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 Though field work gradually made Simpson's classification outdated, it remains the closest thing to an official classification of mammals.
Most mammals, including the six most species-rich
orders, belong to the placental group. The three largest orders in numbers of species are
capybaras and other gnawing mammals;
Chiroptera: bats; and
solenodons. The next three biggest orders, depending on the
biological classification scheme used, are the
Primates including the
even-toed ungulates; and the
Carnivora which includes
seals and allies.
 According to
Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229
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.
The word "
mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the
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
therian mammals (
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
167 million years ago in the
Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.
T. S. Kemp has provided a more traditional definition: "
synapsids that possess a
squamosal jaw articulation and
occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of
Sinoconodon and living mammals.
 The earliest known synapsid satisfying Kemp's definitions is
225 Ma, so the appearance of mammals in this broader sense can be given this
Late Triassic date.
In 1997, the mammals were comprehensively revised by
Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals above the Species Level,
 is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the higher level groupings. The authors worked together as
paleontologists at the
American Museum of Natural History,
New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.
Extinct groups are represented by a
Molecular classification of placentals
Molecular studies based on
DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by
 Classification systems based on molecular studies reveal three major groups or lineages of placental mammals—
diverged in the
Cretaceous. The relationships between these three lineages is contentious, and all three possible different hypotheses have been proposed with respect to which group is
basal. These hypotheses are
Atlantogenata (basal Boreoeutheria),
Epitheria (basal Xenarthra) and
Exafroplacentalia (basal Afrotheria).
 Boreoeutheria in turn contains two major lineages—
Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as
 and varying interpretations of
cladogram above is based on Tarver et al. (2016)
Group I: Superorder
Group II: Superorder
Pilosa: sloths and anteaters (neotropical)
Cingulata: armadillos and extinct relatives (Americas)
Group III: Magnaorder