Fossil remains of Asterophyllites equisetiformis, a genus of plants belonging to the family Calamitaceae, treelike horsetail plants now extinct, that lived some 300 to 350 million years back during the Carboniferous period, in swamps and marshland. This picture shows how whorls of small pointed leafs of the plant are arranged radially along a thin stem. Actual width of this slab of mudstone is 14 cm and is from the late Carboniferous coal measures in the Cantabrian mountains, near Leon, Spain.