Trigonocarpus is an extinct seed from an order of a plant called Medullosales that grew 300 million years ago in the shallow swamps located within present-day Oklahoma, Illinois and Pennsylvania--which was where the Equator rested. The order includes flora species like Alethopteris, Neuropteris and Macroneuropteris.

Medullosans are often miscorrectly called "seed ferns" because the leaves resemble fern leaves, however, these plants produce seeds, not spores, so they are more closely related to conifers and flowering plants. Some grew as trees as tall as 30 feet with leaves reaching 10 feet long, while others vined around other plants. 

It was believed that Trigonocarpus seeds were encased in a fleshy "fruit," similar to an avocado. They are characterized by three prominent ridges that run along the length resembling an over-inflated triangle. The seeds were open at the pointed end to allow pollen to enter when the seeds dropped into the swampy water. 

Trigonocarpus seeds were the largest seeds, reaching lengths of up to 4 inches. 

Type: Plant Seed
Species: Trigonocarpus sp.
Age: Mid Pennsylvanian/Desmoinesian (308 - 306 million years ago)
Formation: Savannah Formation, Krebs Group
Locality: McIntosh County, Oklahoma

Size of Seed: 1.63 x .44 inches
Weight: 2.75 oz.