Population structure of a pioneer and a later stage species in an Ordovician ecological succession

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Abstract
Marine shelf lagoon deposits of Middle Ordovician age in Tennessee contain the record of repeated ecological successions which began on soft, azoic, terrigenous muds and progressed to a complex bryozoan-brachiopod community. The stages of succession formed a skeletal limestone bed. The process led to an interbedded shale-limestone stratigraphic sequence. The pioneer was the brachiopod Strophomena basilica which paved the soft mud with accumulated shells. Encrusting bryozoans colonized this shell pavement and were followed by ramose bryozoans. The final stages of succession included bryozoans, gastropods, pelecypods, and pelmatozoans but were dominated by the brachiopod Rostricellula rostrata.Measurements of total length, width, length to growth lines (Strophomena only), and thickness (Rostricellula only) were used to generate growth-rate, and survivorship curves for these two species. Strophomena shows low juvenile mortality, a moderate, linear growth-rate, and increased mortality in later ages. Rostricellula shows relatively higher juvenile mortality, a rapid non-linear growth rate, and nearly constant mortality. These attributes are contrary to expectations derived from theory, but can be understood in the light of autecological factors for each species. Adaptation to soft substrata was the primary factor controlling population dynamics in Strophomena, and crowding was the main factor in Rostricellula.