What are they?

In today's world, there live only two monotremes ( those that lay eggs ). They are the duck billed platypus and two spiny ant eaters ( echidna ). All of these are found only in Australia and New Guinea, and for good reasons.

Description

There has not been many fossils found till recent. These are the primitive of the mammals. The reasons that support this are as follows: They lay eggs, like the reptiles and birds. They have beaks or snouts - not a jaw. But with these reasons, why are they under mammals?

These 'in-between' animals have:

1. three bones in the ear,
2. hair,
3. high metabolic rate and
4. produce milk to nourish the young.

The way the young are born varies. Though they both lay eggs, the egg is in a pouch till the young hatches in echidnas. In platypus, the egg is kept in a burrow close to a water body. The long nosed echidnas eat worms while the short nosed ones feed on ants and termites.

They have a tongue that extents from the mouth. A platypus finds it's food under the water using it's snout to hunt ( the snout is sensitive ). The diet includes bivalves, crabs and their group ( crustaceans ) and many other stuff. They are kept in a pouch ( cheek pouches ) till it comes to the surface and can eat it peacefully.

Both the varieties have no teeth. The young platypus has teeth, but they are lost as it matures. But to make up for this, the adult has some hard spurs to chew. The platypus is venomous. Though not harmful to humans, they can be harmful to other animals. There are spurs on the heels. They help in the combat to exclusive rights to a female. Echidnas too have spurs - both do in the young, but at maturity, the females loose them. The montoromes that are alive today ( the very few alive today ) are grouped into two kinds:

1. The Ornithorhynchidae - the platypus family and
2. Tachyglossidae - echidnas

The family tree follows:

1. KOLLIKONTIDAE had one species - Kollikondon ritchiei which is extinct.
2. STEROPODONTIDAE had one species - Steropodon glamani which is extinct.
3. ORNITHORYNCHIDAE ( the familiar one ) had four species : Monotrematum sudamericanum - extinct
4. Obdurodon dicksoni - extinct
5. Obdurodon insignis - extinct Ornithorynchus anatinus (modern platypus) - alive

TACHYGLOSSIDAE had five species -

1. Megalibgwilia ramsayi extinct
2. Megalibgwilia robusta extinct
3. Tachyglossus aculeata (short-nosed echidna) - it's alive
4. Zaglossus bruijnii (long-nosed echidna) - it's alive
5. Zaglossus hacketti - it's extinct.

From where did they evolve and when?

They were the survivors of the original mammals - those that laid eggs.