Proliferation of tiering during the Ordovician: a casual history

By the beginning of the Silurian, maximum possible tiering in shallow subtidal soft substrata suspension-feeding communities was up to 100 cm for epifauna and as deep as 12 cm for infauna. These limits, with tier subdivisions, were characteristic for the remainder of the Paleozoic, except for the late Paleozoic increase in infaunal depths. Causes for increased tiering heights or depths included: for epifauna -- feeding efficiency as related to active versus passive suspension feeders and their position within the benthic boundary layer, aspects of constructional morphology, and interspecific competition; and for infauna -- size increase of individuals, and space competition. Organisms that contributed the most to higher epifaunal tiers were passive or weak active/passive suspension feeders. These organisms relied on ambient currents and were best suited for higher tiers in zones of higher velocity currents in the upper part of the benthic boundary layer. Higher tierers were either colonial organisms or stalked echinoderms that could constructionally have easily increased their heights. These morphologies must have been metabolically cost effective and are considered to also have had adaptive value for interspecific competition. Individual size increase among the infauna increased infaunal depths. This also was advantageous for space competition.