Journal of Mammalian Evolution, Vol. 7, No. 2, 2000 1064-7554 / 00 / 0600-0081$18.00 / 0  2000 Plenum Publishing Corporation 81 Endocranial Volume of Mid-Late Eocene Archaeocetes (Order: Cetacea) Revealed by Computed Tomography: Implications for Cetacean Brain Evolution Lori Marino, 1,7 Mark D. Uhen, 2 Bruno Frohlich, 3 John Matthew Aldag, 1 Caroline Blane, 4 David Bohaska, 5 and Frank C. Whitmore, Jr. 6 The large brain of modern cetaceans has engendered much hypothesizing about both the intelligence of cetaceans (dolphins, whales, and porpoises) and the factors related to the evolution of such large brains. Despite much interest in cetacean brain evolution, until recently there have been few estimates of brain mass and / or brain–body weight ratios in fossil cetaceans. In the present study, computed tomography (CT) was used to visualize and estimate endocranial volume, as well as to calculate level of encephalization, for two fully aquatic mid-late Eocene archaeocete species, Dorudon atrox and Zygorhiza kochii. The specific objective was to address more accurately and more conclusively the question of whether relative brain size in fully aquatic archaeocetes was greater than that of their hypothesized sister taxon Mesonychia. The findings suggest that there was no increase in encephalization between Mesonychia and these archaeocete species. KEY WORDS: archaeocete; endocranial volume; encephalization; computed tomography; Cetacea. INTRODUCTION Cetaceans (dolphins, whales, and porpoises) are large brained, fully aquatic mammals second only to modern humans in relative brain size when brain–body allometry is taken into account (Marino, 1998; Ridgway and Tarpley, 1996; Worthy and Hickie, 1986). One measure of relative brain size is the encephalization quotient (EQ), a measure of rela- tive brain size, which takes into account the allometric relationship between brain and body size. The EQ values for at least five species (Grampus griseus, Delphinus delphis, Lagenorhynchus obliquidens, Tursiops truncatus, and Sotalia fluviatilis) within the fam- 1 Neuroscience and Behavioral Biology Program, Psychology Building, Emory University, Atlanta, Georgia 30322. 2 Department of Paleontology and Zoology, Cranbrook Institute of Science, Bloomfield Hills, Michigan. 3 Department of Anthropology, Smithsonian Institution, Washington, D.C. 4 Department of Radiology, University of Michigan, Ann Arbor, Michigan. 5 Department of Paleobiology, Smithsonian Institution, Washington, D.C. 6 U.S. Geological Survey and Department of Paleobiology, Smithsonian Institution, Washington, D.C. 7 To whom correspondence should be addressed. E-mail: lmarino@emory.edu