Echidnas, however, dwell solely on land. The investigators used genetics to come up with an answer. They found that echidnas diverged from platypuses only 19 million to 48 million years ago, meaning that echidnas recently had semiaquatic ancestors and only later recolonized the land. A number of features of echidnas indicate that they may have once had an amphibious platypuslike forerunner—streamlined bodies, rearward-projecting hind limbs that could serve as rudders, and the contours of a ducklike bill during embryonic development.
A previous study of early monotreme fossils had suggested the platypus and the echidna diverged more than million years ago, far longer than the genetic analysis indicates. So far scientists have found no fossil evidence of an echidna transition from water; the fossil record of monotremes remains quite incomplete, Beck says. But a number of fossil sites in Australia are 20 million to 25 million years old, about when the researchers think echidnas evolved.
The presence in monotremes of egg laying and other primitive traits from distant ancestors, such as reptilelike shoulders, is often offered as reasons for their apparent inferiority, Phillips says. These new findings help to recast these archaic features in a positive light—for instance, whereas the reptilelike shoulders are poor for running fast, they provide strong bracing, allowing for huge shoulder and arm musculature to help echidnas dig into the dirt and platypuses maneuver in the water.
Note: This article was originally printed with the title, "Extreme Monotremes. The eggs are then laid in the burrow, and the female curls around them to provide warmth. After 10 more days, they hatch into lima bean-sized, helpless infants. Mother platypuses do not have nipples, but instead their milk comes from pores in their skin and collects in grooves on their stomach.
Babies lap the milk rather than suck. After about four months, the babies are ready to come out of the burrow and learn to swim. Platypus venom is strong enough to kill a small animal like a dog. Although not potent enough to kill a human, it causes extreme pain, which can last for days or even months. The venom is thought to be used during mating season, when males compete for females. Now only two kinds of monotremes are left on the planet — the duck-billed platypuses and the four species of echidnas, or spiny anteaters.
Like all mammals, they possess hair, milk, sweat glands, three middle ear bones and a brain region known as the neocortex. The monotremes were almost totally swept aside when their pouch-bearing marsupial cousins — modern examples of which include the kangaroos — invaded Australia 71 million to 54 million years ago. Marsupials appear to have a number of advantages over monotremes — their bodies seem more efficient at locomotion, and the fact that they give birth to live offspring could provide better care of young.
Moreover, before the marsupials reached Australia, they had migrated from Asia to the Americas to Antarctica. The struggle marsupials presumably had with all the animals on these continents during this journey might have primed them for competition, "while the Australian mammals [including monotremes] that went extinct upon the arrival of marsupials had for the most part been isolated in Australia for a very long time," explained researcher Matthew Phillips, an evolutionary biologist at the Australian National University in Canberra.
All these strengths help explain why marsupials triumphed in Australia. The mystery then is why any monotremes survived. Now Phillips and his colleagues suggest that platypuses and echidnas lived on because their ancestors sought refuge where marsupials could not follow — the water. Platypuses are amphibious creatures, while echidnas — the anteaters — are terrestrial. However, new genetic evidence and comparisons with fossil monotremes suggests that echidnas only diverged from platypuses 19 to 48 million years ago.
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