TRACING THE SHIFT IN DOMINANCE FROM MICROBIAL TO METAZOAN FRAMEWORK-BUILDERS IN LOWER–MIDDLE ORDOVICIAN REEFS, WESTERN UTAH, USA

Following the middle Cambrian extinction of the archaeocyathids, metazoans were absent from reefs and microbial communities were the sole reef builders until the Early Ordovician. This ~40-million-year interval has been called the longest metazoan reef gap in the history of life (Sheehan, 1985; Rowland and Shapiro, 2002). Factors that may have contributed to the delayed return of metazoans to reef-building roles remain poorly understood. In addition, few studies have examined the paleoecology of microbial-metazoan reefs that were constructed immediately following the close of the protracted metazoan reef gap.

The western U.S., a tropical passive margin during the early Paleozoic, is an ideal location for studying this critical interval in the evolutionary history of reef communities. In particular, the Lower–Middle Ordovician succession in western Utah records the establishment of a broad carbonate platform on which multiple episodes of reef building occurred. Well-exposed sections of the Fillmore, Wah Wah, and Juab formations contain at least ten laterally-persistent reef-bearing intervals that showcase the transition from microbial dominance to the re-establishment of metazoans as major reef-framework constituents. Reefs in the lower portion of this succession are exclusively stromatolitic and/or crypto-microbial (mud mounds), but sponges and receptaculitids become common in reefs up-section. In the Middle Ordovician Juab Formation, the typical reef framework consists of a foundational sponge-microbial consortium that becomes sponge-dominated during later reef development. Receptaculitids (currently assigned to Problematica) dominated reef frameworks even earlier than sponges based on examples in the Lower Ordovician Fillmore Formation.

Ongoing field and laboratory investigations are geared toward understanding the paleoenvironmental and geochemical conditions that fostered each episode of reef development and interpreting the ecological factors that led to the gradual shift in dominance from microbial to metazoan reef communities. Of particular interest is the possibility that the pattern of ecological change seen in this succession might mirror the manner in which metazoan-framework reefs became re-established following mass-extinction events later in the Phanerozoic.