One of these three species is the Duck-billed Platypus ( Ornithorhynchus anatinus), which is a small, semi-aquatic mammal that can only be found in the eastern part of Australia. The most ancestral forms among all mammals, today, are the egg-laying mammals and only three existing species grouped into two families (Ornithorhynchidae and Tachyglossidae) and a single order, the Monotremata, remain. Subclass Prototheria (Egg-laying mammals) ![]() The Order Artiodactyla (even-toed ungulates: antelope, hippos, deer, sheep, camels, pigs, cows, etc.) and Cetacea (whales, dolphins) are now falling under the Superorder Cetartiodactyla. However, Metatheira and Eutheria are now considered Infraclasses and fall under the subclass Theira. Mammals used to be placed into three subclasses Prototheira (Egg-laying mammals), Metatheira (Marsupials), and Eutheria (Placental mammals). However, it must be noticed that systematists do not yet agree on the exact number or on how some orders and families are related to others. The Class Mammalia includes over 5000 species placed into two subclasses (Prototheira and Theira) and 26 orders. The demise of the dinosaurs 65 million years ago opened up the feeding niche that had been occupied by the dinosaurs and resulted in an explosive radiation for the mammals, who are now the dominant vertebrate group on land. However, the evolution of the remaining mammals did not end. This time is often called the 'Dark Ages' of mammalian evolution. The numbers of species and the size of the remaining species were greatly reduced during that time and, for the next 135 million years, these early mammals played a very small role in the world. The cynodonts dominated the world 250 million years ago but their era came to an end about 200 million years ago with the appearance of dinosaurs. These early cynodonts evolved mammalian-like teeth and a lower jawbone that foreshadowed the dentary bone that characterizes mammals and from their stock evolved the first true mammals. From the synapsids, a branch called the therapsids appeared 275 to 225 million years ago, and it is during the early evolution of the therapsids that early mammal-like reptiles called cynodonts arose. Mammals evolved from a group of reptiles called the synapsids that arose 310 to 275 million years ago. The Commonwealth of Australia reveres this remarkable mammal so much that it honors the platypus with a place on its 20-cent coin.The Characteristics of the Mammals Middle Ear Bones Production of Milk Mammals in Trouble Baby platypuses hatch after 10 days and nurse for up to four months before they swim off and forage on their own. The female platypus lays her eggs in an underground burrow that she digs near the water’s edge. It has no teeth, so the platypus stores its "catch" in its cheek pouches, returns to the surface, mashes up its meal with the help of gravel bits hoovered up enroute, then swallows it all down. The bill also comes equipped with specialized nerve endings, called electroreceptors, which detect tiny electrical currents generated by the muscular contractions of prey. The watertight nostrils on its bill remain sealed so that the animal can stay submerged for up to two minutes as it forages for food. The platypus is a bottom-feeder that uses its beaver-like tail to steer and its webbed feet to propel itself through the water while hunting for insects, shellfish, and worms. ![]() While the platypus generally inhabits freshwater rivers, wetlands, and billabongs Down Under, it is also known to venture into brackish estuaries (the combined fresh-and saltwater areas where rivers meet the sea). If its appearance alone somehow fails to impress, the male of the species is also one of the world’s few venomous mammals! Equipped with sharp stingers on the heels of its hind feet, the male platypus can deliver a strong toxic blow to any approaching foe. The platypus is a duck-billed, beaver-tailed, otter-footed, egg-laying aquatic creature native to Australia.
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