A MIDDLE ORDOVICIAN SPONGE-ALGAL MOUND FROM WESTERN UTAH: IMPLICATIONS FOR THE RADIATION OF HARD-SUBSTRATE-ATTACHING ORGANISMS

The Ordovician radiation was characterized by rapid diversification of the major marine invertebrate phyla and the rise to dominance of the Paleozoic Evolutionary Fauna. Like the Cambrian radiation, the Ordovician radiation was facilitated by changes in the substrate. Hard substrates became widely available in shallow marine settings during the Early-Middle Ordovician due to a combination of physical and biotic factors, such as a greenhouse climate and an abundance of bioclastic material. Earlier studies have suggested, but not explored, a link between the Ordovician proliferation of hard substrates and the diversification of groups dominated by hard-substrate-attaching forms. As part of an ongoing investigation into connections between substrate change and metazoan radiation during the Ordovician, field studies were conducted at the well-exposed Middle Ordovician (Whiterockian) sections at Fossil Mountain near Ibex, western Utah. Hardgrounds, shell beds, and intraclastic conglomerates are commonly-preserved hard substrates in these sections and have been well-studied by previous workers. Sponge-algal mounds, varying in size from less than a meter to several meters in diameter, are common in the upper portion of the Juab Limestone but have not been observed in the overlying Kanosh Formation (Rigby, 1965; Ross et al., 1989). The mounds in the Juab Limestone have not been thoroughly described (Johns, 1994). Most Middle Ordovician sponge-algal mounds likely formed on stabilized substrates (Li et al., 1993) and in turn served as hard substrates to which other organisms, such as echinoderms, could attach (Johns, 1995). One such mound, 1m in height and >1m in diameter, was observed near the contact between the Juab Limestone and the Kanosh Formation at Fossil Mountain. Primary constituents of the mound include lithistid sponges, algal textures of uncertain affinity, and massive skeletal-rich wackestone. Contacts between the mound and the surrounding strata are sharp, and the underlying beds are undeformed. Two isolated dome-shaped sponges, ranging in diameter from 15-31cm at their base, were found within the mound-bearing interval and in the lower portion of the Kanosh Formation. These individual sponges, like the larger sponge-algal mound, may have served as hard substrates for metazoan attachment.