Coral reefs are declining rapidly due to rising seawater temperatures and ocean acidification. These economically and ecologically important hard reef structures will soon be replaced by softer assemblages of microbes, algae, and sponges. The fossil record of reefs can provide critical insights that allow for predictions about how reef environments transition away from microbial-algal-sponge structures to robust reefs built by skeletonized animals like corals. This project will investigate such a transition during the Ordovician Period by quantifying trends in reef-builders through time. The ecologic impact of the fossil reefs will be compared to that of modern coral reefs to help make predictions about how modern reef environments will function without robustly-skeletonized reef-builders. The products of this work will greatly advance the career of two Latinx geoscientists who teach at predominantly undergraduate institutions. This work will expose ninety first-generation, low-income (FGLI) students to the study of reefs and will involve six to twelve FGLI students in mentored research experiences.<br/><br/>This project will quantify framework builders in reefs of the Early-Middle Ordovician succession near Ibex, Utah using a combination of outcrop, petrographic, and geochemical techniques. A specific aim of this work is to develop geochemical techniques for uncovering the presence of cryptic microbialite fabrics, particularly in mud-mounds. Another aim is to detail how the combined reef mounds within a given interval act as a larger reef unit with respect to impacts on the depositional system. This work will include summer field and laboratory research conducted by FGLI undergraduates participating in two programs at Bryn Mawr College: the STEM Posse and STEMLA Fellows programs. Each summer, research mentees will work with the PI to present short courses on the study of fossil reefs to incoming cohorts of the two programs. The project will involve a field research trip to the Florida Keys to compare Ordovician fossil reef data to the Florida coral reef system.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.