Fossil records of mammals and their early evolution around the Cretaceous/Paleogene boundary

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  • 哺乳類の化石記録と白亜紀/古第三紀境界前後における初期進化
  • ホニュウルイ ノ カセキ キロク ト ハクアキ/コ ダイサンキ キョウカイ ゼンゴ ニ オケル ショキ シンカ

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<p>The topic of macroevolution of mammals has recently been disputed by paleontologists and molecular biologists, specifically in regards to the dispersal timing of crown Placentalia (crown Eutheria) and placental orders around the Cretaceous/Paleogene (K/Pg) boundary (ca. 66 Ma). Except Monotremata, there is no positive fossil evidence that Marsupialia (crown Metatheria) and Placentalia existed in the Mesozoic. Current fossil records from Cenozoic localities indicate that the oldest species included in the placental orders appeared at the beginning of the Paleocene (ca. 60–50 Ma). This strongly supports the explosive divergence model, which claims that crown placentals evolved immediately after the K/Pg boundary. However, this hypothesis is criticized primarily because the interval of diversification after the K/Pg boundary is too short in terms of molecular evolution of mammals. A plausible model that agrees with both paleontological and molecular phylogenetic studies explains that major crown clades of placentals (e.g., Xenarthra and Laurasiatheria) originated in the Late Cretaceous and then ordinal groups dispersed in the early Paleocene.</p>

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