Molecular Clocks and the Timing of the Placental and Marsupial Radiations in Relation to the Cretaceous-Tertiary Boundary

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Paleostratigraphic estimates of divergence time for nine independent cladogenic events within Mammalia, ranging from 14 to 130 million years, were regressed against Tamura-Nei-corrected 12S rRNA transversions. Relative rate-adjusted distances were also regressed against paleostratigraphic divergence times. The resulting equations were used to estimate interordinal divergence times within Eutheria and Metatheria for a data set that includes representatives of all orders in each infraclass. Without the adjustment for rate variation, divergence times range from 34 to 156 million years for placental orders, versus 32 to 86 million years for marsupial orders. With rate adjustments, the range of divergence estimates decreases to 53 to 133 million years for placentals versus 40 to 79 million years for marsupials. The effect of rate adjustments is most noticeable for carnivores and perissodactyls, where rates are slow, and proboscideans, where rates are fast. In agreement with studies based on nuclear genes, both unadjusted and rate-adjusted estimates of sequence divergence indicate that the majority of placental orders originated before the terminal Cretaceous extinction. Exceptions include the perissodactyl-carnivore split and cladogenesis among paenungulate orders. Most marsupial orders, in turn, may have originated in the early Tertiary although didelphimorphs, at least, appear to have split from other lineages in the late Cretaceous. Marsupial divergence times based on 12S rRNA data are in good agreement with estimates based on single-copy DNA hybridization and disagree with the suggestion of Hershkovitz (1992) that Dromiciops separated from other marsupials in the Jurassic.

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