Those who want to look for fossils do not always have to scour abandoned quarries or climb mountains there. Sometimes a quick glance at the kitchen counter, windowsill or curb is enough. Many houses and streets are teeming with millions of years old shells and sea lilies:remnants of a former sea in Western Europe.
Chances are you've recently walked over a coral reef. Or almost tripped over a fossil sea lily. Not while you were snorkeling in some tropical place, but just in the middle of the city. Because in addition to all the brick and concrete, a lot of limestone is also used to embellish buildings and sidewalks. And that limestone is full of fossils.
Former sea
In the Netherlands, Belgian or Irish bluestone is often used for buildings and curbs. This rock-hard limestone variant was formed between 350 and 290 million years ago in a shallow sea that covered Western Europe at the time. The seawater was full of foraminifera, single-celled animals that are indistinguishable with the naked eye and have a skeleton of calcium carbonate (CaCO3 ). Skeletons of dead foraminifera formed a kind of lime mud on the seabed and the lower mud layers petrified by pressure from above into limestone.
The temperature of the former sea was much more pleasant than that of our present North Sea. The bottom was strewn with sea lilies:animals that, as their name suggests, resemble lilies a bit. There were also plenty of coral reefs and brachiopods (shells that were anchored with a 'foot' in the lime mud). After their death, all the animals were soon covered by a layer of lime mud. And in the anoxic seawater, rot did not take place so quickly, so that relatively soft parts were also preserved. Under the influence of chemical reactions and pressure, the dead animals fossilized together with the lime mud to form limestone.
Limestone quarries and canal houses
The sea dried up and the earth's crust was pushed up by plate tectonics, with the result that all those corals and sea lilies suddenly ended up hundreds of meters above the surface, both in the Belgian Ardennes and in Ireland. At first, the fossil-rich limestone layers were still well hidden under younger sediments, but they were exposed through erosion. Today, limestone quarries in Belgium and South East Ireland are full of fossils. The Irish freestone is black and contains many brachiopods and snail shells; in the blue-grey Belgian bluestone you will find many sea lilies and corals. The color differences of the 'matrix' (the petrified lime mud) are determined by the amount of dissolved carbon. The fossils themselves are white in color.
Especially in the 17 e century, Belgian bluestone was very popular with wealthy Dutch citizens. In Amsterdam, stairs, thresholds, windowsills and facades of mansions and canal houses were generously covered with the stone, with the result that you can literally trip over the fossils today. In the Utrecht shopping center Hoog-Catherijne you will find belemnites in the floor:internal skeletons of squids. The concrete curbs in the center of Amsterdam have been replaced in recent years by wide bands of Irish bluestone.
Score species
Every day thousands of people walk over those curbs without noticing the fossils. Admittedly, the sidewalk isn't very attractive to look at either:the tiles are usually littered with dog turds, cigarette butts and empty beer cans. But with some practice you can easily recognize a number of species among the flock.
Brachiopods
If you didn't know better, you could mistake the brachiopods for flattened pieces of chewing gum. The bright white fossils are a few centimeters across and crescent or circular in shape. The Latin name Brachiopodae means 'arm-legs', because of the tentacles with which the shellfish guide the food inside. The shells consist of a large and a small valve and are therefore not symmetrical (like mussels, for example). Today, about 300 species of brachiopods are known.
Corals
The honeycomb corals are easy to identify; they live up to their name. A honeycomb-shaped coral reef actually consists of a colony of countless millimeter-sized animals:the cnidarians or polyps. Each animal has eight tentacles to bring in food.
In addition to honeycomb coral reefs, fossils of extinct rugose corals can also be found. They didn't live in reefs, but on their own. You can recognize a rugose coral as a circle with all stripes that point to the center.
Sea lilies
The stem parts of sea lilies (also called crinoids) are extremely numerous in freestone. The fossilized stems have fallen into pieces. In side section the fragments resemble screws, in cross section they are circles of less than a centimeter in size with a hole in the middle. Today, sea lilies are nearly extinct; only a few species live in submarine caves. Like their relative the starfish, they are fivefold symmetrical.
It's a pity about those city fossils, of course, that you can't just knock off a piece of stone with a hammer. However, you can use another tool during your fossil hunt:a plant sprayer. A little spit or rain also helps – the fossils are best seen when the limestone is wet. And did you discover a counter with fossils? Make sure you never cut lemons on it:acid dissolves the limestone.
- Sea beasts under your feet (De Vuurberg)
- Living Fossils (Knowledge Link Article)