There are about 270 species of marsupials. To tiny, helpless young, similar to modern marsupials first appeared in the Middle Jurassic,Īnd became extinct in the early Oligocene) Order Multituberculata (extinct rodent-like mammals with multi-cusped teeth that gave birth Order Peramelemorphia (bandicoots and bilbies) Order Paucituberculata (shrew-like insectivores) Order Microbiotheria (only one living species, called "monito del monte" (Dromiciops australis)) Order Diprotodontia (10 families and 117 species) (kangaroos, wallaby, wombats, koalas) Order Dasyuromorphia (numbat, extinct Tasmanian wolf) However, monotremes have several important mammalian characters such as hair/fur, the ability to produce milk, a four chambered heart, a single dentary bone, and three middle ear bones. Modern monotremes do not have teeth as adults sutures are hard to see the rostrum is elongate, beak-like, and covered by a leathery sheath and lacrimal bones are absent. The skulls of monotremes look like those of birds. Prototheira probably split from the lineage leading to other mammals sometime in the Mesozoic, 248 million to 65 million years ago. Many characters of their therapsid ancestors can still be seen in egg-laying mammals including a complex pectoral girdle, laying of eggs rather than bearing live young, limbs oriented with humerus and femur held lateral to body, and a cloaca. The other two species are the Short- and Long-beaked Echinata or Spiny Anteaters ( Tachyglossus aculeatus and Zaglossus bruijni). This is the only representative of its family (Ornithorhynchidae) and genus ( Ornithorhynchus), though a number of fossilised relatives have been found, some of them also in the Ornithorhynchus genus. 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 Characteristics of the Mammals Middle Ear Bones Production of Milk Mammals in Trouble
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