Trilete spores
Earth History Research Center

Many genera of mosses, lycopods, and ferns produce spores that are more or less triangular in aspect, and contain on their germinal surface a "Y" shaped suture called a trilete scar. This scar results from the splitting of the spore mother cell into four equivalent reproductive cells in such a manner that the four are all in contact at a single point. When these cells separate, they carry a remnant of the point of commonality and the lines of contact with adjoining cells, forming the "Y" shaped mark or suture. The same effect can be produced by pressing four balls of clay or plasticine tightly together, then carefully separating them again.

This particular spore is from a fern of the large family Polypodiaceae, which includes many common ferns. Ferns range from Devonian to Recent in the fossil record.