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A mammoth is any species of the extinct genus ''Mammuthus'', proboscideans commonly equipped with long, curved tusks and, in northern species, a covering of long hair. They lived from the Pliocene epoch (from around 5 million years ago) into the Holocene at about 4,500 years ago in Africa, Europe, Asia, and North America. They were members of the family Elephantidae, which also contains the two genera of modern elephants and their ancestors. ==Evolution== The earliest known proboscideans, the clade that contains the elephants, existed about 55 million years ago around the Tethys Sea area. The closest relatives of the Proboscidea are the sirenians and the hyraxes. The family Elephantidae is known to have existed six million years ago in Africa, and includes the living elephants and the mammoths. Among many now extinct clades, the mastodon is only a distant relative of the mammoths, and part of the separate Mammutidae family, which diverged 25 million years before the mammoths evolved. The following cladogram shows the placement of the genus ''Mammuthus'' among other proboscideans, based on hyoid characteristics: }} }} }} }} }} }} Since many remains of each species of mammoth are known from several localities, it is possible to reconstruct the evolutionary history of the genus through morphological studies. Mammoth species can be identified from the number of enamel ridges on their molars; the primitive species had few ridges, and the amount increased gradually as new species evolved and replaced the former ones. At the same time, the crowns of the teeth became longer, and the skulls become higher from top to bottom and shorter from the back to the front over time to accommodate this.〔 The first known members of the genus ''Mammuthus'' are the African species ''M. subplanifrons'' from the Pliocene and ''M. africanavus'' from the Pleistocene. The former is thought to be the ancestor of later forms. Mammoths entered Europe around 3 million years ago; the earliest known type has been named ''M. rumanus'', which spread across Europe and China. Only its molars are known, which show it had 8–10 enamel ridges. A population evolved 12–14 ridges and split off from and replaced the earlier type, becoming ''M. meridionalis''. In turn, this species was replaced by the steppe mammoth, ''M. trogontherii'', with 18–20 ridges, which evolved in East Asia ca. 1 million years ago. Mammoths derived from ''M. trogontherii'' evolved molars with 26 ridges 200,000 years ago in Siberia, and became the woolly mammoth, ''M. primigenius''. The Columbian mammoth, ''M. columbi'', evolved from a population of ''M. trogontherii'' that had entered North America. A 2011 genetic study showed that two examined specimens of the Columbian mammoth were grouped within a subclade of woolly mammoths. This suggests that the two populations interbred and produced fertile offspring. It also suggested that a North American form known as "''M. jeffersonii''" may be a hybrid between the two species. By the late Pleistocene, mammoths in continental Eurasia had undergone a major transformation, including a shortening and heightening of the cranium and mandible, increase in molar hypsodonty index, increase in plate number, and thinning of dental enamel. Due to this change in physical appearance, it became customary to group European mammoths separately into distinguishable clusters: # Early Pleistocene – ''Mammuthus meridionalis'' # Middle Pleistocene – ''Mammuthus trogontherii'' # Late Pleistocene – ''Mammuthus primigenius'' There is speculation as to what caused this variation within the three chronospecies. Variations in environment, climate change, and migration surely played roles in the evolutionary process of the mammoths. Take ''M. primigenius'' for example: Woolly mammoths lived in opened grassland biomes. The cool steppe-tundra of the Northern Hemisphere was the ideal place for mammoths to thrive because of the resources it supplied. With occasional warmings during the ice age, climate would change the landscape, and resources available to the mammoths altered accordingly.〔 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「mammoth」の詳細全文を読む スポンサード リンク
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