| Rodhocetus | |
|---|---|
 | |
| Skull ofRodhocetus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Family: | †Protocetidae |
| Subfamily: | †Protocetinae |
| Genus: | †Rodhocetus Gingerich et al. 1994 |
| Species | |
Rodhocetus (fromRodho, the geologicalanticline at the type locality, andcetus, Latin for whale)[1] is an extinct genus ofprotocetidearly whale known from theLutetian of Pakistan.[2] The best-known protocetid,Rodhocetus is known from two partial skeletons that taken together give a complete image of an Eocene whale that had short limbs with long hands and feet that were probably webbed and a sacrum that was immobile with four partially fused sacral vertebrae.[3] It is one of several extinctwhale genera that possess land mammal characteristics, thus demonstrating the evolutionary transition from land to sea.
Rodhocetus was a small whale measuring 2–3 m (6.6–9.8 ft) long.[4] Throughout the 1990s, a close relationship between cetaceans andmesonychians, 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-pulleyastragalus (heel 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 ofRodhocetus balochistanensis, and all three had double-pulley heel bones, thus settling the cladistic controversy.[5]
Through aprincipal components analysisGingerich 2003 demonstrated thatRodhocetus had trunk and limb proportions similar to theRussian desman, a foot-powered swimmer using its tail mainly as a rudder. From this Gingerich concluded thatRodhocetus was swimming mostly at the surface by alternate strokes of its hind feet, and that it was insulated by fur rather than blubber, as areDorudon and modern cetaceans, which made it buoyant and incapable of deep diving.[6]
Theholotype ofR. kasrani,GSP-UM 3012 found in 1992, was described byGingerich et al. 1994: 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, toR. kasrani.[1] The body mass of the holotype has been estimated between 340 and 590 kg (750 and 1,300 lb) based on different techniques.[7]
Derived traits inR. kasrani, relative to older archaeocetes such asPakicetus, include high-crowned cheek teeth, larger auditory bullae, larger mandibular foramen, and mandibular canals. The higher neural spines and shorter femur (60–70%) distinguishRodhocetus from the more primitiveAmbulocetus. The convex posterior surface of the exoccipital, shorter cervical vertebrae, and unfused sacral vertebrae distinguishesR. kasrani fromIndocetus. In contrast to later archaeocetes such asProtocetus and later cetaceans,Rodhocetusretains 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.)[8]
Several cranial features identifiesR. kasrani as an archaeocete: both the premaxillae and the dentaries are elongated, the frontal shield is wide, and the nuchal crest is high. Theauditory bullae are large and dense but, there are no associatedpterygoid fossae or air sinuses. Themandibular foramina are large with a pan bone 90 mm (3.5 in) long and 65 mm (2.6 in) high.[8]
The specific namekasrani comes fromQaisrani, the Baloch tribe inhabiting the type locality.[1] The protocetidQaisracetus is also named after them.
The fossil remains ofR. balochistanensis were found in easternBalochistan,Pakistan in 2001 by Philip Gingerich. Dating from about 47 million years ago, they are one of a series of recent discoveries, including thepakicetids, which have thrown considerable light on the previously mysterious evolutionary origin of whales.[9]
The holotype ofRodhocetus balochistanensis, GSP-UM 3485, is:[10][11]
The five-fingered hand ofR. 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).[12]
With an estimated body weight of 450 kg (990 lb),R. balochistanensis was 13% smaller thanR. kasrani (590 kg (1,300 lb)), but its femur is larger.[10]
