Monotremes were the transition between the reptile-ruled world of the Mesozoic to the mammal-ruled world of the Cenozoic. You can get more information about the differences between mammals and reptiles here. These strange creatures were almost exactly like other mammals-except that they laid eggs.

The platypus was an egg-laying mammal (and still is an egg-laying mammal) that came about in the late Cretaceous. The modern platypus was discovered in Australia in 1798. When sent back to England, a taxidermist (who preserves animals) was accused of attaching a duck's beak to a mammal!

Females lay their 2 eggs in their burrow located at the end of a river. "Premature" (underdeveloped) babies hatch after about 10 days after she lays them. They develop inside the burrow for approximately 4 months, and then they visit the rest of the river. Their curious bills are super-sensitive for discerning food from other things on the river bottom. They are unable to use their eyes very well because they are nocturnal. An amazing thing about the platypus is that the males of the species are one of only a few poisonous mammals. They have hooks behind their ankles where they may inject poison into an unsuspecting and unfortunate victim. This venom has been known to kill dogs!

Only 3 species of monotremes live today, and they only reside in Australia and New Guinea. Echidnas, along with platypuses, are the only remaining of the first types of mammals.