To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.
But by around 23 million years ago, all the whales in this group had baleen, and “all these toothy things go away”, he says.
Researchers still don’t know why baleen and filter-feeding won out over all other eating options in this particular lineage.
Uhen, a vertebrate palaeontologist at George Mason University in Fairfax, Virginia.
Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.
In a way this field, called geochronology, is some of the purest detective work earth scientists do.
There are two basic approaches: relative age dating, and absolute age dating.
So far, most whale fossils have been found in the Northern Hemisphere. selenensis and a variety of older fossils from the Southern Hemisphere, researchers are turning their eyes to the south.
“We thought we knew a lot about this time,” says Pyenson.