Why did ancient chemosynthetic seep and vent assemblages occur in shallower water than they do today?

Cold-seep communities have relatively low diversity, are dominated by one o r two taxa present in high density and high biomass in comparison with the surrounding fauna, and are restricted to aphotic habitats. Their associated heterotrophic fauna are usually distinctive from the fauna of their surrou ndings. In contrast, a more commonplace chemoautotrophically based communit y occurs in shallow photic habitats. The associated heterotrophic fauna inc ludes many of the species typical of the surrounding communities and typica lly dominates abundance, whereas the species with chemoautotrophic symbiont s dominate biomass. All modern seep assemblages are deeper than 550 m. Many fossil seep assemblages occurred in water as shallow as the mid-shelf (<20 0 m). In contrast, communities where species with chemoautotrophic symbiont s are biomass dominants, but not numerical dominants, are common in shallow waters at present but rarely reported in the geological record. We suggest that the absence of cold-seep communities on the continental shelf present ly is due to a combination of predation and competitive exclusion by primar y consumers limiting the presence of species dependent on chemoautotrophic symbionts. We suggest that cold-seep assemblages are more common at shelf d epths in the fossil record for two reasons: (a) The biases of preservation have accentuated their distribution by transforming communities where speci es with chemoautotrophic symbionts dominate by biomass, but not numerically , into cold-seep-appearing assemblages. (b) The importance of predation pre ssure and oligotrophy has varied, with decreased predation pressure accompa nying increased oligotrophy favoring cold-seep communities. We suggest that the paucity of shallow-water assemblages with species harboring chemoautot rophic symbionts as biomass dominants in the fossil record is based on the reliance of paleoecological analysis on numerical abundance data when energ y flow analyses are required to identify these assemblages. The distinctive ness of the fossil seep assemblage is intensified by taphonomic processes t hat bias the assemblage against small individuals and epifaunal species, so that diversity declines, the small heterotrophic component of the assembla ge is significantly reduced, and the epifaunal component is minimized. The final assemblage is usually dominated by the better-preserved large infauna l clams which perchance are also the species with chemoautotrophic symbiont s. In contrast, preservation does not enhance the distinctiveness of these chemoautotroph-harboring species in shallow water.