More than 20 years since the proposal that the end-Cretaceous mass extinction coincided with a major impact, confirmed by the discovery of Chicxulub, nobody has produced convincing evidence for an extraterrestrial culprit for others.  Were geologists implanted with GPS tracking devices as soon as they graduated (no doubt on the cards in new health and safety regulations planned by the Blair government in Britain), then Big Brother would see strong clusters close to a number of boundaries on the geological map of the world.  There would be many at P-T sites.  Electronic tagging would have shown personnel from several US universities (Rochester, Harvard, California) in the Transantarctic Mountains, from time to time in the last few years.  Allegedly, that near-pristine area exposes rocks at the juncture between Permian and Triassic strata over less than a metre.  It is marked by the sudden disappearance of the famous Glossopteris flora, just below a clay breccia, from which this group of scientists have previously extracted evidence for shocked quartz and extraterrestrial fullerenes (football-shaped organic molecules) that contained odd noble-gas isotopes.  Two members of the team have made other finds of fullerenes, at the P-T boundary in China and Japan, the K-T boundary and the ancient Sudbury impact in Canada, whereas other workers have not been so lucky.  In fact, the duo are also the only people to have found fullerenes in meteorites, which is key evidence linking terrestrial finds to possible impact events.  The team has hit the headlines again (Basu, A.R et al. 2003.  Chondritic meteorite fragments associated with the Permian-Triassic boundary in Antarctica.  Science, v. 302, p. 1388-1392).  At first sight their discovery of pristine fragments of forsterite-enstatite rock with probable chondrules at the boundary suggests that indeed a major impact coincided with the biggest of all Phanerozoic mass extinctions.  They even report tiny grains of metallic iron with an astonishing purity, perhaps formed by condensation from the plasma cloud associated with a really big meteorite impact.  What is really odd, however, is that sedimentary rocks a quarter of billion years old should have preserved such highly unstable minerals.  All other finds of fossil meteorite fragments have been highly altered relics, as any geologist would expect.  There is a clamour for the Antarctic samples from other laboratories, so that the results can be confirmed or refuted. 

See also: Kerr, R.A. 2003.  Has an impact done it again?  Science, v. 302, p. 1314-1316, and Oxygen depletion before P-T extinction (above)