morphology was possibly a result of convergent evolution. The oldest known cetacean is the ambulocetid Himalayacetus identified in 1998 and dated to 52.5 million years ago (predating the terrestrial pakicetids), though the exact dating of Himalayacetus and Pakicetus is debated. Ambulocetidae also includes Gandakasia. Himalayacetus and Gandakasia are known only from partial jaw fragments. Ambulocetidae are endemic to the Indian subcontinent, and span the early to middle Eocene. Description Size Upon description, Thewissen and colleagues suggested the holotype specimen may have weighed the same as a male South American sea lion — about — based on the size of the vertebrae, ribs, and limbs. They also estimated a length of roughly . For comparison, the holotype of Pakicetus attocki may have been long. In 1996, they estimated weight of Ambulocetus, using the cross-sections of the long bones, as . Alternatively, they estimated about by using the length of the second upper and lower molars compared to trends between this length and ungulate body mass. They obtained the same result comparing the skull size to those of similarly sized carnivores. In 1998, based on vertebral size, Gingerich estimated a body mass of , similar to modern cetaceans. In 2013, Thewissen suggested that this may be an unreliable mass determinant as the vertebrae are unusually robust in Ambulocetus. Skull Like other archaeocetes which preserve this element, the base of the skull has an undulating contour, probably related to the shape of the nasal canal (and its passage to the throat) and the narrow infraorbital region (the area below the eyes). The base of the skull is wide compared to other archaeocetes, more like that of modern cetaceans. The narrow infraorbital space, made of primarily the pterygoid processes, also occurs in Remingtonocetus and Pakicetus. The pterygoids connect as far back as the middle ear, much farther than other archaeocetes including the more ancient Pakicetus. Most modern cetaceans have a falcate (sickle-shaped) process which juts out prominently halfway between the hypoglossal canal and the ear; Ambulocetus has a similar process continuous of the pterygoid, but it runs alongside and behind the hypoglossal canal. Like many other archaeocetes, the pterygoids, sphenoids, and palatines form a wall lining the bottom of the nasal canal, which causes the palate to extend all the way to the ear. Like other cetaceans, Ambulocetus lacks the postglenoid foramen, which usually is one of the main passageways for veins into the