In a post last month, I pointed out how aerospace engineers can learn a lot from looking at the fossils of ancient flying reptiles. Today’s issue of Nature contains a variation on that theme: ancient swimming reptiles can teach geneticists a lot as well.

Almost all humans have five fingers. Genetic disorders can produce extra fingers and toes, but only rarely. Five fingers is generally the upper limit not just for humans, but for all vertebrates on land. You can find plenty of tetrapods whose ancestors lost one or more of those five fingers. Horses have just one; snakes none. But tetrapods with more than five digits are incredibly rare. In most cases, these aren’t true digits–wrists bones or other