in the fossil record. Evolution is dynamic, 

 but change doesn't happen in a flash. 

 Thus, we can expect to unearth many 

 more missing links. 



Further rungs in the cetacean evolution- 

 ary ladder have already come to hght. Re- 

 cently, paleontologists Hans Thewissen, 

 Taseer Hussain, and Muhammad Arif 

 were working in Pakistan when they found 

 a partial skeleton of a brand-new species 

 of a forty-nine-million-year-old Tethyan 

 archaeocete, with important parts of both 

 front and hind limbs. The femur, or thigh 

 bone, is large, hke that of a land mammal, 

 but the feet are long, like those of a seal. 

 The scientists named the animal Ambulo- 



cetus natans, "the walking whale that 

 swam," in recognition of its amphibious 

 nature. Ambulocetiis was possibly like an 

 otter or seal in its behavior. It most likely 

 came ashore to breed and give birth. Using 

 its flipperlike front limbs, it may have 

 moved about on land by hitching itself for- 

 ward, siinilar to the way a sea Hon moves 

 on land. Its hind legs and feet evidently 

 propelled it through the water when it re- 

 turned to Tethys to feed on marine fare. 



The Tethyan sediments of Pakistan con- 

 tinue to be a mine of ancient whale re- 

 mains. In December of 1992, University 

 of Michigan graduate student Xiaoyuan 

 Zhou found an archaeocete about forty- 



eight million years old in sediments that 

 were deposited in deeper water tiian all 

 older finds. It has a nearly complete verte- 

 bral column, a small femur, and short neck 

 vertebrae, indicating some streamlining of 

 the head and body. Land mammals and 

 some early fossil whales have fused sacral 

 vertebrae and therefore rather stiff hips 

 and tails, but this creature's sacral verte- 

 brae were not fused, giving its back and 

 tail a flexibility approaching that of later 

 whales. It is thus an important link in the 

 transition to fully whalelike swimming, in 

 which the animals undulate their body and 

 move their fluke up and down. 



In the same month, Muhammad Arif 

 and I were again scouring the shallow 

 Tethyan sediments of Pakistan when he 

 found two forty-seven-million-year-old 

 partial skulls and skeletons of a previously 

 poorly known whale ca&tA Indocetus. The 

 new fossils showed that the animal was 

 long necked and still had long hind limbs, 

 a rigid sacrum, and a robust tail. As in our 

 earher Pakicetus, we saw many similari- 

 ties between this primitive animal and 

 land mammals known as mesonychids. A 

 varied group ranging from cat size to bear 

 size, mesonychids lived between sixty and 

 thirty-seven miUion years ago. They were 

 principally carnivorous scavengers. 



Was the first land mammal to return to 

 the sea and start the wheels of whale evo- 

 lution a mesonychid? This theory — origi- 

 nally put forth in the 1960s by Leigh Van 

 Valen, an American Museum graduate 

 student at the time — is based on similari- 

 ties in tooth structure. Subsequent discov- 

 eries, particularly of similarities in whale 

 and mesonychid skeletal structure, have 

 upheld this view. 



As the fossil record of early whales con- 

 tinues to grow, our knowledge of tiie evo- 

 lution of advanced cetacean traits be- 

 comes clearer and more complete. Fossils 

 contradict the notion that whales suddenly 

 appeared full-blown, without intermediate 

 forms. I am a skeptical soul, but I have 

 seen a lot of Tethys and excavated a lot of 

 whales in the past fifteen years. Intermedi- 

 ates, missing links, are everywhere. 



Along the coast of Tethys, an 



ancient warm ocean that 



stretched from Spain to 



Indonesia, an undulating 



Basilosaurus catches fish with 



its four-foot-long jaws and 



battery of sharp teeth. 



Painting by F. Heimberg; coiiection of G. Pilleri 



Natural History 4/94 



