Claire Johnson

Mentor: Katherine Marenco

Re-evaluating the Early Paleozoic “metazoan reef gap”:

Did animals construct reef frameworks in the Early Ordovician?

This research concerns the development of reefs during the Early Ordovician (Ibexian) (~485-470 Ma), an interval of time that coincided with both the most significant biodiversification event in the history of life (the Great Ordovician Biodiversification Event) and the most prolonged “metazoan reef gap” in the Phanerozoic, an ~40 million year interval during which microbes, rather than multicellular animals, were the primary builders of reefs. Little research focused on this metazoan reef gap has been conducted, although recent work has indicated that some of the reef structures built during this time may have been more ecologically complex than previously thought (e.g., Adachi et al., 2011, 2013; Choh et al., 2013). For example, lithistid sponges and other metazoans are often found in reef mounds from the metazoan reef gap interval, but the role, if any, that these animals played in reef construction has not been investigated in detail.

In western Utah, reef mounds built during this metazoan reef gap are present in the Lower Ordovician Fillmore Formation, which is composed of several carbonate-dominated units, one of which contains distinctive sponge-microbial mounds that are one meter tall and up to 2.5 meters wide. Each of these mounds is composed of a stromatolitic (microbial) core surrounded by material that contains lithistid sponges in varying densities. I have chosen to examine one particularly well-preserved mound in which the relationship between microbial and metazoan constituents can be seen clearly. By integrating field data with thin section petrography of the samples that I collected, I will test the hypothesis that sponges constructed a rigid framework, consisting of mineralized skeletal material, that helped support the internal structure of this mound. Features that would support this hypothesis include intact preservation of sponges in an upright growth position and successive generations of sponges growing cemented to one another. If the studied mound displays such evidence of a rigid sponge framework, then this portion of the Early Ordovician likely represents an interruption in the longer metazoan reef gap during which animals again became actively involved in building reefs.