哺乳類は大量絶滅事件直後２度の適応放散を経て様々な環境に進出していった

哺乳類は白亜紀末の大量絶滅事件の影響をうけながらもかろうじて絶滅を免れ、新生代の初期に２段階の適応放散を経て様々な生態系に進出していった。

１回目の適応放散は大量絶滅事件直後の新生代暁新世に起こり、中生代から生き残った古いタイプの哺乳類を起源として多様化が起きた。そして始新世に起きた２回目の適応放散では齧歯類、奇蹄類、偶蹄類など新しいグループが出現し、暁新世に多様化を果たした古いタイプの哺乳類をほぼ完全に駆逐してしまった。

このような過程を経て、様々な食性に適応していった哺乳類は、単純なトリボスフェニック型臼歯を基本形として、多様な形態の臼歯を発達させたのである。

Immediately after the mass extinction event, mammals have through their second adaptive radiation expanded to a new variety of environments.

Even if mammals barely survived the mass extinction event, which occurred at the end of the Cretaceous, they went through a two-step adaptive radiation at the beginning of the Cenozoic and started to conquer a variety of ecosystems.

The first step of this adaptive radiation took place during the Paleocene, immediately after the mass extinction event, where the old species of mammals that survived Mesozoic diversified. Then the second step of the mammals’ adaptive radiation appeared during the Eocene with the emergence of new groups such as Rodent, Perissodactyls (odd-toed ungulate), Artiodactyla (even-toed ungulate), and the almost extinction of the Paleocene mammals old species, which played an important role in the previous diversification.

The basic tribosphenic molars developed and diversified into various forms of molars enabling mammals, through this process, to expand to an even larger variety of food diet.