Was southern Central America an archipelago or a peninsula in the middle Miocene? A test using land-mammal body size

There has been considerable discussion about the complex geological history of southern Central America in the late Cenozoic prior to the closing of the Isthmus of Panama during the Pliocene. It is particularly unclear how far nuclear Central America extended southward as a continuous land connection with North America versus the alternate paleogeographic reconstruction of an extensive island-arc system during this time. In modern faunas, terrestrial mammal body sizes have fundamentally different patterns within regions of continuous land-mass versus island populations as a result of the “island rule.” Variation in body size is relatively limited in species with continent-wide distributions, whereas, relative to mainland populations, those on islands can have as much as a four-fold increase, or decrease, in body size. Using tooth molar dimensions as a proxy for body mass, tooth size is compared from a middle Miocene land-mammal fauna from Panama (Gaillard Cut Local Fauna) relative to contemporaneous faunas in North America. Among six species, there is no significant difference in tooth size between Panama and North America. A second test comparing the artiodactyl Paratoceras wardi between Panama and Texas shows a significant difference in tooth size, with P. wardi from Panama having slightly larger teeth (104% to 112% larger) than P. wardi from Texas. This difference in size, however, is small compared to that predicted from insular evolution and is more consistent with the amount of variation seen in continent-wide species distributions. Results from both tests are consistent with the hypothesis that southern Central America had a dry-land connection to North America during the middle Miocene. Based on these data, there is no support for the alternate hypothesis of an extensive archipelago in southern Central America during this time.