FOSSIL OYSTERS
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.
Gryphaea arcuata
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
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.