Somewhere on the side of your thigh bone, there is a tiny hole. It’s called a “nutrient foramen”. An artery passes through this gap, suffusing the bone with blood and oxygen. The hole is found in all thigh bones, from those of birds to lizards, and it always fulfils the same function. But it can also double as a keyhole into the past, allowing us to peek at the lives of animals long extinct.

Seymour measured the nutrient foramina in the thigh bones of almost a hundred animals, and found that, in general, bigger animals have bigger bones and bigger holes. But those of mammals are around twice as big as those of similarly sized reptiles. Combined with the fact that