the basilosaurid known as Basiloterus. Basilosaurus is considered to be the largest of archeocete whales. Cranium The dental formula for B. isis is . The upper and lower molars and second to fourth premolars are double-rooted and high-crowned. The head of Basilosaurus did not have room for a melon like modern toothed whales, and the brain was smaller in comparison, as well. They are not believed to have had the social capabilities of modern whales. A 2011 study concluded that the skull of Basilosaurus is asymmetrical like in modern toothed whales, and not, as previously assumed, symmetrical like in baleen whales and artiodactyls (which are closely related to cetaceans). In modern toothed whales, this asymmetry is associated with high-frequency sound production and echolocation, neither of which is thought to have been present in Basilosaurus. This probably evolved to detect sound underwater, with a fatty sound-receiving pad in the mandible. In the skull, the inner and middle ear are enclosed by a dense tympanic bulla. The synapomorphic cetacean air sinus system is partially present in basilosaurids, including the pterygoid, peribullary, maxillary, and frontal sinuses. The periotic bone, which surrounds the inner ear, is partially isolated. The mandibular canal is large and laterally flanked by a thin bony wall, the pan bone or acoustic fenestra. These features enabled basilosaurs to hear directionally in water. The ear of basilosaurids is more derived than those in earlier archaeocetes, such as remingtonocetids and protocetids, in the acoustic isolation provided by the air-filled sinuses inserted between the ear and the skull. The basilosaurid ear did, however, have a large external auditory meatus, strongly reduced in modern cetaceans, but, though this was probably functional, it can have been of little use under water. Hind limbs A individual of B. isis had hind limbs with fused tarsals and only three digits. The limited size of the limb and the absence of an articulation with the sacral vertebrae make a locomotory function unlikely. Analysis has shown that the reduced limbs could rapidly adduct between only two positions. Possible uses for the structure have been given, such as clasper-like body functions (compare to the function of pelvic spurs, the last vestiges of limbs in certain modern snakes). These limbs would have been used to guide the animals' long bodies during mating. Spine and movement A complete Basilosaurus skeleton was found in 2015, and several attempts have been made to reconstruct