The logical assumption is that monotremes are an old, specialized offshoot of an early mammal stock that has survived only by being isolated in the Australian area. Platypus have short limbs and a body length between 39-60 cm. Adults weigh between 2.5 and 8 kg. Water proof fur helps insulate against cold temperatures. Platypus use a large, flat, wide bill to capture benthic invertebrates such as crustacean and insects for food.
Male platypus are able to produce and deliver venom through a horny spur on the back of the ankle.
The venom is produced in a gland behind the knee on the upper thigh. Echidnas have the spur structures but venom is not produced. Echidnas have a long cylindrical snout used for scavenging forest litter. A good sense of smell is used to find ants, termites, worms and other insects. Long-nose echidnas compose most of their diet of earthworms.