Monotreme
From EvoWiki
| Monotremata |
|---|
Monotremes are regarded as the most primitive mammals. There are three species of monotremes, the duck-billed platypus and two spiny anteaters, or echidnas (Tachyglossus and Zaglossus). Today, monotremes live only in Australia and New Guinea. The name monotreme means "one-holed," referring to the cloaca, a single hole that serves the urinary tract, anus, and reproductive tract in monotremes.
| See Monotreme in Wikipedia, the free encyclopedia. |
Monotremes are considered very primitive for mammals because, like reptiles and birds, they lay eggs rather than having live birth. Yet also, monotremes seem rather derived, having highly modified snouts or beaks, and modern adult monotremes have no teeth. But like mammals, monotremes have a single bone in their lower jaw, three inner ear bones, high metabolic rates, and hair.
Richard Dawkins provides the following interesting comment on one of these, the duck-billed platypus in The Ancestor's Tale, p.235:
- An early Latin name of the platypus was Ornithorhynchus paradoxus. It seemed so weird when first discovered that a specimen sent to a museum was thought to be a hoax: bits of mammal and bits of bird stitched together. Others have wondered whether God was having a bad day when he created the platypus. Finding some spare parts left over on the workshop floor, he decided to unite rather than waste them.
Even though there are only three existing genera of monotremes today, they attract interest since they seem to be a fusion of mammalian and reptilian features, one of those transitional events in evolution. The ancestors of monotremes split off from the main mammalian line (therians, which include both placental and marsupial mammals) approximately 180 million years ago in the Lower Jurassic. Their geographical isolation provided them the opportunity to continue their line until the present. While monotremes lay eggs they also nurse their young with milk (like mammals, in fact the origin of the name Mammalia), but interestingly never evolved a nipple for delivering the milk and the milk oozes through pores in the mother's skin.
Reference Links
- [1] Introduction to Monotremes
- Monotreme in Wikipedia
- [2] Animal Adaptations - Monotremes
