Journal of Mammalian Evolution

, Volume 7, Issue 2, pp 81-94

First online:

Endocranial Volume of Mid-Late Eocene Archaeocetes (Order: Cetacea) Revealed by Computed Tomography: Implications for Cetacean Brain Evolution

  • Lori MarinoAffiliated withNeuroscience and Behavioral Biology Program, Psychology Building, Emory University
  • , Mark D. UhenAffiliated withDepartment of Paleontology and Zoology, Cranbrook Institute of Science
  • , Bruno FrohlichAffiliated withDepartment of Anthropology, Smithsonian Institution
  • , John Matthew AldagAffiliated withNeuroscience and Behavioral Biology Program, Psychology Building, Emory University
  • , Caroline BlaneAffiliated withDepartment of Radiology, University of Michigan
  • , David BohaskaAffiliated withDepartment of Paleobiology, Smithsonian Institution
  • , Frank C. WhitmoreJr.Affiliated withU.S. Geological Survey and Department of Paleobiology, Smithsonian Institution

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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.

archaeocete endocranial volume encephalization computed tomography Cetacea