Diversity
The
evolution
of the class Mammalia has produced tremendous
diversity
in form and habit. Living kinds range in size from a
bat
weighing less than a gram and tiny
shrews
weighing but a few grams to the largest
animal
that has ever lived, the
blue whale
, which reaches a length of more than 30 metres (100 feet) and a weight of 180 metric tons (nearly 200 short [U.S.] tons). Every major
habitat
has been exploited by mammals that swim, fly, run, burrow, glide, or climb.
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There are more than 5,500
species
of living mammals, arranged in about 125 families and as many as 27?29 orders (familial and ordinal groupings sometimes vary among authorities). The
rodents
(order Rodentia) are the most numerous of existing mammals, in both number of species and number of individuals, and are one of the most
diverse
of living lineages. In contrast, the order Tubulidentata is represented by a single living species, the
aardvark
. The Uranotheria (
elephants
and their kin) and
Perissodactyla
(
horses
,
rhinoceroses
, and their kin) are examples of orders in which far greater
diversity
occurred in the late
Paleogene
and
Neogene periods
(about 30 million to about 3 million years ago) than today.
The greatest present-day diversity is seen in continental tropical regions, although members of the class Mammalia live on (or in seas
adjacent
to) all major landmasses. Mammals can also be found on many oceanic
islands
, which are principally, but by no means exclusively, inhabited by
bats
. Major regional faunas can be identified; these resulted in large part from
evolution
in comparative isolation of stocks of early mammals that reached these areas.
South America
(the Neotropics), for example, was separated from
North America
(the Nearctic) from about 65 million to 2.5 million years ago. Mammalian groups that had reached South America before the break between the continents, or some that “island-hopped” after the break, evolved independently from relatives that remained in North America. Some of the latter became
extinct
as the result of competition with more advanced groups, whereas those in South America
flourished
, some radiating to the extent that they have successfully competed with invaders since the rejoining of the two continents.
Australia
provides a parallel case of early isolation and
adaptive radiation
of mammals (specifically the
monotremes
and
marsupials
), although it differs in that Australia was not later connected to any other landmass. The
placental mammals
that reached Australia (rodents and bats) evidently did so by island-hopping long after the adaptive radiation of the mammals isolated early on.
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In contrast, North America and Eurasia (the Palearctic) are separate landmasses but have closely related faunas as the result of having been connected several times during the
Pleistocene Epoch
(2.6 million to 11,700 years ago) and earlier across the
Bering Strait
. Their faunas frequently are thought of as representing not two distinct units but one, related to such a degree that a single name, Holarctic, is applied to it.