New Eocene rodents from Northwestern Oaxaca, Southeastern Mexico, and their paleobiological significance

AbstractThe Yolomecatl area, northwestern Oaxaca, Sierra Madre del Sur Morphotectonic Province, Mexico, lies between 17°25′N and 17°30′N latitude, 97°29′W and 97°36′W longitude, at 2200–2500 m above sea level, and includes ∼90 km2 of rugged territory, where lithostratigraphic units of Late Jurassic to Quaternary age crop out. The Tertiary succession, which unconformably overlies Mesozoic formations, includes the Nduayaco ‘Group,’ a stack of intermediate lava flows, and the Yolomecatl Formation, a lacustrine/fluvial sequence ∼650 m thick, sparsely interbedded by tuff sheets. One tuff with an 40Ar/39Ar date of 40.3 ± 1.0 Ma places this formation and the southernmost Eocene local fauna from North America in the late middle Eocene (Bartonian). The volcanic Nicananduta ‘Group’ (∼27 Ma) overlies the Yolomecatl Formation. The Yolomecatl local fauna includes, among others, two new taxa: Douglassciurus oaxacaensis, sp. nov., so far the oldest sciurid in the world, and the enigmatic Protozetamys mixtecus, gen. et s...