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Ornithorhynchidae (platypus; subclass Prototheria, order Monotremata) A monospecific family (Ornithorhynchus anatinus) of nocturnal monotremes that are highly adapted for aquatic life. They are now known to be related more closely to the marsupials than to the echidnas. The digits are webbed, the webs of the fore limbs being folded when the animal is not swimming; those of the hind limbs bear strong claws. The snout is covered in thick skin and is shaped like the bill of a duck. A spur on the inside of the hind leg carries venom. The animal feeds on aquatic invertebrates. Breeding takes place in spring, with one to three eggs being laid and incubated in a nest built in a burrow beside water. The animals are distributed throughout southern and eastern Australia and Tasmania. Fossil genera are Obdurodon, from the early Miocene of Queensland, and Monotrematum, from the Palaeocene of Patagonia.