True flight has evolved only once in mammals, the bats; mammals such as flying squirrels and flying lemurs are actually gliding animals.
Live birth also occurs in some non-mammalian species, such as guppies and hammerhead sharks; thus, it is not a distinguishing characteristic of mammals.
Monotremes are mammals that lay eggs, and include the platypuses and echidnas (spiny anteaters).
Whatever the reason, this terminology ties mammals to a feature that is connected to a key mammalian characteristic: parental behavior.
One classification based on molecular studies points to four groups or lineages of placental mammals that diverged from early common ancestors in the Cretaceous.
Most mammals give birth to live young, but the monotremes lay eggs.
The live-bearing mammals can be divided into two further taxa, the marsupials (sometimes labeled as infraclass Metatheria) and the placentals (infraclass Eutheria).
Along with hair, the presence of mammary glands, for feeding milk to their young, is another defining feature of mammals.
During the Mesozoic Period, mammals appeared to diversify into four main groups: multituberculates (Allotherium), monotremes, marsupials, and placentals.
Like birds, mammals are endothermic or "warm-blooded," and have four-chambered hearts.
George Gaylord Simpson's Principles of Classification and a Classification of Mammals (1945) was an original authoritative source for the taxonomy of mammals.
Other systems recognize considerably less orders, families, and genera of mammals.
The Artiodactyla are even-toed mammals and include pigs, camels, cattle, elk, deer, and the American bison, among others.
The Perissodactyla are odd-toed mammals, including rhinoceroses, horses, zebras, and tapirs.
Many mammals are indicated as having blue hair or fur, but in all cases it will be found to be a shade of gray.
Synapsid therapsids, the assumed ancestors of mammals, became common during the Permian period at the end of the Paleozoic era.
Most mammals are terrestrial, but some are aquatic, including sirenia (manatees and dugongs) and the cetaceans.
In 1997, the mammals were comprehensively revised by Malcolm McKenna and Susan Bell, which has resulted in the "McKenna/Bell classification."
Nonetheless, the terms Eutheria and Metatheria remain in common use in paleontology, especially with regards to mammals of the Mesozoic.
During the next eight million years, in the Paleocene period (64–58 million years ago), the fossil record suggests that mammals exploded into the ecological niches left by the extinction of the dinosaurs.
Mammals also have a diaphragm, a muscle below the rib cage that aids breathing.
Placentals generally can be distinguished from other mammals in that the fetus is nourished during gestation via a placenta, although bandicoots (marsupial omnivores) are a conspicuous exception to this rule.
Mammals belong among the amniotes (vertebrates that have membranous sacs that surround and protect the embryo) and in particular to a sub-group called the synapsids.
Marsupials are generally characterized by the female having a pouch in which it rears its young through early infancy, as well as various reproductive traits that distinguish them from other mammals.
Hair and endothermy has aided mammals in inhabiting a wide diversity of environments, from deserts to polar environments, and be active daytime and nighttime.
About 5,500 living species of mammals have been identified.
Mammals are also the only vertebrates with a single bone in the lower jaw.
No mammals have hair that is naturally blue or green in color.
Some other vertebrates have a diaphragm, but mammals are the only vertebrates with a prehepatic diaphragm, that is, in front of the liver.
Evidence from fossils and comparative anatomy suggest that mammals evolved from therapsid reptiles during the Triassic period (approximately 200-250 million years ago).
The presence of hair has helped mammals to maintain a stable core body temperature.
Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis.
Giant Anteater. Linne's Two-toed Sloth. Southern Three-banded Armadillo.Andean Bear. Amur Leopard. Amur Tiger. ... Addax. Babirusa. Bactrian Camel. ... Allen's Swamp Monkey. Black and White Colobus Monkey. ... Matschie's Tree Kangaroo. Opossum. ... Black-tailed Prairie Dog. Capybara. ... California Sea Lion. Harbor Seal.Egg-laying Mammals. Hedgehogs.