A PARADOX OF PLAYTPUSES. Ornithorhynchus anatinus. Duck-billed platypus belong to a sub-group of mammals that lay eggs rather than giving birth to live young. When the first platypus was shipped to Britain from Australia, people thought it was a joke and that someone had sewn a duck's bill to a mammal's body. They are the only venomous mammal, with small poisonous barbs by their feet.