Some fossil oysters attained rather strange-looking shells when compared to modern forms. Some famous examples include:
1) the “devil's toenail”, a Mesozoic-aged fossil oyster called Gryphaea arcuata (Animalia, Mollusca, Bivalvia, Pteriomorphia, Pterioida, Ostreina, Ostreoidea, Gryphaeidae), first named by Jean-Baptiste Lamarck in 1801.
2) another Mesozoic-aged fossil oyster called Exogyra costata (same classification as Gryphaea, down to the family level), first named by Thomas Say in 1820.
Most modern & fossil clams have asymmetrical but equal-sized shells that are mirror images of each other. Oyster shells depart radically from this general rule. The Gryphaea oysters evolved a greatly enlarged, very thick, coiled, highly convex left valve & a greatly reduced, relatively thin, concave right valve.
The thickness of the left shell functioned to protect the oyster from predators such as decapods (crabs) and boring gastropods (snails). The overall shape of the gryphaeid oyster shell was a consequence of its living on soft, fine-grained substrates. All oysters are filter feeders, and practically all are hard substrate encrusters at some point in their ontogeny. The highly coiled Gryphaea oysters are free-living forms as large adults. Their shell shape appears to be the result of repeated downward toppling into the mud along the ventral margin of the left valve by the weight of the shell.
Stratigraphy: Blue Lias, lower Lower Jurassic.
Locality: coastal cliffs in the vicinity of Lyme Regis, far-western Dorset County, southwestern England.
Gryphaea arcuata (6.4 cm across at its widest) - anterior view of left valve, from the Jurassic of England. This view closely approximates the original living position. Most of the shell seen here was submerged beneath the mud, and the sediment-water interface was probably a little below the lip seen in the upper central and left portions of the photo.
Gryphaea arcuata (3.5 cm across at its widest) - view of ventral margin of left valve (foreground) and curved beak area of left valve (background).
Gryphaea arcuata (6.4 cm across) - lateral view of left valve (dorsal to the right; ventral to the left) showing well defined growth lines and some epibiont encrustation scars. The narrow bent area in the bottom of the photo is the posterior sulcus.
Gryphaea arcuata (5.2 cm across) - lateral view of right valve (= sunken area at center & left) and beak area (= knob at right) of left valve. This is a different individual from the one shown in the above three photos.
Exogyra costata is another distinctive, large fossil oyster that’s relatively common in some Mesozoic marine successions. The coiling in Gryphaea (see above) is very close to being planispiral (the shell wraps around itself, within a plane, as it coils). The coiling in Exogyra is more akin to that seen in gastropod shells (snail shells), where coiling is trochospiral (a.k.a. conispiral) (the shell moves outside a plane as it coils).
Stratigraphy: Maastrichtian Stage, upper Upper Cretaceous.
Locality: Myrtle Beach area, eastern-coastal South Carolina, USA.
Exogyra costata (7.0 cm across at its widest) - lateral view of left valve (dorsal at top; ventral at bottom).
Most info. from Stenzel (1971) - Oysters. Treatise on Invertebrate Paleontology, Part N, Mollusca 6, Bivalvia, Volume 3.