The Origin of Whales:

Evolution, Fossils, and Taxonomy

A seminar by Elizabeth Faris Crowell

ABSTRACT

Whales, dolphins, and porpoises, which make up the order Cetacea, are known to be mammals, yet their relationship to other hairy, warm-blooded creatures has long been a mystery. The extensive morphological changes required to make the transition from land to sea make cetacean origins difficult to decipher. Modern toothed and baleen whales are beautifully adapted to their marine environment, a testimony to what 50 million years of evolution can do.

The fossil record of cetaceans is well documented, generally supporting a sister relationship between Cetacea and Mesonychia. The mesonychians are a group of extinct, primitive hoofed mammals. However, the morphological evidence conflicts strongly with molecular evidence, which curiously places cetaceans as a close relative of the artiodactyls, or even-toed ungulates. In the past ten years several molecular and DNA studies have further shaken the phylogenetic tree, suggesting that order Artiodactyla is paraphyletic, and should include the cetaceans. Most of these studies indicate that hippos and whales are sister taxa. While new paleontological evidence published just last year supports the relationship of Cetacea to Artiodactyla, nesting Cetacea within the latter order is still highly contested.

REFERENCES

(A brief summary of the content of each paper or article is included in bold after the citation.)

Thewissen, J.G.M., E.M. Williams, L.J. Roe, and S.T. Hussain. 2001. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature. 413:277-281.

Discovery of Pakicetus attocki and Ichthyolestes pinfoldi fossils with artiodactyl astragali enrich the morphological data set, leading to a consensus cladogram supporting the Artiodactyla hypothesis.

De Muizon, C. 2001. Walking with whales. Nature. 413:259-260.

Christian de Muizon comments on Thewissen’s paper (2001).

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

Discovery of Artiocetus clavis and Rodhocetus balochistanensis fossils with artiodactyl astragali.

Gatesy, J. and M.A. O’Leary. 2001. Deciphering whale origins with molecules and fossils. Trends in Ecology & Evolution. 16:562.

Discussion of the conflict between paleontological and molecular evidence.

Luo, Z. 2000. In search of the whale’s sisters. Nature. 404:235-239.

Zhexi Luo evaluates the reliability of morphological vs. molecular characters and their respective homoplasies.

O’Leary, M.A. and J.H. Geisler. 1999. The position of Cetacea within Mammalia: phylogenetic analysis of morphological data from extinct and extant taxa. Syst. Biol. 48:455-490.

Powerful evidence against trusting molecular evidence based on the inaccessibility of extinct taxa for molecular sampling.

Wong, K. 1999. Cetacean creation. Scientific American.

Summary of molecular and paleontological evidence up to that point and comments on O’Leary’s paper.

Thewissen, J.G.M. and S.I. Madar. 1999. Ankle morphology of the earliest cetaceans and its implications for the phylogenetic relations among ungulates. Syst. Biol. 48:21-30.

Character analysis using tarsals from new fossils show Cetacea Artiodactyla sister group relations.

Gatesy, J., M. Milinkovitch, V. Waddell, and M. Stanhope. 1999. Stability of Cladistic Relationships between Cetacea and Higher-Level Artiodactyl Taxa. Syst. Biol. 48:16-20.

IRBP and vWF genes are added to known DNA data for cetaceans and artiodactyls and the resulting clades are tested for stability.

Nikaido, M., A.P. Rooney, and N. Okada. 1999. Phylogenetic relationships among cetartiodactyls based on insertions of short and long interspersed elements: Hippopotamuses are the closest extant relatives of whales. Proc. Natl. Acad. Sci. USA. 96:10261-10266.

Study of SINEs and LINEs indicates paraphyly of Artiodactyla and Suiformes, supporting a clade for Cetacea and Hippopotamidae.

Thewissen, J.G.M., S.I. Madar, and S.T. Hussain. 1998. Whale ankles and evolutionary relationships. Nature. 395:452.

