We recovered Pleistocene fossils from a lava-dammed river deposit along the Río de Moctezuma in northeastern Sonora, at 29°41′N, 109°39′W, and 605 m elevation. Today the region is semiarid, with a foothills thornscrub community. The impoundment that resulted from the lava dam produced a short-lived marsh with an adjacent savanna. The extraordinary fauna is both diverse and rich, and includes ostracods, mollusks, fish, amphibians, turtles, a crocodilian, snakes, lizards, birds, and mammals, many with tropical affinities today. Most of the animals are either extralimital to the setting today or extinct. The recovery of Bison dictates a Rancholabrean Land Mammal Age for the fauna; a preliminary 40Ar/39Ar age suggests that the deposit is between 570,000 and 310,000 years old. The occurrence of cf. Crocodylus acutus (a crocodilian; generic assignment uncertain) and Pampatherium, the giant armadillo, is unique in the northern interior Sonora setting. We speculate that a well-developed riparian corridor along the Río Yaqui, from the Gulf of California to the mountain-valley setting at Térapa, permitted the animals with tropical affinities to extend 350 km inland.