At http://phys.org/print300440386.html … this story has the title, 'first ever evidence of a comet striking the Earth' – and the event is dated 28 million years ago, leaving evidence behind in the Sahara desert. We may note the date safely shifts the event away from human memory – unlike the less popular idea that something similar happened at the Younger Dryas Boundary, a mere 13,000 years ago.
It is theorised it entered the atmosphere and exploded (somewhat like the recent Chelyabinsk meteor over Russia) and perhaps the researchers as a base line. The explosion caused a downwards shock wave (of fire) which obliterated life forms in its pathway. The research was done by South African scientists with a few others. It is theorised the shock wave heated up the sand in the desert to a temperature of 2000 degrees celsius. The origin of a geological formation consisting of yellow silica glass which lies scattered across 6000 square km of the Sahara. A specimen of the glass, polished, can be seen in a brooch of Tutankhamun and the research is due to be published in Earth and Planetary Science Letters and would be worth getting hold of as it assumes there was a lot of sand in the Sahara as long ago as 28 million years. A black pebble found years ago by an Egyptian geologist in the area of the silica glass was chemically analysed. The researchers came to the conclusion it was part of the comet nucleus – rather than just a meteorite. The explosion also caused nano-diamonds to form – as a result of the shock wave.