The fossil record is the only direct source of data for studying modes (patterns) and rates of morphological change over long periods of time. Determining modes and rates is important for understanding macroevolutionary processes, but just how modes and rates vary within a taxon, and why, remain largely unaddressed. We examined patterns of morphological change in the shell of the Mesozoic marine bivalve genus Buchia over its geographic and temporal range. Most modes conformed to either random walks or stasis, and both modes and rates showed variability between locations. For example, stasis was more common in deeper marine environments, whereas random walks occurred more often at the highest paleolatitudes studied. These results indicate that the environment can play an important role in shaping patterns of evolution.