Cetacean Family Tree

Rodhocetus spp.

Age:

46-47 million years ago, Eocene Epoch.

   
Range:

Two species of Rodhocetus (Rodhocetus kasrani and Rodhocetus balochistanensis) have been named from Pakistan; the fossils were collected from marine sediments deposited on the northern margin of the Paratethys Sea (the precursor to the Mediterranean Sea that retained a broad connection with the Indian Ocean).

   
Size: The nearly completely known skeleton of Rodhocetus kasrani is approximately 2.5 meters long (8 feet).
   
Anatomy:

Rodhocetus, along with most other early whales, would not have resembled any modern mammals. The skull of Rodhocetus is very long and narrow, with differently shaped canines, premolars, and molars (heterodont condition). In modern toothed whales, the teeth are all nearly the same size and shape (homodont condition). Rodhocetus still possessed a well developed set of hindlimbs (which in fact are longer than the forelimbs); whereas modern cetaceans only have a vestigial pelvis and lack functional hindlimbs that protrude from the body. Some reconstructions of Rodhocetus depict it covered in fur (the cover image of the 2001 issue of the journal Science shows such a reconstruction; Gingerich, 2001); however, we show it here as being nearly hairless. Our inference is based on the phylogenetic distribution of hair in extant mammals; cetaceans essentially lack hair and that their closest extant relatives, hippos, have very sparse hair. This pattern suggests that the common ancestor of hippos and cetaceans, and its descendents such as Rodhocetus, also had sparse hair.

   
Locomotion: The skeletons of Rodhocetusare among the most important early whale fossils because of their completeness and the insights they provide into the transition in locomotion from life on land to life in the sea (the holotype of R. kasrani includes most of the vertebral column whereas that of R. balochistanensis has most of the limbs preserved). The skeletons exhibit an interesting combination of traits equipped for swimming or walking on land. The pelvis is attached to the vertebral column, which would enable the hindlimb to support the animal's body weight, although the pelvis is connected to fewer vertebrae in comparison to modern terrestrial mammals. Additionally, the vertebrae of the sacrum (the portion of the vertebral column that is connected to the pelvis) are not fused together, unlike is the condition seen in all land mammals. The unfused sacrum allows the vertebral column to be more flexible during swimming, at the expense of better support of the body on land. The vertebrae of the upper back have very tall spines like those of some land mammals, again suggesting that this whale may have been able to support its weight on land. Small bony crests on the toe bones imply that Rodhocetus had webbed feet (an obvious adaptation for swimming), but the toes themselves appear to have been hoof-shaped. The fore- and hindlimbs of Rodhocetus are relatively short as compared to those of most land mammals, particularly those adapted for running. Rodhocetus was initially thought to have swam like an otter, but more recently, a study of skeletal proportions found that Rodhocetus may move through the water more like a modern water desman (an aquatic relative of moles). Whereas otters undulate most of their wide body to swim, desmans push through the water by extending one webbed foot backward at a time. Additionally, Rodhocetus is the earliest known whale to be collected from sedimentary deposits representing relatively deep “outer shelf” environments, suggesting thatit is one of the earliest ‘open marine’ whale genera to be discovered. In general, the skeleton of Rodhocetus documents an important transitional stage of locomotion in whale evolution; although it inhabited marine environments, it retained the ability to move on land. Whether it definitively did so remains an open question.
   
Sensory Abilities:

Although the skull of Rodhocetus is fairly complete, few details have been published on its skull and ear region. The lower jaw has a large hole near the jaw joint (the mandibular foramen), which in modern toothed whales, is filled with fat ("the mandibular fat pad"). In living odontocetes, sound travels through the thin bone on the side of the jaw and is channeled into the middle and inner ear by the pad of fat. In land mammals (including humans), underwater hearing takes place by sound waves traveling through the bones of the skull to the ears. Sound traveling in this manner arrives at each ear at nearly the same time, and it is not possible to tell the direction from which the sound came. The fat pad in the lower jaw of Rodhocetus and some other cetaceans allows directional hearing when underwater. This indicates that Rodhocetus was well adapted to the aquatic environment.

   
Diet:

Like other early whales, Rodhocetus has distinct canines, premolars, and molars. The teeth in the snout (incisors and canines) are cone-shaped (like many modern fish-eating toothed whales), suggesting that Rodhocetus fed upon fish. The teeth in the back of the jaws (premolars and molars) are triangular, bear a few narrow cusps, and have nearly vertical wear facets that formed when the upper and lower teeth sheared against each other. This suggests that Rodhocetus chewed its meal into smaller pieces before swallowing, unlike most modern toothed whales, which swallow prey items whole. Carbon isotope analysis of the teeth of Rodhocetus suggests that it fed in areas of low salinity (such as estuaries and other shallow marine environments), in contrast to the deep ocean environment of deposition for the sedimentary rocks that entombed it.

   
Author:

Robert Boessenecker

   
References Consulted:

Clementz, M.T., Goswami, A., Gingerich, P.D., and P.L. Koch. 2006. Isotopic records from early whales and sea cows: contrasting patterns of ecological transition. Journal of Vertebrate Paleontology 26:355-370.

Gingerich, P.D. 2003. Land-to-sea transition in early whales: evolution of Eocene Archaeoceti (Cetacea) in relation to skeletal proportions and locomotion of living semiaquatic mammals. Paleobiology 29:429-454.

Gingerich, P.D., Raza, S.M., Arif, M., Anwar, M., and X. Zhou. 1994. New whale from the Eocene of Pakistan and the origin of cetacean swimming. Nature 368:844-847.

Gingerich, P.D., Haq, M. ul, Zalmout, I.S., Khan, I.H., and M.S. Malkani. 2001. Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan. Science 293:2239-2242.


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