Order : Monotremata
The order of monotremes (Monotremata) consists of all mammals (5 species in total) that lay eggs instead of giving birth to live young. Unlike the difference in their reproductive system, they share some similarities with other mammals: they are warm-blooded, have a relatively high metabolism, produce milk for their young, have a single bone in their lower jaw and have 3 middle ear bones (source). Monotremes have a lower body temperature than other mammals, the short-beaked echidna can 'turn off' its body regulating mechanism to keep its energy during colder weather. Today, all living members of Monotremata live in Australia and New Guinea.
Interesting fact: The name 'Monotreme' means 'single opening', because members of the order of Monotremata all just have one opening when other female mammals have three (vagina, the urethra, and the anus).
At this day and age, myths have stood up, and still a lot of people think that these creatures are inferior, they believe that these are the ancestors of the "superior" mammals. Modern studies have almost proved that the modern monotremes are survivor of a long-ago division of the mammal world.
Going back to the eggs, these are maintained for some time, and Monotremes lactate, but nipples are absent. The milk comes out from the mammary glands through holes in the skin, and not only that, these Monotremes have long lives, with low levels of reproduction.
Monotremes' jaws are built differently from the other kinds of mammals, and its jaw muscles are different too. The small bones are totally merged to the skull, and not lying from the jaw as in pre-mammalian synapsids. The outside opening of its ear lies at the jaw's base. They have additional bones at the shoulder area, which other mammal forms do not. They have a reptile-like movement, legs are at the side of the body and not under it. A Monotreme's leg has a spur near the ankle, and this allegedly holds a strong venom, we are talking of the male platypus here.
Metabolic level is significantly low, but whether this is a genuine trait or just a form of survival on their part, could only be speculated. About the remaining forms of Monotremata, they are all found in New Guinea and Australia. But there are proofs existing that in a bygone era, their numbers were abundant. The evidence from fossil findings showed that their line "went out" from the mammalian world approximately 150 million years back. The fossils (jaw) 110 million years old had been discovered somewhere in NSW. You know what the fragments are: from the Steropodon galmani, the oldest recognized fossils of Monotremata.
In '91, a fossil (tooth) of a platypus had been found in Argentina. It was at least 61 million years.
Picture of the platypus by Stefan Kraft, licensed under GFDL
Animals in the order Monotremata
|Long-beaked echidna or New Guinean echidna|