Polymorphis is an extinct genus of litopterns belonging to the family Macraucheniidae. It lived during the Middle Eocene of Argentina.
The type species of Polymorphis is Polymorphis lechei. Both the genus and the species were named by Santiago Roth in 1899. The lectotype of P. lechei is the left half of a mandible with teeth.[1]
Two other species, Megacrodon planus and "Lambdaconus" alius, have been previously assigned to Polymorphis,[2] but are poorly known and may not belong to the genus.[3][4]
Megacrodon, Polyacrodon (which has the junior objective synonym Oroacrodon), Decaconus, and Periacrodon have been considered synonyms of Polymorphis.[5]
Polymorphis is the most basal member of Macraucheniidae.[6][7] It is considered to be either part of the paraphyletic subfamily Cramaucheniinae,[4][8] or in a monotypic subfamily of its own, Polymorphinae.[9][5]
Polymorphis is known from the Eocene of Argentina, belonging to the Mustersan South American land mammal age, and is the earliest known macraucheniid.[10]