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The
Xenartha order is composed of 4 Recent families with
13 genera and 29 species. They are distributed through
the south-central and southeastern United States, Mexico,
Central America and South America. The fossil history
indicates a highly successful, diverse order with more
than ten times the current number of species represented.
It is believed that this order may have inhabited regions
as far away as Europe and Asia during the Eocene and
enjoyed worldwide distribution during the Cretaceous.
Indisputably, however, Xenartha was at its height in
South America throughout the Tertiary when this continent
was an isolated island.
Xenarthrans
are separated from all other mammals by two significant
differences. The lumbar vertebra possesses additional
articulation called xenathrales. The purpose of this
design is to reinforce the lumbar region, primarily
the hips, for digging activity. The other distinction
of this order is the presence of a double vena cava
which returns blood to the heart from the posterior
parts of the body. In all other mammal species, there
is only one vena cava to service the entire lower region.
The
species of Xenartha are quite diverse in appearance
and habits. Some, like the armadillos and giant anteaters,
are terrestrial and active both day and night, whereas
other species, like the sloths are arboreal and nocturnal.
Their diets are varied from herbivorous to omnivorous
to insectivorous. As mammals xenarthrans give birth
to live young and nurse from mammae located either in
the armpit, chest or abdomen. The amount of time spent
nurturing the young depends upon species.
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