快猫短视频

THE mammalian ancestors of both mice and men scurried under the feet of the dinosaurs.

According to the fossil record, our ancestors didn鈥檛 split into modern groups of placental and marsupial mammals until after the dinosaurs bit the dust at the end of the Cretaceous, 65 million years ago. So say John Wible of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and colleagues, who have compared late Cretaceous fossils with modern placental groups.

Biologists consider the first split among modern mammalian groups as marking the origin of placentals, a key point in evolutionary history. Wible鈥檚 comparison found that none of the Cretaceous fossils belong to modern groups (). He concludes that placental groups split away shortly after the Cretaceous, and 鈥渆xplosively evolved鈥� to fill niches opened by the death of the dinosaurs.

That bolsters the traditional view of palaeontologists, but flies in the face of molecular studies of genetic divergence of living species, which put the origin of placentals 80 to 140 million years ago (快猫短视频, 28 March, p 18). 鈥淲e鈥檙e in total discord with the molecular dates,鈥� Wible says. He thinks genetic clocks fail to account for the post-Cretaceous burst of mammalian evolution.

Are palaeontologists missing fossils, or do bursts of evolutionary diversification throw off molecular clocks? You have to take both sides seriously, says Rich Cifelli of the Oklahoma Museum of Natural History in Norman.