Department of Genetics, Evolution & Environment, and Department of Earth Sciences, University College London, Darwin Building, Gower Street, London, WC1E 6BT, UK.
Evodevo. 2012 Aug 10;3(1):18. doi: 10.1186/2041-9139-3-18.
The timing of the placental mammal radiation has been a source of contention for decades. The fossil record of mammals extends over 200 million years, but no confirmed placental mammal fossils are known prior to 64 million years ago, which is approximately 1.5 million years after the Cretaceous-Paleogene (K-Pg) mass extinction that saw the end of non-avian dinosaurs. Thus, it came as a great surprise when the first published molecular clock studies suggested that placental mammals originated instead far back in the Cretaceous, in some cases doubling divergence estimates based on fossils. In the last few decades, more than a hundred new genera of Mesozoic mammals have been discovered, and molecular divergence studies have grown from simple clock-like models applied to a few genes to sophisticated analyses of entire genomes. Yet, molecular and fossil-based divergence estimates for placental mammal origins have remained remote, with knock-on effects for macro-scale reconstructions of mammal evolution. A few recent molecular studies have begun to converge with fossil-based estimates, and a new phylogenomic study in particular shows that the palaeontological record was mostly correct; most placental mammal orders diversified after the K-Pg mass extinction. While a small gap still remains for Late Cretaceous supraordinal divergences, this study has significantly improved the congruence between molecular and palaeontological data and heralds a broader integration of these fields of evolutionary science.
胎盘哺乳动物的辐射时间是几十年来争论的焦点。哺乳动物的化石记录可以追溯到 2 亿多年前，但在 6400 万年前，也就是白垩纪-古近纪（K-Pg）大灭绝之后的大约 150 万年，还没有确认的胎盘哺乳动物化石。因此，当第一批发表的分子钟研究表明胎盘哺乳动物的起源远在白垩纪时，人们感到非常惊讶，在某些情况下，这些研究结果使基于化石的分歧估计值翻了一番。在过去的几十年里，已经发现了一百多种中生代哺乳动物的新属，而分子分歧研究已经从应用于少数基因的简单时钟模型发展到了对整个基因组的复杂分析。然而，胎盘哺乳动物起源的分子和化石分歧估计仍然存在差距，这对哺乳动物进化的宏观尺度重建产生了连锁反应。最近的一些分子研究开始与基于化石的估计值趋同，特别是一项新的系统基因组研究表明，古生物学记录基本正确；大多数胎盘哺乳动物目在 K-Pg 大灭绝之后多样化。虽然晚白垩世超目分歧仍然存在一个小缺口，但这项研究显著提高了分子和古生物学数据之间的一致性，并预示着这些进化科学领域将更广泛地融合。