Fragmentary astragali believed to belong to a Pakicetid and an Ambulocetid show a mix of artiodactyl and mesonychian features.

Ursing, B.M. and U. Arnason. 1998. Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade. Proc. R. Soc. Lond. 265:2251-2255.

Entire mitochondrial genome sequences indicate paraphyly of Artiodactyla and Suiformes, supporting a clade for Cetacea and Hippopotamidae.

Bajpai, S. and P.D. Gingerich. 1998. A new Eocene archaeocete (Mammalia, Cetacea) from India and the time of origin of whales. Proc. Natl. Acad. Sci. USA. 95:15464-15468.

The discovery of Himalayacetus subathuensis.

Monastersky, R. 1998. Fossil jaw tells tale of whale evolution. Science News. 154:229.

Article about the controversy surrounding Himalayacetus.

Shimamura, M., H. Yasue, K. Ohshima, H. Abe, H. Kato, T. Kishiro, M. Goto, I. Munechika, and N. Okada. 1997. Molecular evidence from retroposons that whales form a clade within even-toed ungulates. Nature. 388:666-670.

The first evidence from SINEs of a whale+hippo clade.

Milinkovitch, M.C. and J.G.M. Thewissen. 1997. Even-toed fingerprints on whale ancestry. Nature. 388:622-624.

Milinkovitch and Thewissen comment on Shimamura’s paper and consider how morphological data might be reevaluated.

Gatesy, J. 1997. More DNA support for a Cetacea-Hippopotamidae clade: the blood clotting protein gene gamma-fibrinogen. Molec. Biol. Evol. 14:537-543.

The gamma-fibrinogen gene study further supporting whale+hippo.

Gatesy, J., C. Hayashi, M.A. Cronin, and P. Arctander. 1996. Evidence from milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls. Molec. Biol. Evol. 13:954-963.

The milk casein gene study supporting whale+hippo clade.

Graur, D. and D.G. Higgins. 1994. Molecular evidence for the inclusion of cetaceans within the order Artiodactyla. Molec. Biol. Evol. 11:357-364.

Landmark paper indicating paraphyly of Artiodactyla by showing Cetacea and ruminants form a clade (hippos were not available for sampling in this study.)

Gingerich, P.D., B.H. Smith, E.L. Simons. 1990. Hind limbs of Eocene Basilosaurus: evidence of feet in whales. Science. 249:154-156.

Paraxonic foot shows evidence of relationship to mesonychids and artiodactyls because both these groups have a paraxonic foot.

Van Valen, L.M. 1966. Detratheridia, a new order of mammals. Am. Mus. Nat. Hist. Bull. 132:1-126.

The historical paper where Van Valen attempts to reconcile Boyden and Gemeroy’s molecular evidence with the fossil record, suggesting a mesonychian ancestry of cetaceans.

Boyden, A. and D. Gemeroy. 1950. The relative position of the Cetacea among the orders of Mammalia as indicated by precipitin tests. Zoologica. 35:145.

A landmark study using immunological characters shows a close relationship between artiodactyls and cetaceans.

BOOKS

Thewissen, J.G.M. 1998. The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea. New York: Plenum Publishers.

A great book with sections contributed by most of the leading researchers who authored the articles listed above.

Freeman, S. and J.C. Herron. 2001. Evolutionary Analysis 2nd edition. Upper Saddle River, NJ: Prentice Hall.

An entire chapter devoted to exploring parsimony analysis using the problem of placing Cetacea in Artiodactyla as an example.

ONLINE RESOURCES

For photos and a comprehensive taxonomy of modern cetaceans: The Tethys Research Laboratory. http://www.tethys.org.

Brief survey of selected fossil archaeocetes: Introduction to the Cetacea. http://www.ucmp.berkeley.edu/mammal/cetacea/cetacean.html.

Dr. Thewissen’s research group online: http://neoucom.edu/Depts/ANAT/Thewissen.html.

Dr. Uhen’s Cetacean Research: http://www-personal.umich.edu/~uhen/CetRes.html.