Five living mammals that still lay eggs
Monotremes are the only surviving egg-laying mammals: one platypus and four echidnas. All living species are native to Australia and New Guinea, though Late Cretaceous and Paleocene fossils from southern South America imply they once ranged via Antarctica. Greek monós + trêma names the shared cloaca.
Brain and skull details set them apart from therian mammals. Some authors fold them into Australosphenida with Jurassic–Cretaceous fossils from Madagascar, South America, and Australia; others dispute that clade, so taxonomy stays contested.
They share core mammal traits: warm blood, body hair, milk from mammary glands, a single lower-jaw bone and three middle-ear bones. Like marsupials they lack a corpus callosum linking the brain's hemispheres, relying instead on the anterior commissure. Adults have no teeth, though fossil forms and young platypuses carry three-cusped molars. Extra shoulder bones, an interclavicle and coracoid, and a sprawling, reptile-like gait with legs set at the sides set them further apart.
The platypus genome revealed five pairs of sex chromosomes that act together as a single XY system. Eggs stay inside the mother for a while, drawing nutrients from her, and hatch within about ten days of laying; the tiny, undeveloped newborns are called puggles. Milk seeps from skin openings rather than teats and contains a potent antibacterial protein found in no other mammal. Their embryos divide only partially, as in birds and reptiles, and they have lost the gastric glands most mammals have in their stomachs.
The platypus runs at about 31 °C, against roughly 35 °C for marsupials and 37 °C for placental mammals, yet it holds that temperature steady even in icy mountain streams. Echidnas were once thought to have no REM sleep, but a study found it made up about 15 percent of sleep at 25 °C. Both groups carry ankle spurs on the hind legs: vestigial in echidnas, venomous in the platypus. The main venom component arose before the two lineages split.
Source: Monotreme
