The Cenozoic is Earth's current geological era, representing the last 66 million years of Earth's history. It is characterised by the dominance of mammals, birds, and angiosperms. It is the latest of three geological eras, preceded by the Mesozoic and Paleozoic. The Cenozoic started with the Cretaceous–Paleogene extinction event, when many species, including the non-avian dinosaurs, became extinct in an event attributed by most experts to the impact of a large asteroid or other celestial body, the Chicxulub impactor.
A carnivore, or meat-eater, is an animal or plant whose food and energy requirements derive from the consumption of animal tissues whether through hunting or scavenging.
A mammal is a vertebrate animal of the class Mammalia. Mammals are characterized by the presence of milk-producing mammary glands for feeding their young, a neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 29 orders.
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a pouch. Living marsupials include kangaroos, koalas, opossums, Tasmanian devils, wombats, wallabies, and bandicoots among others, while many extinct species, such as the thylacine, Thylacoleo, and Diprotodon, are also known.
Ungulates are members of the diverse clade Euungulata which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to a distant clade Afrotheria. Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.
Multituberculata is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, and reached a peak diversity during the Late Cretaceous and Paleocene. They eventually declined from the mid-Paleocene onwards, disappearing from the known fossil record in the late Eocene. They are the most diverse order of Mesozoic mammals with more than 200 species known, ranging from mouse-sized to beaver-sized. These species occupied a diversity of ecological niches, ranging from burrow-dwelling to squirrel-like arborealism to jerboa-like hoppers. Multituberculates are usually placed as crown mammals outside either of the two main groups of living mammals—Theria, including placentals and marsupials, and Monotremata—but usually as closer to Theria than to monotremes. They are considered to be closely related to Euharamiyida and Gondwanatheria as part of Allotheria.
Placental mammals are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the fetus is carried in the uterus of its mother to a relatively late stage of development. The name is something of a misnomer considering that marsupials also nourish their fetuses via a placenta, though for a relatively briefer period, giving birth to less developed young which are then nurtured for a period inside the mother's pouch. Placentalia represents the only living group within Eutheria, which contains all mammals more closely related to placentals than to marsupials.
Synapsids are one of the two major clades of vertebrate animals in the group Amniota, the other being the sauropsids, which include reptiles and birds. The synapsids were the dominant land animals in the late Paleozoic and early Mesozoic, but the only extant group that survived into the Cenozoic are the mammals. Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye orbit, leaving a bony arch beneath each; this accounts for their name. The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period, when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.
A therapsid is a member of the clade Therapsida which is a major group of eupelycosaurian synapsids that includes mammals and their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more underneath the body, as opposed to the sprawling posture of many reptiles and salamanders.
Eutheria, also called Placentalia sensu lato or Pan-Placentalia, is the clade consisting of placental mammals and all therian mammals that are more closely related to placentals than to marsupials.
Cynodonts are eutheriodont therapsids belonging to the clade Cynodontia that first appeared in the Late Permian, and extensively diversified after the Permian–Triassic extinction event. Cynodonts occupied a variety of ecological niches, both as carnivores and as herbivores. Mammals are cynodonts, as are their extinct ancestors and close relatives (Mammaliaformes), having evolved from advanced probainognathian cynodonts during the Late Triassic. All other cynodont lines went extinct, with the last known non-mammaliaform cynodont group, the Tritylodontidae, having its youngest records in the Early Cretaceous.
Theria is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians and the metatherians but excludes the egg-laying monotremes and various extinct mammals evolving prior to the common ancestor of placentals and marsupials.
Allotheria is an extinct clade of mammals known from the Mesozoic and early Cenozoic. Shared characteristics of the group are the presence of lower molariform teeth equipped with longitudinal rows of cusps and enlarged incisors. Typically, the canine teeth are also lost. Allotheria includes Multituberculata, Gondwanatheria, and probably Haramiyida, although some studies have recovered haramiyidans to be basal mammaliaforms unrelated to multituberculates. Allotherians are often placed as crown group mammals, more closely related to living marsupials and placentals (Theria) than to monotremes or eutriconodonts.
Mammaliaformes is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia. Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium as well as the Triassic Tikitherium, the earliest known member of the group.
Mammal Species of the World: A Taxonomic and Geographic Reference is a standard reference work in mammalogy giving descriptions and bibliographic data for the known species of mammals. It is now in its third edition, published in late 2005, which was edited by Don E. Wilson and DeeAnn M. Reeder.
The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes. Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.
Fur is a thick growth of hair that covers the skin of almost all mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket that keeps the animal warm.
Trechnotheria is a group of mammals that includes the therians and some fossil mammals from the Mesozoic Era. It includes both the extinct symmetrodonts and the living Cladotheria.
Cladotheria is a clade of mammals. It contains modern therian mammals and several extinct groups, such as the "dryolestoids", amphitheriids and peramurids. The clade was named in 1975 by Malcolm McKenna. In 2002, it was defined as a node-based taxon containing "the common ancestor of dryolestids and living therians, plus all its descendants". A different, stem-based definition was given in 2013, in which Cladotheria contains all taxa that are closer to Mus musculus than to the "symmetrodont" Spalacotherium tricuspidens.
Monotremes are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts, compared to the more common mammalian types. Although they are different from almost all mammals in that they lay eggs, like all mammals, the female monotremes nurse their young with milk.
