Rodhocetus was a small whale measuring long. Throughout the 1990s, a close relationship between cetaceans and
mesonychians, an extinct group of cursorial, wolf-like ungulates, was generally accepted based on morphological analyses. In the late 1990s, however, cladistic analyses based on molecular data clearly placed Cetacea within the Artiodactyla near the hippopotamus. One of the diagnostic characteristics of artiodactyls is the double-pulley
astragalus (ankle bone), and palaeontologists, unconvinced by the data from the labs, set themselves out to find archaeocete single-pulley heel bones. Hind legs from three archaeocete species were recovered within a few years, among them those of
Rodhocetus balochistanensis, and all three had double-pulley heel bones, thus settling the cladistic controversy. Through a
principal components analysis demonstrated that
Rodhocetus had trunk and limb proportions similar to the
Russian desman, a foot-powered swimmer using its tail mainly as a rudder. From this Gingerich concluded that
Rodhocetus was swimming mostly at the surface by alternate strokes of its hind feet, and that it was insulated by fur rather than blubber, as are
Dorudon and modern cetaceans, which made it buoyant and incapable of deep diving.
R. kasrani The
holotype of
R. kasrani,
GSP-
UM 3012 found in 1992, was described by : a cranium with two dentaries, most of the vertebral column as far as the anterior tail (C2–C7; T1–13; L1–6, S1–4, Ca1–4), most ribs, parts of the sternum, both hip bones, and a left femur. Gingerich et al. 1994 referred a specimen collected in 1981, GSP-UM 1852 two dentaries with teeth, to
R. kasrani.
Derived traits in
R. kasrani, relative to older archaeocetes such as
Pakicetus, include high-crowned cheek teeth, larger auditory bullae, larger mandibular foramen, and mandibular canals. The higher neural spines and shorter femur (60–70%) distinguish
Rodhocetus from the more primitive
Ambulocetus. The convex posterior surface of the exoccipital, shorter cervical vertebrae, and unfused sacral vertebrae distinguishes
R. kasrani from
Indocetus. In contrast to later archaeocetes such as
Protocetus and later cetaceans,
Rodhocetus retains external nares above upper canines, high neural spines on anterior thoracic vertebrae, and four sacral vertebrae with sacroiliac joints similar to those in land-mammals (suggesting a hip joint that could support the body weight.) Several cranial features identifies
R. kasrani as an archaeocete: both the premaxillae and the dentaries are elongated, the frontal shield is wide, and the nuchal crest is high. The
auditory bullae are large and dense but, there are no associated
pterygoid fossae or air sinuses. The
mandibular foramina are large with a pan bone long and high. The holotype of
Rodhocetus balochistanensis, GSP-UM 3485, is: • A weathered braincase found at the surface next to a partial dentary with an unfused mandibular symphysis, a characteristic of protocetids. • Large parts of the axial skeleton including cervical, thoracic and proximal caudal vertebrae, but excluding sacral vertebrae. • Forelimb material including the left distal humerus, radius and ulna, and two virtually complete hand skeletons including all carpal bones, unfused and lacking an os centrale, and phalanges. • Parts of the pelvis including an acetabular rim. • Hind limb material includes the right femur, patella, tibia, and possible partial fibula; two virtually complete foot skeletons include tarsal and metatarsal bones and phalanges. The astragalus (heel bone) is characteristic of artiodactyls with a deep tibial trochlea restricting lateral movements and a large calcaneal tuber (posterior part of heel bone) providing leverage for powerful extension. The metatarsals and phalanges are very long and thin and can not have been weight-bearing, suggesting that
Rodhocetus was predominantly aquatic and on land must have walked on the plantar surface of the tarsals. The shape of the metatarsal and phalanges reveal that these bones could be tightly compressed during flexion and widely separated during extension. The five-fingered hand of
R. balochistanensis is mesaxonic (i.e. has a central digit) with three weight-bearing central digits equipped with nail-like hooves, flanked by two more slender digits lacking hooves (distal phalanges preserved on first, second, and fourth digits). The four-toed foot is paraxonic (i.e. central axis passes between the two central digits), with all four digits ending in pointed nails (distal phalanges preserved on second and third digits). With an estimated body weight of ,
R. balochistanensis was 13% smaller than
R. kasrani (), but its femur is larger. ==See also==