Alphadon (meaning “first tooth”) was a genus of small, primitive mammal that was a member of the metatherians, a group of mammals that includes modern-day marsupials. Its fossils were first discovered and named by George Gaylord Simpson in 1929.
Not much is known of its appearance as it is only known from teeth. It probably grew to about 12 in (30 cm) and may have resembled a modern opossum. Judging from its teeth, it was likely an omnivore, feeding on fruits, invertebrates and possibly small vertebrates.
Alphadon lived during the end of the late Cretaceous period, alongside dinosaurs such as Tyrannosaurus and Triceratops. Its fossils have been found across North America, ranging from as far north as Alberta, Canada, to as far south as New Mexico in the United States.