Order
Cetartiodactyla
Even-toed ungulates . . . and
whales! |
The even-toed ungulates are the most successful group of large herbivores
on earth today, having outpaced the formerly widespread perissodactyls in
the Oligocene. Artiodactyls are indigenous to every zoogeographic region
(including several species on Sulawesi and other islands in the Australasian
region); they are not native to the continents of Antarctica and Australia,
but many species have been introduced into areas outside of their natural
range, including Australia, New Guinea, and the islands of Oceania. The
approximately 240 ungulate members of this order show incredible diversity
in size, form, dietary preferences, and climatic tolerance. This order
contains the majority of domesticated mammal species, including cattle, reindeer,
camels, pigs, goats, and sheep.
Recent molecular evidence has radically reorder the classification of this
order, notably proving that whales and dolphins (Order Cetacea) belong WITHIN
this order. To accommodate the merging of these order, a hybridized
name "Cetartiodactyla" has been created. (Some authors, to avoid the
taxonomic implications of including the Cetacea within Artiodactyla, retain
the two orders, but group them in a superorder, also titled the Cetartiodactyla).
Since they don't have hooves, whales and dolphins aren't included in
this website - but you can check out the
links section for some great cetacean websites!
There are three well-established suborders:
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Tylopoda - camels
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Suiformes - pigs and peccaries (and formerly hippos)
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Ruminantia - the ruminants,
containing the majority of modern artiodactyl species
Like the hybridized name Cetartiodactyla, the close allying of the
hippopotamuses and cetaceans has created the need for a new suborder:
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Cetancodonta - hippos, dolphins, and whales.
Cet·ar·ti·o·dac·ty·la
(seet är'tee-oh dak ti'lah)
From Latin cetus, a large sea creature or whale, combined with
Greek artios, complete, of numbers even; daktulos, a finger
or toe |
Evolution
The evolutionary history of "artiodactyls" (the ungulate members of the
Cetartiodactyla) is relatively well known, since, due to their large bones,
fossils are plentiful. However, the inclusion of cetaceans - a group
with a relatively scant fossil history - in this order has caused much debate.
The first artiodactyl fossils (including the rabbit-sized
Diacodexis and Protodichobune) appear around 54 million years
ago, in the early Eocene deposits of North America and Europe. These
early even-toed ungulates had the full placental complement of low-crowned
teeth (44 in total), four distinct toes on each foot, and no cranial appendages.
Arising at a time when perissodactyls dominated the large herbivorous
niches, artiodactyls remained relatively unspecialized until the Oligocene,
when an explosive radiation is apparent (primarily in Eurasia). This
order has remained abundant and diverse since that time (18 extinct families
of ungulate cetartiodactyls are known in addition to the ten modern families),
while the formerly highly successful Perissodactyla have continually declined.
The highly specialized cetaceans are believed to have evolved from the
Archaeoceti, a group of primitive whales known from fossils as early as 50
million years ago. The Archaeocetes were formerly thought to have arisen
from Mesonychian ungulates based on morphological features. However,
Mesonychians are known from fossil deposits 60 million years old - much earlier
than the first artiodactyls. While it is now generally agreed that
these shared characters are convergent, the fossil history of the cetaceans
has yet to be fully resolved in relation to the evolution of the
artiodactyls.
Diagnostic Characteristics
The primary distinguishing feature of all of the ungulates within this order
is the paraxonic limb structure, in which the symmetry of the foot passes
between the two middle digits (III and IV). The first digit (the "thumb"
or pollex in the hand and the hallux on the hind limb) is absent in all modern
artiodactyls, with the result that all species possess an even number of
toes on each foot (with the exception of the Tayassuidae, in which the hind
foot only has three digits). Two main types of foot structure are
recognized: a "cloven hoof" with two weight-bearing toes, and a spreading
foot with four digits. In all cases the third and fourth digits are
well developed, while the second and fifth are reduced, vestigial, or absent.
All ungulate cetartiodactyls have pulley-shaped articulating surfaces
on both ends of the astragalus (fossil evidence of cetaceans indicates that
primitive whales also possessed this feature).
The nasal bones of the artiodactyls are not expanded caudally, nor is there
an alisphenoid canal. Teeth are variable, but the upper incisors are
always reduced or absent. Canines are usually small or not present
at all, although in some species they are greatly enlarged into tusks. Two
main types of molars are recognized - the brachyodont (low-crowned) teeth
of the pigs, peccaries, and hippos, and the hypsodont (high-crowned) teeth
of the camels and some ruminants. A postorbital bar is present in all
species.
The Cetartiodactyl
Family Tree
(branch lengths are not proportional to
time)
Click on each family above to learn
more,
or jump to the Cetartiodactyla
Species List
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Literature Cited
Hernandez-Fernandez, M., and E. S. Vrba. 2005. A complete estimate
of the phylogenetic relationships in Ruminantia: a dated species-level supertree
of the extant ruminants. Biological Review; 80: 269-302.
Nikaido, M., A. P. Rooney, and N. Okada. 1999. Phylogenetic relationships
among cetartiodactyls based on insertions of short and long interspersed
elements: Hippopotamuses are the closest extant relatives of whales. Proceedings
of the National Academy of Sciences (USA); 96: 10261-10266.
Nowak, R. M. [Editor]. 1991. Walker's Mammals of the World. Fifth Edition.
Johns Hopkins University Press, Baltimore.
Price, S. A., O. R. P. Bininda-Emonds, and J. L.Gittleman. 2005. A
complete phylogeny of the whales, dolphins and even-toed hoofed mammals
(Cetartiodactyla). Biological Review: 80: 445-473.
Thenius, E. 1990. Even-toed ungulates: Phyogeny. In
Grzimek's Encyclopedia of Mammals, Volume 5. Edited by S. P. Parker.
New York: McGraw-Hill. pp. 4-15.
Vaughan, T. A., J. M. Ryan, and N. J. Czaplewski. 2000. Mammalogy. Fourth
Edition. Saunders College Publishing, Philadelphia.
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