4" Fossil Pennsylvanian Horsetail (Asterophyllites) Plate

Asterophyllites is the form genus of leaf clusters of tree-like sphenophytes (horsetails) known as Calamites.

Leaves on Calamites trees formed in clusters of 8 or more, called whorls, at spaced nodes along stems. Asterophyllites leaf clusters curved upward into a sort of cup shape, though the leaves themselves could be any shape. They can also be identified by midline veins throughout the leaf from node to leaf tip.

We use these terms to describe the shapes of fossil imprints. Exact species are often difficult to determine, but these form taxa help in distinguishing the prolific species of sphenophytes that defined the Carboniferous Period.