Purgatorius

• It includes some 6,000 species who live in the oceans as well as on land.
• My team’s study used a new method to investigate this question and our findings may help settle this debate.

Estimating origins

• The first is reading the fossil record - the oldest fossil in a group determines the date when it first evolved.
• For placental mammals, there are a couple of fossils from around 65 million years ago, just after the mass extinction of dinosaurs.
• Another way of estimating when groups first evolved is through molecular clock dating studies.
• Two species that are similar in their genetic makeup probably have family trees that split from each other fairly recently.

Rocks or clocks?

• For young lineages that evolved only a few hundred thousand years ago, this sampling rate may be quite high, because we have more fossils from younger rocks.
• Younger rocks are more intact, and often closer to the surface.
• But for older groups, the sampling rate may be quite low because geological processes degrade and destroy rocks and fossils over time.

The origins of placental mammals

• The BBB model estimated the age of placental mammals to be within the Cretaceous period, around 70-80 million years ago – possibly up to 20 million years before the asteroid impact.
• From an origin at the feet of dinosaurs, placental mammals have soared to become the most dominant animals on Earth.