Reconstructing Cetacean Brain Evolution Using Computed Tomography
Total Page：16
File Type：pdf, Size：1020Kb
WellBeing International WBI Studies Repository 5-2003 Reconstructing Cetacean Brain Evolution Using Computed Tomography Lori Marino Emory University Mark D. Uhen Cranbook Institute of Science Nicholas D. Pyenson University of California - Berkeley Bruno Frohlich The Smithsonian Institution Follow this and additional works at: https://www.wellbeingintlstudiesrepository.org/acwp_vsm Part of the Animal Studies Commons, Other Animal Sciences Commons, and the Other Ecology and Evolutionary Biology Commons Recommended Citation Marino, L., Uhen, M. D., Pyenson, N. D., & Frohlich, B. (2003). Reconstructing cetacean brain evolution using computed tomography. The Anatomical Record Part B: The New Anatomist, 272(1), 107-117. This material is brought to you for free and open access by WellBeing International. It has been accepted for inclusion by an authorized administrator of the WBI Studies Repository. For more information, please contact [email protected]. THE ANATOMICAL RECORD (PART B: NEW ANAT.) 272B:107–117, 2003 FEATURE ARTICLE Reconstructing Cetacean Brain Evolution Using Computed Tomography LORI MARINO,* MARK D. UHEN, NICHOLAS D. PYENSON, AND BRUNO FROHLICH Until recently, there have been relatively few studies of brain mass and morphology in fossil cetaceans (dolphins, whales, and porpoises) because of difficulty accessing the matrix that fills the endocranial cavity of fossil cetacean skulls. As a result, our knowledge about cetacean brain evolution has been quite limited. By applying the noninvasive technique of computed tomography (CT) to visualize, measure, and reconstruct the endocranial morphology of fossil cetacean skulls, we can gain vastly more information at an unprecedented rate about cetacean brain evolution. Here, we discuss our method and demonstrate it with several examples from our fossil cetacean database. This approach will provide new insights into the little-known evolutionary history of cetacean brain evolution. Anat Rec (Part B: New Anat) 272B:107–117, 2003. © 2003 Wiley-Liss, Inc. KEY WORDS: computed tomography; CT; Cetacea; imaging; fossil; endocranial; brain; evolution; encephalization INTRODUCTION Mysticeti (comprising 11 living species popotamids are their extant sister of baleen whales) are first found in the group (Nikaido et al., 1996; Shimamura The origin and evolutionary history of fossil record in the latest Eocene et al., 1997; Gatesy, 1998; Milinkovitch Cetacea (dolphins, whales, and por- (Mitchell, 1989) and Odontoceti (com- et al., 1998). Recent fossil morphologic poises) represents one of the most dra- evidence confirms an artiodactyl–ceta- matic and provocative transformations prising 66 living species of toothed cean link from both early Eocene pro- in the fossil record. Cetacea consists of whales, dolphins, and porpoises) are tocetid (Gingerich et al., 2001) and pa- one extinct and two modern suborders. first found in the fossil record in the kicetid (Thewissen et al., 2001) whales The Eocene suborder, Archaeoceti, early Oligocene (Barnes et al., 1985). (Geisler and Uhen, 2003). See Figure 1 contained approximately 30 (described) Cetacean terrestrial ancestry is closely for a phylogenetic tree depicting the genera (updated from Thewissen, 1998) tied to that of ungulates (hooved mam- ranges and phylogenetic relationships and survived from the early Eocene, mals) and particularly Artiodactyla, the of extinct and extant cetacean families. around 53 million years ago (Ma) until “even-toed” ungulates. For many years, the late Eocene, around 38 Ma (Barnes molecular evidence has indicated that et al., 1985; Bajpai and Gingerich, 1998; cetaceans are embedded in the Uhen, 1998). Of the modern suborders, paraphyletic Artiodactyla and that hip- Some of the most significant evolutionary Dr. Marino is a senior lecturer in the Neu- Integrative Biology and Museum of Pale- changes that occurred roscience and Behavioral Biology Pro- ontology at the University of California, gram at Emory University in Atlanta, GA, Berkeley, is interested in evolutionary de- among cetaceans are in and a Research Associate at both the Liv- velopment, paleoecology, and major evo- ing Links Center for the Advanced Study lutionary adaptations. Dr. Frohlich is an brain size and structure. of Human and Ape Evolution at Yerkes anthropologist in the Department of An- Regional Primate Research Center and thropology at the National Museum of the National Museum of Natural History at Natural History, The Smithsonian Institu- the Smithsonian Institution in Washington, tion. His research includes the studying of D.C. Her research program is focused on mortuary practices in the Middle East and Skeletal fossils document the major comparative brain and behavioral evolu- in