A fifty-foot Eocene whale, Basilosaurus isis,/ram the Zeuglodon Valley 

 of Egypt, Itad tiny hind limbs, shown in detail above. 



Adapted from Science, vol. 249, 13 July 1990 



matched life's genealogy by spanning the 

 gaps in sequential steps. Consider the four 

 main events in chronological order. 



Case One: Discovery of the oldest 

 whale. Paleontologists have been fairly 

 confident, since Leigh Van Valen's 

 demonstration in 1966, that whales de- 

 scended from mesonychids, an early 

 group of primarily carnivorous running 

 mammals that spanned a great range of 

 sizes and habits from eating fishes at river 

 edges to crushing bones of carrion. 

 Whales must have evolved during the 

 Eocene epoch, some fifty million years 

 ago, because late Eocene and Oligocene 

 rocks already contain cetaceans so fully 

 marine that we must judge them as past 

 any point of intermediacy. 



In 1983, my colleague Phil Gingerich, 

 of the University of Michigan, along with 

 N. A. Wells, D. E. Russell, and S. M. 

 Ibrahim Shah ("Origin of Whales in Epi- 

 continental Remnant Seas," Science, vol. 

 220, pp. 403^06), reported their discov- 

 ery of the oldest whale, named Pakicetus 

 to honor its country of present residence, 

 from Middle Eocene sediments some fifty- 

 two million years old in Pakistan. In terms 

 of intermediacy, one could hardly have 

 hoped for more from the limited material 

 available, for only the skull of Pakicetus 

 has been found. The teeth strongly resem- 

 ble those of terrestrial mesonychids, as an- 

 ticipated, but the skull, in feature after fea- 

 ture, clearly belongs to the developing 

 lineage of whales. 



Both the anatomy of the skull, particu- 

 larly in the ear region, and its environment 

 of deposition testify to transitional status. 

 The ears of modem whales contain modi- 



fied bones and passageways that permit di- 

 rectional hearing in the dense medium of 

 water. They have also evolved enlarged si- 

 nuses that can be filled with blood to main- 

 tain pressure during diving. The skull of 

 Pakicetus lacks both these featiu^es, and 

 this first whale could neither dive deeply 

 nor hear directionally with any efficiency 

 in water. 



hi 1993, J. G. M. Thewissen and S. T 

 Hussain ("Origin of Underwater Hearing 

 in Whales," Nature, vol. 361, pp. 444-45) 

 affirmed these conclusions and added 

 more details on the intermediacy of skull 

 architecfiire in Pakicetus. Modem whales 

 do much of their hearing through their 

 jaws, as sound vibrations pass through the 

 jaw to a "fat pad" (the technical literature, 

 for once, invents no jargon and employs 

 the good old English vernacular in naming 

 this structure) and thence to the middle 

 ear. Terrestrial mammals, by contrast, de- 

 tect most sound through the ear hole 

 (called the "external auditory meatus," in 

 more refined language). Since Pakicetus 

 lacked the enlarged jaw hole that holds the 

 fat pad, this first whale probably continued 

 to hear through the pathways of its terres- 

 trial ancestors. Gingerich concluded that 

 "the auditory mechanism oi Pakicetus ap- 

 pears more similar to that of land mam- 

 mals than it is to any group of extant ma- 

 rine mammals." 



As for place of discovery, Gingerich 

 and colleagues found Pakicetus in river 

 sediments bordering an ancient sea {see 

 "The Whales of Tethys," Natural History, 

 April 1994) — an ideal place for the first 

 stages of such an evolutionary transition 

 (and a good explanation for lack of diving 



specializations if Pakicetus inhabited the 

 mouths of rivers and adjacent shallow 

 seas). They judged Pakicetus as "an am- 

 phibious stage in the gradual evolutionary 

 transition of primitive whales irom land to 

 sea.... Pakicetus was well equipped to 

 feed on fishes in the surface waters of shal- 

 low seas, but it lacked auditory adapta- 

 tions necessary for fully marine exis- 

 tence." 



Verdict: In terms of intermediacy, one 

 could hardly hope for more from the lim- 

 ited material of skuU bones only. But the 

 limit remains severe, and the results there- 

 fore inconclusive. We know nothing of the 

 limbs, tail, or body form of Pakicetus, and 

 therefore cannot judge its transitional sta- 

 tus in these key features of anyone's ordi- 

 nary conception of a whale. 



Case Two: Discovery of the first com- 

 plete hind limb in a fossil whale. In the 

 most famous mistake of early American 

 paleontology, Thomas Jefferson, while not 

 engaged in other pursuits usually judged 

 more important, misidentified the claw of 

 a fossil ground sloth as that of a lion. My 

 prize for second worst error must go to R. 

 Harlan who, in 1834, named a marine fos- 

 sil vertebrate Basilosaurus in the Transac- 

 tions of the American Philosophical Soci- 

 ety. Basilosaurus means "king lizard," but 

 Harlan's creature is an early whale. 

 Richard Owen, England's greatest 

 anatomist, corrected Mr. Harlan before the 

 decade's end, but the name sticks — and 

 must be retained by the official rules of zo- 

 ological nomenclature. (Remember that 

 the Linnaean naming system is a device 

 for information retrieval, not a guarantor 

 of appropriateness. The rules require that 

 each species have a distincfive name, so 

 that data can be associated unambiguously 

 with a stable tag. Often, and inevitably, the 

 names originally given become literally 

 inappropriate for the unsurprising reason 

 that scientists make frequent mistakes, and 

 that new discoveries modify old concep- 

 tions. If we had to change names every 

 time our ideas about a species altered, tax- 

 onomy would devolve into chaos. So 

 Basilosaurus will always be Basilosaurus 

 because Harlan followed the rules when he 

 gave the name. And we do not change our- 

 selves to Homo horribilis after Auschwitz, 

 or to Homo ridiculosis after Tonya Hard- 

 ing — but remain, however dubiously. 

 Homo sapiens, now and into whatever for- 

 ever we allow ourselves.) 



Basilosaurus, represented by two spe- 

 cies, one from the United States and the 

 other from Egypt, is the "standard" and 

 best-known early whale. A few fragments 



12 Natural History 5/94 



