PARALLEL ADAPTIVE RADIATIONS IN FOUR LINEAGES OF APLODONTOID RODENTS: A CASE STUDY IN THE EVOLUTION OF HYPSODONTY

The evolution of high-crowned teeth, or hypsodonty, is well studied in a variety of large mammals. It is generally attributed to consumption of grasses, increased grit on food due to the spread of open habitats, or both. Similar trends toward increasing tooth crown height are present in several small mammal lineages; however, they have been left out of most discussions of the evolution of hypsodonty. There are good reasons for this exclusion; differences between large and small mammals in diet, life span, and other aspects of life history mean the causes of increasing tooth crown height may be different for small mammals. Nevertheless, the nearly simultaneous increase in hypsodonty in several lineages of large and small mammals suggests some commonality of causation.

This study focused on several lineages of small mammals in the clade Aplodontoidea (including rodents from the families Aplodontidae and Mylagaulidae), to determine whether the trends toward increasing hypsodonty in four lineages within this clade show a common evolutionary pattern. The four lineages included in this study are the subfamilies Meniscomyinae, Allomyinae, and Aplodontinae, and the family Mylagaulidae. These four lineages are of roughly the same rank within the larger clade, and are monophyletic with respect to one another, diverging from a brachydont common ancestor. Divergence time, time of maximum diversification, degree of development of hypsodonty, lineage duration, and morphological diversity were compared among the four groups. It appears that all these clades diverged in the late Oligocene, and all increased in hypsodonty at about the same time. However, there are substantial differences between lineages in time of peak diversification, degree of hypsodonty, lineage duration, and morphological diversity. It appears that the dissimilarity between clades in diversification patterns may be due to autecological differences between members of the various clades. Despite these ecological differences, for these small mammals, as in previously studied large herbivores, there appears to be simultaneous, parallel trends toward increasing tooth crown height. Such consistency in evolutionary trends between independent lineages suggests a common causation.