In Search of Rotten Fish

Fossil hunting in the shale

O20001 - first published 8th August 2020

About 330 million year ago, the land that is now West Lothian lay on the edge of giant continent, somewhere near the equator. Coastlines and sea level changed over the course of millions of years, and on several occasions a freshwater lagoon was formed, extending over an area roughly between Tarbrax and Burntisland. At certain times some rather strange conditions occurred in this lagoon that led to the formation of seams of oil shale. It is thought that unusual environmental conditions led to huge blooms of microscopic algae that formed a thick scum and then sunk to the bottom of the lagoon where a lack of oxygen limited its decay. Through the passage of time, this slimy stinky mess was transformed into oil shale. Fossils found within the shale provide clues to the other creatures that lived in this strange black lagoon.

The impressions of twigs, leaves and bark that fell into the lagoon are sometimes found when splitting open the leaves of shale, but the most interesting fossils are found encased within hard iron-rich nodules, which occurred in layers within the seams of productive shale. Miners were paid according to the weight of shale they produced, so hated the nodules which had to be loaded into hutches and hauled to the surface without earning income.

Nodules seem to have been formed by some rather strange chemistry. A lump of something organic may have sunk to the bottom of the lagoon and started to rot, but in the absence of oxygen, reactions took place with sulphur and iron that eventually coated the organic matter in a hard stony shell. Pressure beneath the earth often squashed these stony balls into flatter lens shapes.

Each of these rusty lens-shaped nodules is a mystery parcel, and you never know what you’re going to find when you hit it with a large hammer and reveal what lies at its centre. Usually it’s just a bit of black mush; the remains of something so rotten as to be unrecognisable. When freshly split open, this black centre is surrounded by a halo of golden iron pyrites and accompanied by the nasty niff of sulphur. Often the organic matter in the middle is a coprolite – fossil fish poo – sometimes retaining the pointy end where it was extruded from the fish's bottom. If you are lucky, the black mass at the centre contains bones and scales and other recognisable components. If you are really fortunate you may find a complete fish, preserved as a collection of bones and beautifully detailed scales.

The old miners came to recognise which nodules were likely to contain the richest prizes, and made useful beer money by selling these to local gentleman collectors. Many specimens found their way to the collections of the Royal Museum of Scotland whose keeper of natural history – Dr. Traquair – was a world expert on ancient fish.

Today it is still possible to go hunting for ancient fish. Heaps of black waste shale can still be found at some old mine sites and are likely to contain nodules among the stone and rejected shale. Nodules can be also be picked up on the shores of the Forth (particularly at Queensferry) and along the course of rivers. It’s probably not worthwhile searching among the red blaes of shale bings whose content has been toasted at high temperatures to release the oil.

On the most recent expedition, a couple of hours was spent hammering open tough iron-rich nodules, but only one revealed anything vaguely fishy. But you never know what prize the next one might hold. Like angling for real fish, it’s all a matter of chance and patience.