he Who 

 T< 



les 



by Philip D. Gingerich 



In Greek mythology, Tethys is the wife 

 of Oceanus and a sea goddess in her own 

 right. About a hundred years ago, geolo- 

 gists appropriated her name for the ancient 

 sea that once divided the earth's great 

 northern and southern continents. Today 

 the Mediterranean is a mere suggestion of 

 what Tethys must have been in its time. 

 Stretching from what is now Spain to In- 

 donesia, Tethys was an ocean when trilo- 

 bites and other early forms of life flour- 

 ished, and it lasted more than 500 million 

 years, through the Age of Dinosaurs and 

 into the Age of Mammals. The inexorable 

 drift of continental plates finally obliter- 

 ated Tethys. India and central Asia con- 

 verged and raised the Himalayas; Arabia 

 pushed into western Asia and uplifted the 

 Zagros; Africa encroached on Europe and 

 raised the Alps. Tethyan sea sediments 

 now lie dry and exposed in the Sahara 

 Desert and in the folded foothills of the 

 Himalayas and the Alps. 



Extensive and relatively shallow, the 

 waters of Tethys would have been warm 

 and well stocked with fish and moUusks. It 

 must also have been inviting to mammals 

 that lived at its edge. Three hundred mil- 

 lion years after vertebrates first colonized 

 land, some mammals reversed their pat- 

 tern and returned to Tethys. Today, the de- 

 scendants of those seagoing pioneers — 

 toothed porpoises and dolphins and the 

 toothed and baleen whales that make up 

 the order Cetacea — have adapted fully to 

 life in water. All have a streamlined body, 

 a blowhole or pair of holes on the top of 

 the skull for breathing, simplified teeth 



(replaced in some by keratinous baleen), a 

 specialized system for underwater hear- 

 ing, and locomotion powered by a fluked 

 tail instead of by limbs or flippers. These 

 advanced cetacean features were acquired 

 in steps over time, but the prototype was a 

 land mammal living on the shores of 

 Tethys. 



The oldest-known fossil whales come 

 from the Kuldana Formation, a stratum of 

 rocks in northern Pakistan deposited by 

 ancient rivers and sandwiched between 

 Tethyan marine formations. In the Eocene, 

 some fifty million years ago, Tethys could 

 not have been far downriver from this site. 

 In 1979, 1 led an international team from 

 the United States, France, and Pakistan to 

 search the Kuldana Formation for fossils 

 of early land mammals. One December 

 day, Jean-Louis Hartenberger, a rodent 

 specialist, hammered open a rock with 

 what looked like a small bone on its sur- 

 face; the bone turned out to be a crest on 

 the back of a beautifuUy complete fossil 

 skull. Because the skull was relatively 

 large but the braincase was small, we 

 began to suspect that he had found an ar- 

 chaeocete, a member of an ancient family 

 of whales. These now-extinct relatives of 

 today's toothed and baleen whales had 

 some modem features, such as a dense 

 tympanic bone for hearing, but lacked 

 many ofliers, including a blowhole on the 

 top of the skull. 



When I returned to my lab at the Uni- 

 versity of Michigan, my coUeagues and I 

 removed the remaining rock from the new 

 skull. The configuration of bones in the 



Basilosaurus, an ancient whale that lived some forty 



million years ago, had front flippers and tiny 



but functional hind limbs, complete with thigh, femur, 



and three toes. It may have used its feet to guide 



its fifty foot-long body during copulation. A fossil 



of the whale was unearthed in 1989 in what is 



now the Egyptian Sahara. 



Painting by Marianne Collins; © 1993, W. W. Norton and Company 



skull base confirmed that it was indeed an 

 archaeocete; we named it Pakicetus in- 

 achus. We later speculated that this dog- 

 sized whale first entered Tethys from its 

 riverside home to take advantage of easy 

 fishing in the warm waters. 



Pakicetus, which lived about fifty mil- 

 lion years ago, had not evolved the ability 

 to hear directionally, or perhaps to hear 

 well at all, in water, a hallmark of modem 

 whales. Archaic features such as this, 

 along with its discovery among remains of 

 land animals, makes Pakicetus a very 

 primitive whale indeed. In time and in its 

 morphology, Pakicetus is perfectly inter- 

 mediate, a missing link between earlier 

 land mammals and later, full-fledged 

 whales. 



Our unexpected discovery and our sub- 

 sequent investigation of Pakicetus made 

 me realize how little was known about the 



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86 Natural History 4/94 



