Alroy J
Department of Paleobiology, Smithsonian Institution, MRC 121, Washington, D.C. 20560, USA.
Syst Biol. 1999 Mar;48(1):107-18. doi: 10.1080/106351599260472.
Paleontologists long have argued that the most important evolutionary radiation of mammals occurred during the early Cenozoic, if not that all eutherians originated from a single common post-Cretaceous ancestor. Nonetheless, several recent molecular analyses claim to show that because several interordinal splits occurred during the Cretaceous, a major therian radiation was then underway. This claim conflicts with statistical evidence from the well-sampled latest Cretaceous and Cenozoic North American fossil record. Paleofaunal data confirm that there were fewer mammalian species during the latest Cretaceous than during any interval of the Cenozoic, and that a massive diversification took place during the early Paleocene, immediately after a mass extinction. Measurement data show that Cretaceous mammals were on average small and occupied a narrow range of body sizes; after the Cretaceous-Tertiary mass extinction, there was a rapid and permanent shift in the mean. The fact that there was an early Cenozoic mammalian radiation is entirely compatible with the existence of a few Cretaceous splits among modern mammal lineages.
长期以来，古生物学家一直认为，哺乳动物最重要的进化辐射发生在新生代早期，即便不是所有真兽类都起源于白垩纪后的单一共同祖先。尽管如此，最近的几项分子分析称，由于在白垩纪发生了几次目间分裂，所以当时有一次主要的兽亚纲辐射正在进行。这一说法与来自采样充分的最新白垩纪和新生代北美化石记录的统计证据相冲突。古动物群数据证实，最新白垩纪时期的哺乳动物物种比新生代的任何时期都要少，并且在一次大灭绝之后紧接着的古新世早期发生了大规模的物种多样化。测量数据表明，白垩纪哺乳动物平均体型较小，且体型范围狭窄；白垩纪-第三纪大灭绝之后，平均体型出现了迅速且永久性的变化。新生代早期存在哺乳动物辐射这一事实与现代哺乳动物谱系中在白垩纪出现的一些分裂现象完全相符。
