Rodent Regatta

I want to learn more about how these folk are dating their finds. It’s been a while since 7th grade science, and I need a refresher in how we know something is x million years old. What is the premise we’re starting with. At one point in this article they mention a plant that they know was alive y million years ago. How? I’m not skeptical; I just want to understand the methods!

Fossils point to asteroid causing dinosaurs’ demise. Analysis of ancient leaves boosts claims that a massive asteroid strike obliterated 70 per cent of life 65 million years ago [New Scientist]