Mammals evolved after the extinction of dinosaurs, with the first true mammals appearing 195 million years ago. There are three major groups: monotremes, marsupials, and placental mammals. Monotremes lay eggs, while marsupials and placental mammals give birth to live young. Placental mammals are the most successful group, with over 5,500 species and a high metabolic rate. They include rodents, bats, carnivores, and herbivores, and gave rise to humans.
Mammals are a diverse group that have dominated terrestrial ecosystems for 65 million years, ever since the previous rulers, the dinosaurs, were obliterated by an incoming meteor, leaving only birds as descendants. When they evolved depends on what a person considers a mammal. The first animals that could be considered, the mammalians, evolved during the Late Triassic, about 210 million years ago. The first true mammals evolved at the very beginning of the Jurassic, about 195 million years ago. Hadrocodium wui, a shrew-like animal the size of a paper clip, had the first true mammalian jaw. It prowled in the shadow of dinosaurs, consuming insects.
For a long time, mammals were all quite similar, all resembling small shrews. About 125 million years ago, they diversified, resulting in today’s three major groups: monotremes (Prototheria, including the platypus, echnidna, and extinct relatives), marsupials (Metatheria, including kangaroos, koalas, and many obscure living and extinct forms), and placental mammals (Eutheria, including mice, elephants, whales, and humans, among many others). Although all had diverged from one another by 120 million years ago, those with placentas and marsupials are more closely related, both being members of the class Theria (meaning “beast”), while monotremes are more distantly related.
The monotremes (Prototheria) are a group that was once much more successful, but are now almost completely extinct, leaving only five species: one platypus and four echnidnas. The name means “single hole” in Greek, referring to their all-purpose cloaca, which they have in common with marsupials. Monotremes are the only group that lay eggs rather than feed their young directly, although they are warm-blooded, provide milk for their young, have only one bone in their lower jaw, and are covered with a layer of fur. Monotremes maintain their body temperatures at about 90°F (32°C), compared to about 95°F (35°C) for marsupials and 100°F (38°C) for placental mammals.
Marsupials (Metatheria) are a modestly successful group comprising 334 species. They once had a range that included much of the planet, but today they are only really successful in Australia where they are the dominant form. Australia and nearby islands include over 200 species of marsupials, ranging from the familiar koala and kangaroo to the more unusual bandicoots and Tasmanian devils. In South America there are about 100 species, mainly opossums, but also unique specimens such as the diminutive Monito del Monte. There are 13 species that moved to Central America when it crashed into South America across the Isthmus of Panama, and a single species made it to North America: the Virginia opossum. Marsupials differ from other groups in their mode of reproduction, presence of cloaca, and metabolic rate.
Placental mammals (Eutheria) are the most successful group on the planet today and occupy a number of ecological niches occupied by dinosaurs and pelycosaurs before them. They boast over 5,500 species. There are four main groups: Rodentia (mice, rats, and other small animals with fast-growing teeth), Chiroptera (bats), Carnivores (dogs, cats, bears, and other carnivores), and Cetartiodactyla (many herbivorous species, such as deer, sheep, goats, buffaloes and whales). Placental mammals are found on every continent except Antarctica, and many have close mythological and symbolic ties to human culture. They are distinguished by a high metabolic rate and their reproductive mode, where the embryo grows to maturity in a uterus before being born. The animals of this group are probably the most intelligent that ever lived on the planet and very recently (200,000 years ago) gave rise to the species Homo sapiens or humans.
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