case to be made that developments in the plankton gave rise to both the evolutionary and the biogeochemical perturbations that characterize the Proterozoic-Phanerozoic transition.
The fossil record of Proterozoic-Cambrian protists has been most recently reviewed in detail by Knoll (1992, 1994). Simple, small to moderately sized spheromorphic acri-