The fossil record of North American Mammals: Evidence for a Paleocene Evolutionary radiation

John Alroy*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

224 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)107-118
Number of pages12
JournalSystematic Biology
Issue number1
Publication statusPublished - Mar 1999
Externally publishedYes


  • Body mass
  • Cenozoic
  • Cretaceous
  • Diversification
  • Extinction
  • Mammalia
  • Molecular clock


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