ECOGRAPHY 26: 567–572, 2003 Tooth row counts, vicariance, and the distribution of the sand tiger shark Carcharias taurus Luis O. Lucifora, Alberto L. Cione, Roberto C. Menni and Alicia H. Escalante Lucifora, L. O., Cione, A. L., Menni, R. C. and Escalante, A. H. 2003. Tooth row counts, vicariance, and the distribution of the sand tiger shark Carcharias taurus.– Ecography 26: 567 – 572. Geographic variation in tooth row counts among sand tiger sharks Carcharias taurus (Chondrichthyes), from the SW Atlantic, NW Atlantic and the East China Sea is analyzed in this paper. We found significant differences between sand tigers from the SW Atlantic (Southern Hemisphere population) and each of the other two (Northern Hemisphere) regions in the number of upper lateral tooth rows, and between individuals from the SW Atlantic and the East China Sea in the total number of upper tooth rows. Sand tiger sharks from the two Northern Hemisphere populations did not differ in any of the studied variables. Our results agree with comparisons of vertebral counts between sand tiger sharks from Southern and Northern Hemi- spheres. Both lines of evidence suggest that Southern and Northern Hemisphere populations of C. taurus were isolated to a larger extent than populations of the Northern Hemisphere. The fossil record of the genus Carcharias begins in the Early Cretaceous and C. taurus is certainly known since the Late Miocene. During the Miocene, the Tethys Sea separating northern and southern land masses was still present and it provided a continuous temperate shallow sea that could allow dispersal of sand tiger sharks along Northern Hemisphere seas. Independent observations on the distribution and evolutionary history of the genera Myripristis, Neoniphon, Sargocentron and Aphanius, and genetic studies on the temperate shark genus Mustelus that indicate a close relationship between the Indo-Pacific M. manazo and the Mediterranean M. asterias suggest that this hypothesis is plausible and deserves to be tested. L. O. Lucifora (lolucif@mdp.edu.ar), Inst. Nacional de Inestigacio ´n y Desarrollo Pesquero, Casilla de Correo 82, Correo Central, Mar del Plata 7600, Argentina.– A. L. Cione, Museo de La Plata, Dept Cientı ´fico Paleontologı ´a Vertebrados, Paseo del Bosque s /n, La Plata 1900, Argentina.– R. C. Menni, Dept Cientı ´fico Zoologı ´a Vertebrados, Paseo del Bosque s /n, La Plata 1900, Argentina.– A. H. Escalante, Uni. Nacional de Mar del Plata, Dept de Biologı ´a, Funes 3250 3er. Piso, Mar del Plata 7600, Argentina. The origin of disjunct geographic distributions is often difficult to explain. Usually they are the result of histor- ical and ecological processes operating together to pro- duce the observed pattern (Ridley 1993). Barriers (others than land) and continuities are more difficult to locate in the marine environment than on the land (Norris 2000). Hence, most examples of disjunct distri- butions due to vicariance come either from terrestrial or freshwater organisms. Examples include characoid fishes splitted when Gondwana broke up (Helfman et al. 1997) as well as exchanges of fauna through the Bering land bridge and the Great American Biotic Interchange (Marshall 1988, Cione and Tonni 1995). Disjunct distributions are common in the marine environment and there are several examples among sharks. Reif and Saure (1987) attempted to explain major patterns of shark distribution, offering a vicari- ance hypothesis to explain the disjunct distribution patterns of the genera Lamna Cuvier and Somniosus Le Sueur. They stated that vicariance could have played a Accepted 24 March 2003 Copyright © ECOGRAPHY 2003 ISSN 0906-7590 ECOGRAPHY 26:5 (2003) 567