Y: So here we are in the natural history museum, and what I'm looking at appears to be the skeleton of some kind of extinct giant seal dinosaur? Don, come take a look. What is this thing?

D: I bet we can ask someone who works here, and they'll tell us all about it.

Y: What I'd like to know is how scientists know anything about these skeletons they dig up. It must be hard enough to put the bones together. How do they figure out even basic facts such as whether an animal walked on land or swam in the sea?

D: Well, that got a little easier for scientists after researchers developed an index that determines if a mammal lived on land, in the water, or both. The index uses measurements of the mammals' rib cages to determine how well they resist vertical compression, which lets us know if an animal can support its weight against gravity while on land. A mammal's ribs need to be strong for it to walk or crawl on land, but not if they get around by swimming.

Y: I'm guessing they created this index using measurements from modern-day mammals?

D: Yup, twenty-six of them, as different as the giraffe and polar bear. Then they applied the index to four extinct mammals, and were able to confirm or reject earlier ideas about their lifestyles. They figured out that an early ancestor of the whale, for example, definitely couldn't have supported itself on land, so it was definitely fully aquatic.

Y: Sounds useful. But it doesn't really help me figure out what I'm looking at. Come on, let's go find a guide.