which might endure well beyond the period when the toxic sulphide gas is present, that could pose the principal ecological prob- lem. An event, first clearly noted on 17 May 2001, was still plainly visible as late as 6 June 2001. Scarla J. Weeks * , Bronwen Currie†, Andrew Bakun‡ *Ocean Space Ltd, and ‡IRD, IDYLE Project, Oceanography Department, University of Cape Town, Rondebosch 7701, South Africa e-mail: oceanspace@icon.co.za †National Marine Research and Information Center, Swakopmund, Namibia 1. Bailey, G. W., Beyers, C. J. deB. & Lipschitz, S. R. S. Afr. J. Mar. Sci. 3, 197–214 (1985). 2. Mas-Riera, J., Lombarte, A., Gordoa, A. & Macpherson, E. Marine Biol. 104, 175–182 (1990). 3. Hamukuaya, H., O’Toole, M. J. & Woodhead, P. M. J. S. Afr. J. Mar. Sci. 19, 57–59 (1998). 4. Wooster, W. S. & Reid, J. L. in The Sea Vol. 2 (ed. Hill, M. N.) 253–280 (Interscience, New York, 1963). 5. Tyrrell, T., Holligan, P. M. & Mobley, C. D. J. Geophys. Res. 104, 3223–3241 (1999). Competing financial interests: declared none. Biomechanics Dinosaur locomotion from a new trackway A rdley Quarry in Oxfordshire, UK, contains one of the most extensive dinosaur-trackway sites in the world, with individual trackways extending for up to 180 metres. We have discovered a unique dual-gauge trackway from a bipedal thero- pod dinosaur from the Middle Jurassic in this locality, which indicates that these large theropods were able to run and that they used different hindlimb postures for walk- ing and running. Our findings have impli- cations for the biomechanics and evolution of theropod locomotion. The Ardley trackways are preserved on a single horizon of the Middle Bathonian (163 million years old) 1 white-limestone formation and include those from large theropods and at least two types of sauro- pod dinosaur. Three of the trackways (numbered 13, 29 and 80; J.J.D., un- published observations) comprise large tridactyl (‘three-toed’) prints, with narrow claw impressions typical of theropod dinosaurs (Fig. 1). All three trackways (except for a section of track 13) are wide- gauge, with the prints indicating that the hind feet were placed sequentially in a zig-zag arrangement separated widely from the midline (Fig. 1a). Stride length (2.70 m) and pace angulation (117°–132°; Fig. 1) remain constant throughout the wide- gauge tracks. This pattern contrasts strong- ly with the more usual narrow-gauge form of theropod trackway 2,3 in which pace- angulation values range between 160° and 170°: the hind-foot impressions are located close to the midline of the trackway. Trackway 13 is unique because one section shows the theropod gait switching from wide to narrow gauge (the stride length increases to 5.65 m and pace angula- tion increases to 173°; Fig. 2). The orienta- tion of the feet also changes from positive rotation (toes directed forwards and inwards; Fig. 1a) during wide-gauge locomotion, to a small negative rotation (toes directed slightly outwards) during the narrow-gauge phase (Fig. 1b). Estimating the speed of movement of the track-maker requires information on 494 NATURE | VOL 415 | 31 JANUARY 2002 | www.nature.com reflective precipitated microgranules of sul- phur resulting from the oxidation of sul- phide ions near the oxygenated sea surface. In the days that followed, the feature was observed from satellite pictures to be advected northwards and offshore in the prevailing equatorward geostrophic current flow and offshore-directed surface-wind drift fields, which are characteristic of such eastern-ocean coastal upwelling systems 4 , to reach the position and configuration seen in Fig. 1b. In the image of 3 April (Fig. 1c), even while the earlier offshore feature continues to maintain a coherent identity, another totally new hydrogen sulphide emission event is seen to have started abruptly within the coastal upwelling zone north of Lüderitz. Once the earlier feature had separated from the coast, it took on an appearance superficially similar to that in satellite views of very large coccolithophore blooms 5 . However, the continuity of the feature in the available satellite views and the advec- tive context, as inferred from the evolution of satellite-sensed ocean surface tempera- ture, make it clear that the offshore feature is the same as that previously observed as an intense episode of sulphide emission nearer the coast. The zone of intense coloration in later images (Fig. 1b) extends over an expanse of total sea surface greater than 20,000 km 2 . Such episodes have been recorded some- where along the coast of Namibia more often than not during the recent months since the satellite observational capability was recognized. Where verification on the ground has been available, the early stages of these events have likewise been confirmed to be associated with sulphide emissions. In addition to its directly toxic effects, hydrogen sulphide strips dissolved oxygen from the water column, leaving behind a subsurface hypoxia that is manifested visibly as the sulphur-infused sea surface. It is this subsurface hypoxia, brief communications Figure 1 Sea-surface distribution of suspended sulphur granules. Quasi-true-colour images for the region at latitude 12° E–16° E, longitude 22° S–27° S, off south-central Namibia; images were generated from the OrbView-2 SeaWiFS satellite during March–April 2001. The milky turquoise coloration represents high concentrations of suspended sulphur granules. Data were collected on a, 18 March; b, 29 March; and c, 3 April 2001. 13° E 14° E 15° E 23° S 24° S 25° S 26° S 13° E 14° E 15° E 23° S 24° S 25° S 26° S 13° E 14° E 15° E 23° S 24° S 25° S 26° S Walvis Bay Walvis Bay Walvis Bay Luderitz .. Luderitz .. Luderitz .. a b c Figure 1 The dual-gait features of theropod trackway 13 from Ardley Quarry, Oxfordshire, UK. a, Wide-gauge section; b, narrow-gauge section. Digits are numbered and footprints are identified as left (L) and right (R); ANG, pace angulation; SL, stride length. L R L L R L ANG ANG SL SL 4 3 2 2 3 4 a b © 2002 Macmillan Magazines Ltd