 Lundomys molitor , also known as Lund 's amphibious rat or the greater marsh rat , is a semiaquatic rat species from southeastern South America . Its distribution is now restricted to Uruguay and nearby Rio Grande do Sul , Brazil , but it previously ranged northward into Minas Gerais , Brazil , and southward into eastern Argentina . The Argentine form may have been a separate species, Lundomys argentinus .
The body is small, with a body length and a tail length . The Argentine form may have a larger body than the Brazilian form, but is a smaller form with a smaller tail length and a smaller weight . The body is usually solitary, but