make ooids a particularly valuable paleoenvironmental proxy and a stable frame of reference for comparing carbonate platforms throughout Earth's history. Carbonate platforms contain many subenvironments such as oolitic shoals, beaches, lagoons, and tidal channels. Each of these settings has unique chemical and physical conditions, which produce ooids with environment-dependent sizes and shapes (Figures 1a and 1b; Mariotti et al., 2018;Trower et al., 2018). Consequently, the morphology of ooids can be used to improve paleoenvironmental reconstructions by refining facies models of ancient carbonate platforms and providing insights into paleohydraulic and geochemical conditions.While some features of ooid growth remain enigmatic, there is general agreement that ooids grow from the precipitation of calcium carbonate onto a nucleation surface (whether biologically mediated or not), such as a carbonate grain or an existing ooid (Bathurst, 1975). The ooid continues to grow until it is either buried, too big to be transported (Sumner & Grotzinger, 1993), or precipitation and abrasion rates reach a dynamic equilibrium (Figure 1b; Trower et al., 2017). These models provide useful heuristics for understanding the relationship between environmental conditions and ooid size by indicating that increased precipitation rates and more frequent ooid transport lead to larger ooids.Like size, the shape of the ooid reflects the conditions under which the ooid formed. The shape of an ooid is the result of surface-normal growth from the precipitation of calcium carbonate onto the ooid combined with three types of abrasion: (1) collisional abrasion, which primarily occurs during saltation and produces more spherical shapes, (2) frictional abrasion, which occurs when ooids are rolling and sliding on the