|
The even-toed ungulates (order Artiodactyla) are ungulates (hoofed animals) whose weight is borne approximately equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulate (perissodactyls), such as horses. The name Artiodactyla comes from (Greek: ἄρτιος (ártios), "even", and δάκτυλος (dáktylos), "finger/toe"), so the name "even-toed" is a translation of the description. This group includes pigs (family Suidae), peccaries (family Tayassuidae), hippopotamuses (family Hippopotamidae), camels (genus ''Camelus'' in family Camelidae), llamas (genus ''Lama'' in family Camelidae), chevrotains or mouse deer (family Tragulidae), deer (family Cervidae), giraffes (family Giraffidae), pronghorn (genus Antilocapra in family Antilocapridae), antelopes (sections of family Bovidae including subfamily Antilopinae), goat-antelopes (which include sheep, goats and others; subfamily Caprinae in family Bovidae), and cattle (genus ''Bos'' in family Bovidae). Some exclude whales (infraorder Cetacea), although DNA sequence and anatomical data indicate they share a common ancestor, making the group paraphyletic. This school of thought names the joint group Cetartiodactyla (from Cetacea and Artiodactyla). Conversely, Grove and Grubb, 2011, citing Heglen and others, include Cetacea as a sister group of Hippopotamidae, since they are "deeply nested" within the phylogeny of the Artiodactyla, rather than a sister group which could be comprehended by a parent "Cetartiodactyla". They further contend, on the same authority, that just as Carnivora embraced Pinnipedia without a name change, no name change is required for Artiodactyla. Of the roughly 220 artiodactyl species, many are of great dietary, economic, and cultural importance to humans. A further distinguishing feature of the group is the shape of the astragalus (talus), a bone in the ankle joint, which has a double-pulley structure. This gives the foot greater flexibility. ==Taxonomy and phylogeny== The classification of artiodactyls is hotly debated. This is because there is a morphologically defined order, yet cetaceans have evolved from them, and some groups (hippopotamuses) are more closely related to them than to other even-toed ungulates. This makes the artiodactyls a phylogenetic (defined through the tribal development) classification - which is significant in recent times -in essence, a paraphyletic taxon, that is, a group which, although descended from a common ancestor, does not include all descendants. The phylogenetic classification is recognized as only a monophyletic taxa, that is, groups that are descended from a common ancestor and include all descendants including their ancestors; in essence, the order Artiodactyla and infraorder Cetacea are summarized in the taxon Cetartiodactyla. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Even-toed ungulate」の詳細全文を読む スポンサード リンク
|