 |
Ruminants - so named for their habit of ruminating or "chewing
the cud" (regurgitating and rechewing their food) - are considered to be
the most advanced artiodactyls, and they are certainly the most numerous
and widespread of the world's modern-day ungulate fauna. Their great
success is due to a very specialized digestive tract, which allows these
ungulates to thrive on relatively poor vegetation.
Learn more about the process of rumination
HERE
All ruminants have a four-chambered stomach. The dental formula is
generally I 0/3, C 0/1, P 3/3, M 3/3 x 2 = 32, although in members of the
Tragulidae, Moschidae, and some Cervidae the upper canine may be present
(total teeth 34). The cheek teeth have selenodont (crescent-shaped)
ridges, which grind food efficiently with the side-to-side chewing motion
characteristic of this group. The bones in the feet (metapodials) are
fused to form a cannon bone, although in Hyemoschus (Tragulidae)
this does not occur until after maturity. The navicular and cuboid
bones are always fused. Only the third and fourth digits are well
developed; the second and fifth are vestigial or absent.
Within the Ruminantia, two infraorders are recognized
-
Tragulina - the chevrotains
-
Pecora - the "horned" ruminants
The infraorder Tragulina displays several ancestral characteristics (notably
a lack of fusion of the limb bones until adulthood and a poorly-developed
omasum), and the chevrotains probably resemble the ancestors of the other
ruminants. The Pecora are a much more diverse group, all of which have
an advanced ruminating stomach. All members of the Pecora display cranial
appendages (except for two genera: Hydropotes and Moschus),
but this characteristic has evolved independently among the different Pecoran
families.
The basal divergence between the Pecora and Tragulina is well-supported by
morphological and molecular evidence. All six of the ruminant families
are accepted as monophyletic, but the interrelationships within the infraorder
Pecora are still controversial and under review. Different analyses
have arrived at vastly different relationships between the five modern Pecoran
families; this phylogenetic instability is likely the result of rapid
diversification in the late Oligocene and early Miocene, and is compounded
by parallel evolution within the groups (e.g., the evolution of cranial
appendages and the loss of upper canines). Although the family tree
shown below is a general consensus, recent studies have suggested that the
Moschidae may ally more closely with the Bovidae than the Cervidae. |
