Heart of stone

Small fossil of archaecyatha found on Saterday ASNAT geology trip in the Black Mountain, South of France.

Archaeocyatha (or archaeocyathids, meaning ancient cups) is a taxon of extinct reef-building marine organisms of warm tropical and subtropical waters that lived during the early (lower) Cambrian Period, more than 500 million years ago. They became the planet's very first reef-building animals and are an index fossil for the Lower Cambrian worldwide. Almost all species became extinct by the Middle Cambrian, with the final-known species disappearing just prior to the end of the Cambrian period.

Some archaeocyathans were built like nested bowls, while others were long. Some were solitary organisms, while others formed colonies.

The typical archaeocyathid resembled a hollow horn coral. Each had a conical or vase-shaped porous skeleton of calcite similar to that of a sponge. The structure appeared like a pair of perforated, nested ice cream cones. Their skeletons consisted of either a single porous wall (Monocyathida), or more commonly as two concentric porous walls, an inner and outer wall separated by a space. Inside the inner wall was a cavity. At the base, these pleosponges were held to the substrate by a holdfast. The body presumably occupied the space between the inner and outer shells (the intervallum).

Archaeocyathan morphology allowed them to exploit flow gradients, to draw water through the pores, removing nutrients, and expelling spent water and wastes through the pores into the central space.

The archaeocyathans inhabited coastal areas of shallow seas, like this place in South of France was during early Cambrian period before moving to its current place.

Their widespread distribution over almost the entire Cambrian world, as well as the taxonomic diversity of the species, might be explained by surmising that, like true sponges, they had a planktonic larval stage that enabled their wide spread.