Living large in the Cretaceous

摘要

Discoveries of large, carnivorous mammals from the Cretaceous challenge the long-held view that primitive mammals were small and uninteresting. Have palaeontologists been asking the wrong questions? Although more than two-thirds of mammalian evolutionoccurred between about 180 million and 65.5 million years ago, many people think that these early mammals were not very exciting. Mesozoic mammals are usually portrayed as rat-sized, nocturnal prey animals, ecologically marginalized and constrained fromevolving diverse body types and sizes until the extinction event at the end of the Cretaceous removed non-avian dinosaurs from the scene. Two fascinating discoveries of near-complete fossil skeletons, described by Hu et al. on page 149 of this issue, overturn this outdated view. Neither is of a small mammal. One is more than a metre long. The other appears to have a dismembered juvenile dinosaur in its stomach.