A hundred million year old pet rock | Day 92

Today I was given a piece of fossil by someone who recently went to Morocco. This north-west African country is partially located in the Sahara desert, which was once a warm, shallow sea teeming with ancient sea creatures. As a result, quite a bit of commercial fossil trading takes place here, with specimens making it into coffee tables, sinks, bookends and other decorative items, shaped and polished out of ‘Moroccan fossils marble’.

My own little fossil is a polished round of an ammonite – a predatory, squidlike creature that swam through the seas encased in its coil-shaped shell and snared tiny fish with its tentacles. One day, perhaps some 240 million years ago, this prehistoric mollusc died and sunk down onto the seabed. Bacteria ate away at its flesh, but left the shell, which became safely buried in the fine sediment at the bottom of the sea. Here decomposition couldn’t find it, and the shell stayed intact for months, that turned into years, that turned into centuries. Over millions of years the sediment slowly turned into rock, with minerals percolating through the layers and replacing the molecules of the ammonite shell with molecules of calcite and other minerals. Eventually, all that was left was just rock – shaped exactly like the original shell.

Ammonites are amongst the most commonly found fossils today, and they are so widely distributed and so easy to recognise, they have become known as excellent ‘index fossils’ – species of fossil that help paleontologists and geologists identify the age of a rock sediment.

However, even a common fossil is still a rare and incredible occurrence. The vast majority of the organisms that have populated our planet for the last 550 million years died and decomposed without leaving a trace. Only the occasional creature went through the fortunate process that allowed rock to preserve its shape – or bones – for posterity. That percentage is so tiny, today we are only finding a minuscule fraction in the shape of fossils. And we will never learn anything about the potential hundreds of ancient species that vanished without even a fossil trace.

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