The rise and fall of the penis bone

From cats and dogs to primates and rodents, the males of many mammal species have a special genital bone called the baculum... Females often possess a bone analogous to the baculum called the baubellum or os clitoris...
After looking across 954 mammal species to check for the presence or absence of a baculum, Dean and his colleagues determined that the enigmatic bone independently evolved nine times and was subsequently lost in 10 different lineages. This means that the baculum is not an ancestral trait, but something that has popped up over and over again in mammalian history...
“There is nothing in common among species with a baculum versus species without,” Dean says. And solving the mystery “is not some weird niche of science.” The rapid and repeated evolution of bacula, Dean says, “is an absolutely fundamental pattern of evolution in almost all sexually reproducing organisms. More at the Washington Post.