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Modern day camelids are the only living representatives of the suborder
Tylopoda (from the Greek words tylo, meaning callous, and pous,
genitive podos, meaning foot). The four Recent species are well
adapted to arid environments. A unique wide pad of connective tissue
on the base of the feet provides support on soft, sandy soils. The
fossil record of this subfamily begins in the mid-Eocene deposits of
North America (the center of camelid evolution). The camels radiated
out from North America to Eurasia in the Pliocene, and to South America during
the Pleistocene, disappearing from North America around 10,000 years
ago.
The feet of modern camels seemingly defy the definition of "ungulate": the
two digits in the feet (III and IV) sit almost flat on the ground (i.e.,
are digitigrade) as part of a wide pad. Instead of hooves, the distal phalanges
bear nails on their upper surface. Nevertheless, the fossil record
distinctly shows that camels are ungulates: fossilized camels from the Oligocene
have the same two-digit foot structure, but were nearly unguligrade and likely
had true hooves. These early camels display one of the characteristic
features of Tylopods: the metapodials (foot bones) are fused to form a cannon
bone, but the distal ends separate and flare outward, forming a "Y" of bone
(the arms of the "Y" lead to the two digits). A general climatic shift
to more arid conditions occurred in the Miocene, and was coupled with a secondary
development of a digitigrade foot posture. All camelids since the Pliocene
have been digitigrade.
The position of this family within the order Cetartiodactyla was formerly
confounded by the fact that camelids ruminate (regurgitate and rechew their
food) - at was thus assumed that they were more closely related to the ruminants
than the Suiformes. However, molecular evidence suggests that this
feature evolved independently from the
Ruminantia; the stomach of the Camelidae is
only three-chambered. All members of this family are renowned for
"spitting" when upset, although it is actually stomach contents which are
ejected at annoying objects.
The upper lip of all members of the Camelidae is split into two finger-like
projections which aid in the collection of tough forage. The dental
formula is I 1/3, C 1/1, P 2-3/1-2, M 3/3 x 2 = 30-34. There is no
gall bladder. Unique among mammals, the red blood cells of the Camelidae
are oval in shape. There is no connective skin between the hind legs
and the belly, increasing the apparent length of the hind legs. Camelids
walk with a "pacing" gait, where both the front and hind legs on one side
of the body are moved forward together. The result is that the body
weight is shifted from one side to the other, creating a rocking motion -
camels are thus often called "ships of the desert".
All four species of recent camels have been domesticated. The two domestic
forms from South America (the llama and alpaca) were formerly considered
as species in their own right; today, they are lumped together with their
wild counterparts (the guanaco and vicuña, respectively). While
domestic camelids are found in large numbers, the wild species are all listed
on the IUCN's Red List. Indeed, the dromedary or Arabian camel
(Camelus dromedarius), is now extinct in the wild (although feral
populations exist, notably in Australia). |
