<Tr> <Td> Rodentia Chiroptera Soricomorpha Primates Carnivora Artiodactyla Diprotodontia Lagomorpha Didelphimorphia Cetacea Dasyuromorphia </Td> <Td> Afrosoricida Erinaceomorpha Cingulata Peramelemorphia Scandentia Perissodactyla Macroscelidea Pilosa Monotremata Proboscidea </Td> </Tr> <P> Mammal classification has been through several iterations since Carl Linnaeus initially defined the class . No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums . George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century . Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics . Though field work gradually made Simpson's classification outdated, it remains the closest thing to an official classification of mammals . </P> <P> Most mammals, including the six most species - rich orders, belong to the placental group . The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras and other gnawing mammals; Chiroptera: bats; and Soricomorpha: shrews, moles and solenodons . The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes, monkeys and lemurs; the Cetartiodactyla including whales and even - toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals and allies . According to Mammal Species of the World, 5,416 species were identified in 2006 . These were grouped into 1,229 genera, 153 families and 29 orders . In 2008, the International Union for Conservation of Nature (IUCN) completed a five - year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species . According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 recently extinct . </P> <P> The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor . Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century . If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others . Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group . </P>

The number of orders in class mammalia is