A fossilized meteorite is suspected to provide the key to one or more of the major events in Earth's evolutionary history.

Most meteorites are stony objects called chondrites.  Chondrites are divided into different categories based on metal content, which is thought to represent the original asteroid from which they came. In particular, L chondrites represent more than a third of recent arrivals on Earth. They are characterized by low iron but moderate content of other metals and most are thought to come from a single large asteroid that broke up 470 million years ago.

That break up left a lot of smaller rocks floating around the inner solar system, many of which collided with the Earth. Some of the larger remnants of this break-up are thought to have created craters still visible today.

We know how much debris rained down upon the Earth because a large number of small items fell on the sea bed in what is now southern Sweden and became trapped in limestone being laid down. According to a paper in Earth and Planetary Science Letters “101 meteorites, 1–21 cm in diameter, have been found, representing∼98% of all fossil meteorites known to science” in the Thosberg limestone quarry. It is thought large asteroid strikes from the same source triggered the diversification of species that happened at the time after a long period of stagnancy.

Until now all Thosberg meteorites were chondrites, and probably all L chondrites resulting from this break up. 

However, the paper reveals that one meteorite found at Thosberg in 2011 has such different characteristics that that it was dubbed Mystery Object (MO). MO has a chemical nature similar to the much rarer winonaite meteorites, but the authors conclude “The combined data...show that the MO is of a meteorite type that has no documented equivalent among the ~49,000 recent meteorites known today.” It also appears to have been part of a larger body until shortly before landing on Earth.

^{B Schmitz. This fossil meteorite is unlike any seen before, winning the interim name Mystery Object}

The paper proposes three explanations for the origins of the MO. The most interesting, and also the one the authors consider most likely, is that it “represents a fragment of the body that hit and broke up the L-chondrite parent body.”

Lead author Professor Birger Schmitz of Lund University has proposed the name Österplana for the MO after a church near the quarry. Schmitz and his colleagues modeled the impact of a 20km wide asteroid with a 100km wide originator for the L chondrites. They found that such events would be expected to occur in the asteroid belt every 250 million years, and that 1% of the meteorites would come from the smaller body.