Most male mammals have a bone in the penis, a baculum. Most of our closest relatives have such a bone, but humans do not. The closest relatives of Ida’s species also had one. The first find of a primate in the Messel quarry had one. Because Ida’s skeleton is so complete, we can clearly see that there is no baculum between her thigh bones.
 The X-ray of Ida’s lower body clearly shows that there is no penis bone. Photo: PloS One |
 The lower body of Europolemur koenigswaldi, the first primate found in the Messel quarry. Photo: Wighart von Koenigswald, University of Bonn |