This 2 inch Trigonocarpus is a seed of a Medullosa (pteridosperms seed plants) which grew in shallow swamps abour 306-308 million years ago when St. Clair, PA was located near the Equator. Leaves associated with this seed & tree include Alethopteris and Neuropteris. It is thought that these seeds were encased in fleshy "fruit" like an avocado. The seeds were open at the pointed end to allow pollen to enter when the seeds dropped into the water in the shallow swamps where these trees grew. These were the largest Carboniferous seeds, growing up to 4 inches.