Isognomon is known in the fossil record from the Permian period to the Quaternary period (age range: 254.0 to 0.012 million years ago). Fossils of species within this genus have been found all over the world.[2]
This genus is placed in the family Isognomonidae. However previous molecular phylogeny studies have shown that these tree oysters belong in the family Pteriidae.[3] [4]
These oysters grow to be about 4.75 inches (12.1 cm) in overall length, producing a highly irregular shell with a blue-gray and often heavily encrusted exterior but a smooth and pearly white interior. They use their byssus to completely immobilize themselves to the roots of mangrove trees, corals, and other substrates.[5] It is because of the preference for mangroves that these are sometimes called tree oysters[6]