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The mammals are the class of vertebrate animals Mammals
primarily characterized by the presence of
mammary glands in the female which produce milk Scientific classification
for the nourishment of young; the presence of Kingdom: Animalia
hair or fur; and which have endothermic or "warm
blooded" bodies. The brain regulates endothermic Phylum: Chordata
and circulatory systems including a four chamber Class: Mammalia
heart. Humans are mammals. Mammals embrace more
than 5,000 genera, distributed in 425 families Orders
and 46 orders. ╩Monotremata
Phylogenetically, the Mammalia are defined as the ╩Paucituberculata
last common ancestor of monotremes (e.g. ╩Microbiotheria
echidnas) and therian mammals (e.g. hedgehogs), ╩Dasyuromorphia
and all of this last common ancestor's ╩Peramelemorphia
While most mammals give birth to live young, ╩Afrosoricida
there are a few mammals - the monotremes - that ╩Macroscelidea
lay eggs. Live birth also occurs in a variety of ╩Tubulidentata
non-mammalian species; thus it is not a ╩Hyracoidea
diagnostic characteristic for class Mammalia. ╩Proboscidea
Endothermy is also present in many non-mammals, ╩Sirenia
primarily birds. While monotremes do not have ╩Xenarthra
nipples, they do have mammary glands, meaning ╩Dermoptera:
that they do meet all conditions for inclusion in ╩Scandentia
the class Mammalia. It should be noted that the ╩Primates
current trend in taxonomy is to emphasize common ╩Rodentia
ancestry; the diagnostic characteristics are ╩Lagomorpha
useful for identifying this ancestry, but if, for ╩Insectivora
example, a cetacean were found that had no hair ╩Chiroptera
at all, it would still be classed as a mammal. ╩Pholidota
Mammals have three bones in each ear and one (the ╩Perissodactyla
dentary) on each side of the lower jaw; all other ╩Artiodactyla
vertebrates with ears have one bone (the stapes) ╩Cetacea
in the ear and at least three on each side of the
jaw. A group of therapsids called cynodonts had three bones in the jaw, but
the main jaw joint was the dentary and the other bones conducted sound.
Mammals belong among the amniotes, and in particular among a group called
the synapsids, distinguished by the shape of their skulls. Within this group
they developed from the therapsids, and more specifically the eucynodonts,
220 million years ago during the Triassic period. During the Mesozoic period
they diversified into the three main groups found today, i.e. monotremes,
marsupials, and placentals. They remained small and shrew-like throughout
the era, but radiated rapidly after the Cretaceous-Tertiary extinction event
65 million years ago.
The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory
that Placentalia were descendants of Marsupialia, which were in turn
descendants of Monotremata, but this theory has been refuted. However,
Eutheria and Metatheria are often used in paleontology, especially with
regards to mammals of the Mesozoic.
Most mammals are terrestrial, but a number are secondarily aquatic,
including whales which are the largest of all animals. One order, the bats,
have developed flight, and are among the only animals to have done so.
The Classification of mammals (Class Mammalia) represented in the box at the
right and in the listing below reflects George Gaylord Simpson's classic
Principles of Classification and a Classification of Mammals (AMNH Bulletin
v. 85, 1945). Simpson laid out a systematics of mammal origins and
relationships that was universally taught until the end of the 20th Century.
Since Simpson's 1945 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 (q.v.). Though field work gradually
made Simpson's classification outdated, it remained the closest thing to an
official classification of mammals.
In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and
Susan K. Bell, which has resulted in the McKenna/Bell classification.
McKenna and Bell, Classification of Mammals: Above the species level, (1997)
is the most comprehensive work to date on the systematics, relationships,
and occurrences of all mammal taxa, living and extinct, down through the
rank of genus. The new McKenna/Bell classification was quickly accepted by
paleontologists. The authors work 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 extict taxa that reflects the historical
genealogy of Mammalia.
The McKenna/Bell hierarchical listing of all of the terms used for mammal
groups above the species includes extinct mammals as well as modern groups,
and introduces some fine distinctions such as legions and sublegions
(categories which fall between classes and orders) that are likely to be
glossed over by the layman.