SCLEROBIONT COMMUNITY DEVELOPMENT DURING THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT: PATCHY COLONIZATION OF EXTENSIVE HARD SUBSTRATES IN THE POGONIP GROUP, WESTERN UTAH

Hard substrates were widely available in shallow marine environments by the onset of the Great Ordovician Biodiversification Event (GOBE). Sclerobionts, organisms adapted to living attached to hard substrates, radiated dramatically and their communities flourished during the GOBE. In order to address the probable link between increasing hard substrate availability and the Ordovician radiation of sclerobionts, preserved instances of hard-substrate attachment were documented and sampled stratigraphically in the Fillmore, Wah Wah, Juab, Kanosh, and Lehman formations of the Pogonip Group (Ibexian-Whiterockian) near Ibex, western Utah.

Hard substrates are both common and laterally extensive in the studied portion of the Pogonip Group, but diverse and ecologically-complex level-bottom communities of sclerobionts are preserved only within the Kanosh Formation. Kanosh hardground communities included encrusting bryozoans and stalked echinoderms occupying both exposed and cryptic niches (Wilson et al., 1992). In the other studied Pogonip Group units, in situ evidence of level-bottom hard-substrate colonization is limited to isolated echinoderm holdfasts, uncommon small sponges and receptaculitids, and rare articulated stalked echinoderms. Organic buildups (stromatolite reefs and mounds, carbonate mud mounds with few sponges, and sponge-microbial mounds) also occur in several intervals within the Fillmore, Wah Wah, and Juab formations but are absent in the Kanosh and Lehman formations. The availability of hard substrates undoubtedly facilitated the development of these organic buildups, and the reef structures themselves served as attachment surfaces for other sclerobionts.

The patchy development of sclerobiont communities and reefs in the Pogonip Group, despite the abundance of hard substrates, likely reflects a combination of environmental and preservational factors. The intra-platform basin setting of the Kanosh Formation, which likely experienced periodic lulls in storm activity and sediment accumulation, may have been particularly conducive to both early seafloor cementation and hard-substrate encrustation. In contrast, frequent storm events during Fillmore deposition may have precluded the establishment and preservation of most sclerobiont communities.