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Ischyromyidae (order Rodentia, suborder Sciuromorpha) An extinct family comprising the most ancient ancestors of rodents, which lived from the late Palaeocene to the Middle Eocene, probably as scampering, squirrel-like animals. The skull and teeth retain many primitive features. There are more teeth than in later rodents, but the front upper premolar is reduced and lost in some species. The jaw lacks modifications found in later rodents.