Earliest Placental Mammal Ancestor Pinpointed
Placental mammals - as opposed to the kind that lay eggs, such as the platypus, or carry young in pouches, such as the kangaroo - are an extraordinarily diverse group of animals with more than 5,000 species today. They include examples that fly, swim and run, and range in weight from a couple of grams to hundreds of tonnes.
A wealth of fossil evidence had pointed to the notion that the group, or clade, grew in an “explosion” of species shortly after the dinosaurs’ end about 65 million years ago.
But a range of genetic studies that look for fairly regular changes in genetic makeup suggested that the group arose as long as 100 million years ago, with mammals such as early rodents sharing the Earth with the dinosaurs.