Play Pokéman Go - with fossils

The hunt for traces of ancient life

Gees Voorhees

By Nobert Peeters

You don’t actually need a deserted quarry to find fossils – Leiden is brimming with palaeontogical 
treasures. “Every tile is a video frame, a frozen image of a tropical sea teeming with life.”

For years, my colleagues, students and I haunted the alcove at the entrance of Plexus Student Centre. During opening hours, we took it in turns to shelter on the tiny limestone terrace. When it was my turn, I would blow out smoke rings and sip my coffee, my gaze drawn to the tiles I was standing on. One of tiles seemed to have something scribbled on it, rather resembling a Keith Haring figure.

Over the years, the spot never disappeared, only changing colour due to the litres of coffee spilt on it.
But it was only after reading Kijk waar je loopt! Over stadspaleontologie [Watch where you put you foot; About urban palaeontology] by Jelle Reumer, an endowed professor of vertebrate palaeontology, that I realised that this strange stain is actually a “wolf paw”, an extremely rare fossil of a glass sponge (Asteractinella).

The glass sponge at Plexus is an exceptional discovery. Although in later geological eras, glass sponges were very common fossils, there is only one other limestone “wolf paw” in a Dutch city that we know of (to be precise, it is in the pavement in front of the house at Lange Nieuwstraat 56 in Utrecht).

However, Reumer tells us about urban fossils that we can find on practically any street. To go on a fossil hunt, you don’t need to find a remote quarry - towns are actually open-air museums of palaeontology. Besides bricks, cement, glass and metal, towns and cities are built of natural stone in abundance and they reveal a profusion of fossil life. Once you get a feel for it, you can see how these petrified animals and plants now form the building blocks of our towns. It’s like playing Pokémon Go – the palaeontogical version with Reumer’s book as a very useful Pokédex.

With Reumer’s help, you will soon learn that the tiles in the alcove at Plexus are made from a very popular kind of natural stone, also known as Belgian bluestone, ashlar, carboniferous limestone or petit granit. The last name is misleading: granite is a type of igneous rock and doesn’t contain fossils. By contrast, bluestone is a kind of sedimentary rock: a petrified sea floor dating from the late Devonian period (between 383 and 359 million years ago) and the early Carboniferous period (between 359 and 331 million years ago).

Dutch towns, particularly in their historic centres, have ample bluestone. In Leiden´s centre, there is bluestone, most of which was quarried in the Belgian Ardennes, in practically every street: flagstones, window ledges, doorsteps and decorative elements.

In fact, every bluestone tile is a video frame, a frozen image of a shallow tropical sea teeming with life in which particularly organisms with hard calcium layers or shells have been preserved well. Although the alcove at Plexus only contains a few snapshots, in and around the Academy Building, there is an entire film reel of early life in a bluestone sea. The floors of the image gallery and in the cloakroom reveal a polished cross section of the sea floor, with thousands of beautiful fossilised remains of brachiopods, sea lilies and colonies of corals and polyps. One of the former convent´s window ledges provides a bird’s-eye view of a piece of coral. From the side, you can see the parallel tubes of the fossilised calcium skeleton while the view from above clearly shows the honeycomb structure.

Rapenburg features more treasures from the Carboniferous period. You can find a number of rare fossils, including the white outlines of two shells (filled with quartz crystal) in the bluestone that girds the Dutch National Museum of Antiquities, diagonally opposite the Academy Building. When I posted a photograph on Twitter, Reumer immediately identified them as specimens of an extinct group of creatures: Rostroconchia.
These hingeless bivalves probably lived buried in the sediment of the sea floor, where they could filter nutrients out of the water. Last year, the Swiss Journal of Palaeontology published an article devoted to these urban fossils.

The largest concentration of fossilised life can be found in the university library. Of course, I’m not referring to the stony researchers who study dusty folios but to the floor tiles from a region of Bavaria known as the Franconian Jura. Reumer informs us that this beige Bavarian limestone comes from the geological Treuchtlingen formation, dates from the Late Jurassic period and is approximately 157 to 152 million years old. Again, here is an impressive cross section of tropical-sea sediment.

You must have noticed, while pacing about and talking on the phone, the ammonites in the tiles, with their spiral shells divided into ever-decreasing segments. The well-cut specimens clearly reveal the transverse partitions (septa) that separate the shells into compartments. You can also see that each compartment is dissected by a tube (sipho) connecting the separate compartments. The mollusc used the tube to regulate the water and gas in the compartments and by doing so, it could control its buoyancy, more or less like a submarine does.

Sometimes, the floors of the university library reveal another kind of cephalopod: the belemnite. Although at first, this fossil looks more like a black rubber smudge, you can actually see the fossilised inner shell (rostrum) of this ten-armed squid. This feature is also reflected in the fossil’s names: the Greek work belemnon means “javelin”.

In contrast to sea snails, ammonites and belemnites moved through the water at great speed. Just like squids (which we recognise more easily when they are fried and served up as calamari), both species had a siphon (hyponome) with which they could produce a powerful squirt of water, a feature that meant that both these molluscs were formidable predators in the Jurassic sea. In turn, these molluscs were preyed on by the mosasaurus and ichthyosaurus.

As we have learnt from this brief palaeontogical exploration of Leiden’s town centre, Leiden is an unprecedented Fundgrube for fossils. There are still plenty of spots for you to discover that I haven’t mentioned here: from the walls of the Museum voor Volkenkunde and the Kamerlingh Onnes Building to Mayor Van Der Werf’s pedestal and the floor of Pieterskerk.

While you might not be allowed to get out your hammer and chisel to them in town, you can still take pictures of them. For students who are even remotely fascinated by fossils, this palaeontogical Pokémon Go is more fun and more educational than collecting digital, imaginary animals. All you need is a Euro coin (to show the scale) and a smartphone, and you’re off on a fossil hunt!

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