The Rise of Whales
The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea. J. G. M. Thewissen, ed. 448 pp. Plenum, 1998. $115.
Just after the extinction of the dinosaurs, modern mammals began their evolutionary trek to the air, sea and new opportunities on land from a small, primitive insectivore-like ancestor. The whales and dolphins in the sea and the bats in the air show remarkable morphological divergence from this archetypal ancestor. In this half of the century, we have learned an extraordinary amount from discoveries, especially in whales, that challenge the adage that there are no missing links.
J. G. M. Thewissen has done an extraordinary service to cetacean paleontology in his compendium of evidence for missing links, evolutionary scenarios and phylogenetic relations. In 16 chapters by 24 authors, The Emergence of Whales maps the landscape surrounding one of the most fascinating examples of evolutionary change into new habitats. The book reviews both the origins of this group of the largest living mammals and the phylogenetic relations of ungulate mammals from a molecular perspective.
The first whales are known from the Indo-Pakistan region of the ancient Tethys seaway in early Eocene sediments dating to about 50 million years ago. Although a large margin of all Eocene material consists of skull and dental remains, enough skeletal limbs and vertebrae are known to demonstrate that the earliest whales retained hind legs that could have enabled them to move on land as well as in water. A trend toward reduction and loss of the hind limbs is seen in the later Eocene.
The most recent complete review of the primitive whales, the Archaeoceti, was done in 1936 by Remington Kellegg. In 1966 Leigh Van Valen hypothesized that whales had evolved from mesonychids, a primitive group of ungulates. The wealth of paleontological discoveries since the work of Van Valen and Kellogg is thoroughly reviewed by Ellen Williams and Mark Uhen. Here we find that whale paleontology is in its formative stage, with a burgeoning roster of discoveries that have been made only recently. Sunil Bajpai, Jans Thewissen and Richard Hulbert provide in-depth reviews of particular primitive whales and faunas.
Maureen O'Leary examines the information to be gained from the dentition of both the primitive whales and primitive animals in the mesonychids or closely allied to the group. She suggests that the evidence for mesonychid relations is not as compelling as previously supposed but that new knowledge on poorly known genera from the eastern Tethys region may hold the key to greater resolution.
John Gatesy, Michel Milinkovitch, Martine Berube and Per J. Palsboll provide strong molecular evidence that the whales are monophyletic and may be nested in the modern even-toed ungulate clade and order, the Artiodactyla. This in part contradicts the hypotheses garnered from the paleontological data that the mesonychids were whale ancestors and begs the question of whether the dental and other morphology of mesonychids and the early whales are simple convergences.
The Emergence of Whales ends with excellent summaries about our knowledge to date of early whale evolution. The book is a very good source of information on the early whales and is necessary reading for anyone involved in whale paleobiology and evolution. My only complaint is that the best artistic reconstructions of the early whales appear only on the cover. More illustrations of what these early whales looked like would have added much to a better understanding of how they changed through time.—Richard K. Stucky, Collections and Research, Denver Museum of Natural History