small, thick rib fragment. Later in the field season, while surveying the upper Kuldana Formation, Thewissen discovered a femur (thigh bone) and proximal portion of the tibia (upper portion of the shin) which clearly belonged to a mammal. An hour later, Arif discovered the rest of the skeleton, and the two began excavation the next day. At first, Thewissen speculated the fossils belonged to an anthracobunid (a large semi-aquatic mammal), until he found the teeth near the end of the field season, which were characteristically cetacean (living cetaceans are whales, dolphins, and porpoises). Thewissen, at the time, could not afford to excavate and store everything, so he took the skull with him to the United States, while Arif kept the rest in two crates which used to hold oranges. In October 1992, Thewissen presented his research of the skull to a vertebrate palaeontology convention in Toronto, Canada. The next year, American palaeontologist Philip D. Gingerich paid for the rest of the skeleton to be shipped to the United States. In 1994, the formal description of the remains was published by Thewissen, mammal palaeontologist Sayed Taseer Hussain, and Arif. They identified the remains as clearly belonging to an amphibious cetacean, and so they named it Ambulocetus natans. The genus name comes from Latin ambulare "to walk" and cetus "whale", and the species name natans "swimming". The Kuldana Formation is constrained to sometime during the Lutetian stage of the Early Eocene, and the remains may date to 48–47 million years ago. The holotype specimen, HGSP 18507, is a partial skeleton initially discovered preserving an incomplete skull (missing the snout), some elements of the vertebral column and ribs, as well as portions of the fore- and hind-limb. Other specimens initially found were HGSP 18473 (a second premolar), HGSP 18497 (a third premolar), HGSP 18472 (a tail vertebra), and HGSP 18476 (lower portion of a femur). The holotype was found in a silt and mudstone bed over a area. Further excavation recovered most of the holotype's skeleton—most notably the hip, sacrum, and most of ribcage and thoracolumbar series (the spine excluding the neck, sacrum, and tail). These left the holotype about 80% complete by 2002, making it the most completely known cetacean from the time period. In 2009, some more elements of the holotype's jawbone were identified from a then-recently prepared matrix block. Though it was known that cetaceans descended from land mammals before