The cycle is therefore partly decoupled from the oxidation of organic matter or the reduc- tion of electron acceptors. The thiosulfate shunt can help account for the large isotopic fractionation of 34S to 32S between sulfate and sulfides in sediments. It has long been an enigma why the apparent isotopic fractionation during sulfate reduc- tion in marine sediments is commonly be- tween 35 and 60 per mil whereas pure cultures of sulfate reducing bacteria cause a fractionation of only 10 to 30 per mil (15). Recycling via thiosulfate will tend to in- crease the isotopic difference between sulfate and reduced S, mostly because the inner (+6) S atom of thiosulfate is strongly en- riched in 34S relative to the outer (-2) S atom (16). When the S2032- is dispropor- tionated, the 34S-rich inner atoms are con- verted to s042- while the 34S-poor outer atoms are converted to HS-. The thiosulfate shunt will thereby tend to recycle lighter S in the reduced state and convert heavier S to sulfate. REFERENCES AND NOTES 1. B. B. J0rgensen, Nature 296, 643 (1982); P. M. Crill and C. S. Martens, Geochim. Cosmochim. Acta 51, 1175 (1987); J. P. Christensen, Cont. Shelf Res. 9, 223 (1989). 2. B. B. J0rgensen, M. Bang, T. H. Blackburn, Mar. Ecol. Prog. Ser. 59, 39 (1990). 3. R. W. Howarth, Biogeochemistry 1, 5 (1984); R. C. Aller and P. D. Rude, Geochim. Cosmochim. Acta 52, 751 (1988). 4. Yu. I. Sorokin, J. Cons. Cons. Int. Explor. Mer 34, 423 (1972); F. J. Millero, Mar. Chem. 18, 121 (1986); C. 0. Moses, D. K. Nordstrom, J. S. Herman, A. L. Mills, Geochim. Cosmochim. Acta 51, 1561 (1987). 5. F. Bak and H. Cypionka, Nature 326, 891 (1987). 6. F. Bak and N. Pfennig, Arch. Microbiol. 147, 184 (1987). 7. H. Fossing and B. B. J0rgensen, in preparation. 8. B. B. Jorgensen, unpublished data. 9. and F. Bak, unpublished data. 10. M. Kramer and H. Cypionka, Arch. Microbiol. 151, 232 (1989). 11. F. Bak, thesis, University of Konstanz, GFR (1988). 12. F. Widdel, in Biology of Anaerobic Microorganisms, A. J. B. Zehnder, Ed. (Wiley, New York, 1988), pp. 469-585. 13. D. P. Kelly, in Autotrophic Bacteria, H. G. Schlegel and B. Bowien, Eds. (Science Tech, Madison, 1989), pp. 193-218. 14. G. W. Luther III et al., Science 232, 746 (1986). 15. L. A. Chambers and P. A. Trudinger, Geomicrobiol. J. 1, 249 (1979). 16. F. Uyama, H. Chiba, M. Kusakabe, H. Sakai, Ceochem. J. 19, 301 (1985). 17. Results are from a single but representative slurry experiment. The isotopes were added to two sepa- rate but identical slurries kept under N2 at 22'C. The sandy mud had an organic content of 1.8% dry weight. In situ temperature was 4'C and salinity was 15 per mil. The in situ s042- concentration was 13 mM, and S2032- was below the detection limit by ion chromatography, 10 F.M. An unlabeled pool, 75 ,uM, of S203 - was therefore added to the two identical slurries. The sediment contained 81 p.M HS- plus 8.2 ,umol of FeS and 19.9 p.mol of FeS2 per gram of slurry. Subsamples were taken without air contact, fixed with ZnCI2, and centrifuged. Ra- diolabeled S2032- and s042- were separated in the supernatant by ion chromatography and counted. Reduced 35S (Sred) comprised HS-, S52-, S', FeS, and FeS2 and was all collected by H2"S distillation from washed sediment by the chromium reduction technique (18) and counted. Radioactivities are ex- pressed as percent of added 35S2032- radioactivity. 18. N. N. Zhabina and I. I. Volkov, in Environmental Biogeochemistry and Geomicrobiology, W. E. Krumbein, Ed. (Ann Arbor Science, Ann Arbor, MI, 1978), vol. 3, pp. 735-746; J. T. Westrich, thesis, Yale University, New Haven (1983). 