A Pebble from Lake Constance Tells the Story of Microfossils Nearly 50 Million Years Old

During a brief visit to Lake Constance, favourable conditions made it possible to collect a water sample for microscopic examination. With the water level exceptionally low and continuing to fall, the shoreline near Langenargen was easily accessible. Spring also offers the best opportunity to discover diatoms. While searching for suitable samples, attention naturally turns to the countless pebbles along the shore—one of which immediately stood out.

An Unassuming Pebble with a Fascinating Story

Dry and heavily weathered, the pebble initially appears unremarkable. However, faint outlines of so-called Nummulites can be discerned—large foraminifera, also known as forams. These single-celled creatures typically inhabit warm seas and construct shells, usually from calcium carbonate. In Nummulites, these shells are formed of multiple chambers arranged in a flat spiral. The name derives from the Latin nummulus, meaning small coin. In ancient Egypt, Nummulites were even used as a form of currency.

Nummulites – Giant Single-Celled Organisms

Modern large foraminifera can reach diameters of up to 13 cm—an impressive size for a single-celled organism. The Nummulites found in this pebble measure around 1–2 cm. During the Eocene epoch, between 56 and 33.9 million years ago, such organisms were widespread and in some cases even larger than today. Many of their shells settled in sediments and fossilised into what is now known as Nummulite limestone.

How Did This Fossil Pebble End Up at Lake Constance?

This approximately 45-million-year-old Nummulite limestone is geologically relatively young. Sediments from the Cretaceous and early Tertiary periods that gave rise to the rock were later uplifted to form part of the Western Alps, a region geologists refer to as the Helvetic zone.

A similar find was reported in the article “Nummuliten-Kalksandstein – ein exotisches Geröll vom Bodensee“ in „Fossilien“ 04/2019. The exact origin and transport route of the pebble described there remain unknown—the same applies to our specimen. It is likely that gravel from the glacier was transported further by a river. Nevertheless, the discovery remains a captivating one.

Colourful Micro-Worlds Hidden in Stone

Under the microscope, the pebble reveals an astonishing variety of foraminifera—mainly from the genera Nummulites sp., Assilina sp., and several smaller forms. The fossils are embedded in dark grey sedimentary rock, which reveals a striking array of colourful minerals under magnification. A high iron content, indicated by reddish-brown dust during polishing, is complemented by bright green and yellow mineral inclusions both inside and outside the calcareous shells. Who would have expected such vibrant details in an otherwise plain gray stone?

Images captured using the Analyth STR microscope and a Samsung Galaxy S23 smartphone. Some images have been cropped.

BRESSER Analyth STR 10x - 40x Stereo Microscope