Dolphins, along with whales and porpoises, are thought to be descendants of terrestrial mammals, most likely of the Artiodactyl order. Modern dolphin skeletons have two small rod shaped pelvic bones thought to be vestigial hind legs. In October of 2006 an unusual Bottlenose Dolphin was captured in Japan that had small fins on either side of the genital slit, which scientists believe to be a more pronounced development of these vestigal hind legs. Dolphins are often regarded as one of Earth's most intelligent species, though it is hard to say just how intelligent dolphins are as straightforward comparisons of species' relative intelligence are complicated by differences in sensory apparatus, response modes, and nature of cognition.