Alaska, and the application of computed transformations in cranial and post- tion studies of cetaceans (dolphins, tomography to the study of archaeological whales, and porpoises) and primates. Dr. artifacts, human remains, and paleobio- cranial morphology that occurred Uhen is the Head of Science and Curator logical specimens. throughout cetacean evolution (Gaskin, of Paleontology and Zoology at Cranbrook *Correspondence to: Dr. Lori Marino, De- Institute of Science. Dr. Uhen’s research partment of Psychology, Emory Univer- 1982; Barnes, 1985; Oelschlager, 1990; focuses on the origin and evolution of sity, Atlanta, GA 30322. Fax: 404-727-0378; Buchholtz, 1998; Luo, 1998). In addi- cetaceans (whales and dolphins), major E-mail: [email protected] tion, some of the most significant evo- evolutionary transitions in general, func- tional morphology, use of stratigraphic lutionary changes that occurred among data in phylogenetic analysis, and theoret- DOI 10.1002/ar.b.10018 cetaceans are in brain size and struc- ical aspects of diversification. Mr. Pyen- Published online in Wiley InterScience son, a graduate student in the Dept. of (www.interscience.wiley.com). ture. Numerous lines of evidence indi- cate that the terrestrial ancestors of © 2003 Wiley-Liss, Inc. 108 THE ANATOMICAL RECORD (PART B: NEW ANAT.) FEATURE ARTICLE Figure 1. Phylogenetic relationships among families of Cetacea. Families of the paraphyletic suborder Archaeoceti are shown in green, whereas those of Mysticeti are shown in blue, and those of the Odontoceti are shown in red. Ranges of families are shown in solid colored lines, and phylogenetic links are shown in dashed lines. Some families (such as the Basilosauridae) are paraphyletic in that they are thought to be ancestral to other groups of cetaceans (in this case, the Neoceti [Odontocetiϩ Mysticeti]). This figure is based on Figure 1 of Barnes et al. (1985). cetaceans were not particularly highly eral cetacean groups with EQs in the problem has been that the data and encephalized and possessed typically range of 4.0 to 5.0, possess enceph- analysis techniques for determining organized mammalian brains alization levels significantly higher the pattern of that dramatic change (Edinger, 1955; Gingerich, 1998). The than all other mammals except mod- from the terrestrial ancestral form to encephalization quotient (EQ) quanti- ern humans with EQs of 7.0 (Marino, the present form have not been avail- fies the actual brain size of an animal 1998) and evince evidence of a sub- able. compared to the expected brain size stantial degree of morphologic diver- of an animal of that body weight gence and cortical reorganization re- CETACEAN NEUROANATOMY within a given reference group. EQs sulting in a different elaborative mode FROM ENDOCASTS higher than 1 are greater than ex- from other mammals (Glezer et al., pected, and those less than 1 are lower 1988). Therefore, cetacean brains There have been relatively few esti- than expected. Presently, when com- have changed significantly through- mates of brain mass and/or brain– pared to other modern mammals, sev- out their evolution. A longstanding body mass ratios in fossil cetaceans FEATURE ARTICLE THE ANATOMICAL RECORD (PART B: NEW ANAT.) 109 because of difficulty accessing the lated over the decades from examina- ceans transformed from a terrestrial matrix that fills the endocranial cav- tions of either natural or artificial en- (Thewissen et al., 2001), to a semia- ity of fossil cetacean skulls, which is docasts. In the past few years, quatic (Gingerich et al., 2001), to a often very hard and difficult to re- computed tomography (CT) imaging fully aquatic creature (Uhen, 1998). move. Also, even when the matrix is has become a breakthrough investiga- Previous studies indicate that Eocene removed, it is difficult and time con- tive tool in the study of fossil endocra- cetaceans (archaeocetes) are not par- suming to make accurate artificial nia because it allows for nondestruc- ticularly encephalized when com- endocasts from which volume mea- tive visualization and measurement of pared with modern odontocetes (Gin- surements can be made. Several endocranial features and digital re- gerich, 1998; Marino et al., 2000). early estimates of brain mass from construction of specimens. CT in- Marino et al. (2000) was the first study endocranial casts have been pub- volves the application of a collimated of cetacean encephalization to use CT lished (Dart, 1923; Marples, 1949; series of x-rays through the target ob- methodology to visualize, measure, Breathnach, 1955). More recently, ject to produce a series of sectional and reconstruct endocranial features Gingerich (1998) used natural endo- images, called tomographs, which re- of archaeocete fossils. The value of CT casts to reinterpret brain and body flect the radiographic densities