19. Sediment cores were incubated for 18 hours at the in situ temperature (5.5'C). The sediment was sandy mud from 33-m water depth; organic content was 4% dry weight. The 35S-tracer techniques are de- W HALES ARE REMARKABLE among mammals in being fully aquatic, and their transition from land to sea is among the most interesting problems of evolution (1-3). Most mam- mals use limbs, particularly hind limbs, in locomotion. Modem cetaceans live in water and lack hind limbs entirely, retaining only rod-like vestiges of pelvic bones, femora, and rarely tibiae embedded in musculature of the ventral body wall (4, 5). Limbs are important for understanding the early evo- lution of whales. Hind limb buds have long been known in embryonic cetaceans up to 32-mm crown-rump length (6), and adults with extemally projecting rudiments are also known (7). We now describe evidence of functional hind limbs in a cetacean. Basilosaurus is a large serpentine Eocene vertebrate discovered early in the 19th cen- tury when it was described as a reptile and named "king lizard" (8). Richard Owen demonstrated the mammalian characteristics of Basilosaurus and, within mammals, its cetacean affinities (9). Two species are known: B. cetoides from the late Eocene of the southeastern United States and B. isis from the late middle Eocene of Egypt (10, 11). The most complete Basilosaurus speci- mens known previously were two partial P. D. Gingerich, Museum of Paleontology, University of Michigan, Ann Arbor, MI 48109-1079. B. H. Smith, Museum of Anthropology, University of Michigan, Ann Arbor, MI 48109-1079. E. L. Simons, Duke University Primate Center and Department ofAnatomy and Anthropology, 3706 Erwin Road, Durham, NC 27705. scribed in (7, 20). 20. H. Fossing and B. B. J0rgensen, Biogeochemistry 8, 205 (1989). 21. I thank D. T. Thomsen for technical assistance and F. Bak and H. Fossing for inspiring discussions. I am grateful to B. Kruse for an invitation to join a cruise on RV Gunnar Thorson in Kattegat. Financial support was given by the Danish Ministry of the Environment and the Danish Natural Science Re- search Council. 23 February 1990; accepted 2 May 1990 skeletons of B. cetoides collected by C. Schu- chert in Alabama in 1894 and 1896 for the U.S. National Museum (USNM). One of these specimens, USNM 12261, includes left and right innominate bones of the pelvis and a partial femur (12, 13); these remains were considered vestigial and functionless (14), an interpretation consistent with loss of functional hind limbs in modem whales. In 1987 and 1989 we mapped 243 partial skeletons of B. isis and 77 partial skeletons of smaller archaeocetes (15) in the desert of Zeuglodon Valley (ZV), 50 km west of Fayum oasis in north central Egypt (16). All occur in a 90-m-thick stratigraphic section of shallow marine sandstones and shales of the Gehannam and Birket Qarun formations of late middle Eocene age (17). Excavations in 1989 yielded several nearly complete skel- etons combined in the reconstruction shown in Fig. IA. These indicate that B. isis had 7 cervical, 18 thoracic, and 42 lumbar and caudal vertebrae (Fig. 2), 9 more than the number of vertebrae shown in reconstruc- tions of B. cetoides (13, 14). Limb and foot bones described here were all found in direct association with articulated skeletons of B. isis and undoubtedly represent this species. Specimens are conserved in the Cairo Geo- logical Museum (CGM) and University of Michigan Museum of Paleontology (UM). The innominate (Fig. 1B) is a straplike coossification of an elongated pubis and a relatively small ilium and ischium, each con- tributing to a well-defined acetabulum. The pubis and ischium surround a small obtura- tor foramen. Left and right pubes fit togeth- SCIENCE, VOL. 249 Hind Limbs of Eocene Basilosaurus: Evidence of Feet in Whales PHILIP D. GINGERICH, B. HOLLY SMITH, ELWYN L. SIMONS New specimens of middle Eocene Basilosaurus isis from Egypt include the first functional pelvic limb and foot bones known in Cetacea. These are important in corroborating the intermediate evolutionary position of archaeocetes between general- ized Paleocene land mammals that used hind limbs in locomotion and Oligocene-to- Recent whales that lack functional pelvic limbs. The foot is paraxonic, consistent with derivation from mesonychid Condylarthra. Hind limbs of Basilosaurus are interpreted as copulatory guides. 154- on December 20, 2012 www.sciencemag.org Downloaded from