Heterastridium is a genus of fossil marine Hydrozoan from the Upper Triassic. They are mostly discoid or spherical and some forms found in the Karakorum mountains are called Karakorum stones. They vary in diameter from 1 to 35 cm and appear to follow Cope's rule for the prehistoric climate
Hydrozoa (hydrozoans, from ancient Greek ὕδρα, hydra, "sea serpent" and ζῷον, zoon, "animal") are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most living in salt water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in fresh water. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.
Some examples of hydrozoans are the freshwater jelly (Craspedacusta sowerbyi), freshwater polyps (Hydra), Obelia, Portuguese man o' war (Physalia physalis), chondrophores (Porpitidae), "air fern" (Sertularia argentea), and pink-hearted hydroids (Tubularia)
This comes from Cyprus 200 myllion years old.
Photos Troodos 8/12/2014 by George Konstantinou