The 25-centimetre (10-inch) fragment has been found in the Morokweng impact crater in northwest South Africa, where a massive object slammed into Earth around 145 million years ago.

The find is unique, for large asteroids or comets are believed to vaporise or melt completely within a few seconds after they hit the Earth, so enormous is the energy of their impact.

As a result, their composition is identified indirectly, by looking at chemical telltales left in the crater soil.

The Morokweng rock falls into the category of a "stony" meteorite. But it is chemically quite unlike other meteorites of this type, which are of a far more recent vintage. The South African rock is rich in iron silicates and iron-nickel sulphides yet poor in metal.

More work is needed to see if there could be other meteorites around the world that match this unusual signature, say the authors, led by Wolfgang Maier of the University of Quebec in Chicoutimi, Canada.

But the implication is that the composition of space rocks that have reached the Earth differs over the timescale of impact.

This, in turn, raises the question that there may be a bigger than suspected variety in the chemistry of rocks circling the Sun or perhaps some alteration to the rocks themselves that occurs during the aeons in orbit.

The study appears on Thursday in Nature, the weekly British science journal.

The Morokweng crater measures more than 70 kilometres (44 miles) across. It lay unidentified until 1996 because the distinctive crater ring had been worn away by the millions of years or lay hidden under sand.

The impact coincided with the end of the Jurassic era 145 million years ago, when there was a mass extinction among marine life and reptiles.

This has caused some scientists to draw a parallel with an impact that occurred in modern-day Mexico around 65 million years ago that is believed to have ended the long reign of the dinosaurs.

Impacts by very large asteroids or comets, according to this theory, can deliver so much energy that clouds of dust can be kicked into the atmosphere, cooling the heat from the Sun and killing off species of vegetation and the chain of animals that depend on them.

Meteorites are the remains of meteors -- rubble that collides with Earth and show up as streaks of light as they burn up through friction with atmosphere.

These remnants can be very big, but until now they have only been found in craters with a diameter of four kilometres (2.5 miles) or less. The smaller the rock, the less energy is released, which means there is less risk that the object will be consumed upon impact.

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