Chronoclinal variation and sexual dimorphism in Mammut americanum (American mastodon) from the Pleistocene of Florida

Abstract

This study quantitatively and statistically analyzes chronoclinal variation in Mammut americanum fossils over a broad temporal range (~1. 5 Ma to 10,000 year BP). A large sample of elements (third molars, mandibles, limb bones) from the Irvingtonian and Rancholabrean of Florida were measured and statistically compared for significant differences. Rancholabrean elements are generally found to be significantly larger than Irvingtonian elements. Florida M. americanum shows directional selection for increased body mass from the Irvingtonian to the Rancholabrean. Anterior mandibular height is the only measurement that demonstrates significant sexual dimorphism in Rancholabrean individuals, with males having a larger anterior mandibular height than females. Mandibular tusk presence/absence ratios. in Florida M. americanum show directional selection against these small tusks from the Irvingtonian to the Rancholabrean. M. americanum demonstrates a heterochronic increase in growth rate at the onset of puberty from the Irvingtonian to the Rancholabrean. In addition, Rancholabrean M. americanum populations also exhibit holobiotic heteromorphism, with males growing at a faster rate than females. The results of this study demonstrate that throughout its ~1.5 Ma range in Florida, M. americanum was not static in its evolution.