December 2025: Stromatoporoid - Bayerische Staatssammlung für Paläontologie und Geologie

SNSB-BSPG 1971 III
cf. Actinstroma sp.
Middle Devonian (Givetian), ca. 385 million years, Sötenich syncline, Sötenich,
Eifel hills, NW-Germany
dimensions: 13 x 10 x 6 cm

Stromatoporoids are an important group of fossil invertebrates. They form calcareous, layered skeletons and are found in sedimentary limestone sequences from the Ordovician to the Neogene. Stromatoporoids are most prevalent in the Middle Paleozoic (Silurian-Devonian). During this period, they are known for their significant role in reef growth and formation.

In the late 1960s and early 1970s, remarkable calcifying sponges were described from deeper forereef areas off Jamaica. These living sponges possess a calcareous skeleton composed of the minerals aragonite or calcite and form encrusting or massive structures. They also have a skeleton of siliceous spicules. The tissue of these forms exhibits a system of channels that facilitate water outflow and also leave traces in the skeleton, resembling the astrorhizae (star-shaped structures on the surface, often associated with wart-like projections called mamelons) of stromatoporoids. Based on the presence of spicules, stromatoporoids were subsequently classified as sponges (Porifera). Spicules have since been found in many Mesozoic and a few Paleozoic stromatoporoids.

The fossil of the month beautifully displays typical characteristics of stromatoporoids. These include the wart-like protrusions on the surface, which formed the centers of water outflow during the sponge’s lifetime. Another characteristic is the layered structure, consisting of horizontal skeletal elements called laminae and vertical, pillar-like structures called pilae.

Stromatoporoids of the Middle Paleozoic could grow to sizes of up to 5 meters and often were the dominant reef builders; unlike their modern counterparts, which are rather small and slow-growing and are primarily found in reef caves.

In Central Europe, a limited marine basin, the Rhenohercynian Ocean, existed along the southern edge of the Old Red continent during the Middle and Late Devonian. Massive stromatoporoid coral reefs formed on the nearshore shelf within this ocean, and these reefs are now exposed in the low mountain ranges of the Eifel, Sauerland, Bergisches Land, and Harz Mountains.

Martin Nose, Munich