Evolutionary trends of body size and hypsodonty in notoungulates and their probable drivers

Members of the Order Notoungulata are among the most diverse and common mammals in South America during the Cenozoic. Several lineages within notoungulates (e.g., suborders Typotheria and Toxodontia) show a tendency for increased body sizes and hypsodonty during the last 50 Myr. However, the timing, evolutionary mode, and drivers of such tendencies are not entirely understood. In this paper, we use an extensive database of notoungulate fossil occurrences and body mass and hypsodonty estimates to characterize the evolutionary mode of these two phenotypic traits over time, test the extent to which several factors (e.g., development of open environments in the south of South America) have influenced it through time, and investigate whether large trait values were selected through elevated origination or reduced extinction rates. Our results demonstrate that most of the major notoungulate clades evolved toward larger body sizes (up 1500 kg) and higher tooth crown, from a small and low-crowned tooth ancestor, in a punctuated mode. We also show that body mass and the hypsodonty in typotherians and toxodonts had a coupled evolutionary history. Species sorting was a relevant macroevolutionary process in some notoungulate clades, as taxa with high teeth crown and body mass had lower extinction rates. Finally, the development of the hypsodonty in notoungulates must reflect repeated and quick instances of adaptive responses to the increased availability of volcanic or other terrigenous particles, within the broad context of the SSA Cenozoic Andean mountain building.