Repeated climate-driven dispersal and speciation in peripheral populations of Pleistocene mastodons

Recent ancient DNA work has shed some light on the responses of mastodons to Pleistocene glacial/interglacial cycling but focused primarily on their expansion into Beringia. However, genetics has complicated our understanding of the relationships within Mammut , specifically between Pacific and American mastodon phylogeography and questioned whether these are in fact two separate species or regionally localized morphotypes. Here we expand on both avenues by sequencing and contextualizing the mitochondrial genome of a Pacific mastodon, as well as from several North American eastern specimens throughout the last 800 thousand years. We show that Pacific mastodons fall within a previously established, and deeply divergent mitochondrial clade, extending the range of this species into western Canada and potentially Mexico. We also present evidence for at least three discrete expansion events into northeastern coastal regions (i.e. Nova Scotia and the eastern continental shelf), and identify two new mastodon clades, which contain temporally distinct, but geographically co-occurrent specimens. This work sheds further light on mastodon taxonomy and phylogeography across North America throughout the Pleistocene, highlighting interglacial range expansion into northeastern America mirroring the effects on the western side of the continent (Beringia).