REEF-BUILDING AT THE DAWN OF THE GOBE: THE RISE OF METAZOAN FRAMEWORK CONSTITUENTS IN LOWER-MIDDLE ORDOVICIAN REEFS, WESTERN UTAH, USA (Invited Presentation)

The Great Ordovician Biodiversification Event (GOBE) began during the longest metazoan reef gap in Earth history, a ~40-million-year interval following the middle Cambrian extinction of the archaeocyath sponges (Sheehan, 1985; Rowland and Shapiro, 2002). During this reef gap, microbial communities were the dominant constructors of reef fabrics, and metazoans, when present alongside the microbes, typically played minor roles in reef building. Although the diversity of metazoan reef builders was comparatively low during this interval (e.g., Kiessling, 2009), reef settings likely contributed to the initiation of the GOBE. Reefs built during the Paleozoic harbored higher genus-level origination and turnover rates across major benthic metazoan groups than post-Paleozoic reefs, and this newly-generated biodiversity likely spread to non-reef environments (Kiessling et al., 2010). Investigating the changing character of reefs built before and during the GOBE is necessary for elucidating how these settings might have influenced broader evolutionary patterns.

Reefs built during the Early–Middle Ordovician in present-day western Utah record the later portion of the post-archaeocyath-extinction reef gap. Stratigraphic sections in the Ibex area bear numerous intervals of reef mounds that are 1-2 meters in height and 1-3 meters in diameter. In addition, a more prominent and geographically-extensive set of interconnected mounds, “Hintze’s Reef” (Miller et al., 2012), occurs in the lower part of the succession. Microbial fabrics dominate mounds low in the succession, but lithistid demosponges become increasingly abundant, large, and morphologically diverse in mounds up-section. In addition, receptaculitids are the primary non-microbial reef constituents within two of the mound intervals.

The re-establishment of metazoan reef-builders in the Ibex area is broadly consistent with patterns observed in other Laurentian Lower–Middle Ordovician reef-bearing successions (Webby, 2002). However, metazoan framework reefs returned earlier and attained higher diversity earlier in South China (e.g., Zhu et al., 1995; Liu et al., 1997; Adachi et al., 2011; Li et al., 2016). The underlying causes of these geographic variations in reef development are likely relevant to other aspects of the GOBE as well.