ROLE OF VOLCANIC ACTIVITY IN THE EVOLUTION OF EVER-GROWING CHEEK TEETH IN APLODONTID RODENTS

The evolution of hypsodonty has been well-studied in large herbivores, both within lineages and in ecological associations. The evolutionary course of hypsodonty in small mammals, however, remains relatively unknown. In large herbivores, the convergent evolution of hypsodonty is rampant in the mid-Tertiary. However, ever-growing teeth (hypselodonty) very rarely appear in large mammals. Small mammals, on the other hand, have several times evolved hypselodonty not only in the incisors, but also in the cheek teeth responsible for processing foliage. Driving factors hypothesized for the evolution of hypsodonty include grazing on abrasive plant parts and ingestion of exogenous grit. It is important in studying the evolution of hypsodonty in small mammals to consider the differences in how such drivers might affect them relative to larger herbivores. Ideally, a study aimed at understanding the drivers for the evolution of hypsodonty would consider all aspects of the ecological context both within a lineage and in the environment in which it occurred, with reference to phylogenetic history. While few small mammals have adequate phylogenetic data to enable such consideration, the Aplodontoidea offer an ideal study system, with well-studied ecology, phylogeny, and ecological context. Within this clade, hypsodonty has evolved in parallel twice, once in mylagaulids and once in aplodontines. Detailed study of the evolution of hypselodonty in aplodontines shows that the most extreme hypsodonty and eventual hypselodonty evolved in only a single part of the geographic range, in Oregon and Northwest Nevada during the Middle Miocene. Populations from even a small distance to the south, even in dry habitats, were characterized by much smaller degrees of hypsodonty at the same time. The origin of hypselodonty in this lineage, then, seems to have been a response to a relatively local phenomenon, possibly the extensive volcanic activity from the Yellowstone Hotspot and the Columbia River Basalts. Whether this is a phenomenon unique to this lineage or a response to volcanic activity common to a number of small mammals remains to be studied in other lineages.