Paleontologists trace the existence of oysters back to at least the Triassic (about 248 to 213 million years ago). Oysters have always been plentiful in the oceans of the world. Some species preferred brackish water and others liked higher salinity. There's a pretty town in the French region of Bordeaux called Sainte-Croix-du-Mont where caves (called Le banc d'huîtres de Sainte Croix du Mont) lead through enormous ground layers of fossilized oysters. As recently as 2005, paleontologists discovered thousands of new and peculiar oyster fossils - of all places, near a dreamy little town called Stetten in Austria. The oysters have been dated at about 17 million years old. They are monstrous in size, some as long as 3 feet and, in some cases weighing in at more than 20 pounds. These huge fossilized oysters from Austria look a lot like absurdly overgrown Pacific oysters. Overnight, the little town of Stetten, far removed from any sea, suddenly became the oyster capital of Austria. Paleontologists meticulously excavated the discovery area. A marvelous oyster exhibit was subsequently built at great expense and these spectacular primordial oysters are available for public viewing.
Oyster fossils also prove that oysters always had more than their fair share of natural predators. For instance, many of them turn up with small drill holes which evidence the activity of the rasp-like tongue ("radula") of some primordial gastropods (snails). At some point, a number of large mammals evolved that had massive crushing and grinding teeth, ideally suited to turn even the toughest oyster or clam fortress into something reminiscent of pancake batter.
Strange and Familiar Oyster Fossils
Some oyster fossils look pretty strange. Many primordial oyster species tended to display more or less pronounced coiling in their shell design. Some members of the genus Gryphaea reduced or eliminated their tendency to coil at a certain size. Others seem to have changed away a bit from the pronounced "pot with a lid" type of shell design and then back again. Others again, perhaps by design, had a tendency to break off their original settling point at a certain size and drop to the sea floor instead of staying attached in one spot all their life. The cupped shell design of those oysters allowed their continued growth on a soft sea floor. Members of Exogyra frequently tended to coil aggressively which led to some quite unusual looking oyster fossils. Some could also turn into spiraled horns that either laid flat or ended up standing upright on the sea floor.
The strong coiling or spiraling tendency of Gryphaea and Exogyra may have contributed to their ultimate extinction, as the cupped part of the shell ultimately started curling over so much that the rather flat "lid" shell counterpart could no longer open properly, particularly once such oysters grew older and became larger. Although this shell design draw-back worked for a very, very long time, some substantial environmental change at some point may have been too much for the design to bear.
The famous marine biologist Charles Maurice Yonge (1899-1986), a leading 20th century authority on oysters, wrote in his marvelous book Oysters (1960) on page 134: "The process from flat to excessively cupped oysters would seem to have occurred several times during the Mesozoic era which is the reason why we consider Gryphaea as a recurrent grade rater than a genus. In every case temporary success was followed by extinction. But always there was a persistent stock of flat oysters (an example of what J.S. Huxley calls stasigenesis) which was exposed to none of the dangers due to over-elaboration of structure. From this stock modern oysters of all types are descended."
C.M. Yonge also points out the very central
position of the adductor muscle in Gryphaea and Exogyra.
Its position may well have compromised or possibly excluded the
existence of a chamber in the soft body tissue inherent in today's
somewhat similar "cup and lid shelled" oyster species.
They belong to the genus Crassostrea, examples of which
are the Eastern oyster (Crassostrea virginica) and
the Pacific oyster (Crassostrea gigas). This chamber,
formed by a local separation of the oyster mantle from the underlying
body tissue, may aid in the ability to survive in more turbid
water conditions and may also perfom other important functions,
perhaps even enhancing the reproductive system. It is called
the "promyal chamber" and was discovered by the brilliant
marine biologist and oyster specialist Thurlow Christian (T.C.)
Nelson (1890 - 1960). However, this chamber does not exist in
oysters of the genus Ostrea, to which, for instance, today's
famous European oyster (Ostrea edulis) or the delectable
Olympia oyster (Ostrea lurida a.k.a. Ostrea conchaphila)
The fact that many of the fossilized oyster species did not change at all in the course of dozens of millions of years before they expired does not help paleontologists either. Thus, the mere discovery of some oyster fossils alone is often no fast track for paleontology detectives in assigning specific geological time periods. The oysters may merely be a precious piece in a puzzle of (hopefully) other available fossilized remains of other life assignable to a particular era. Paleontologists may seek so called "index fossils" which fairly reliably evidence the geologic age of the rock formation in which they are discovered.
On the bright side, the extremely compact design of oyster shells has lead to superbly preserved fossil specimens. Fossilized internal molds are also not uncommon and frequently of great interest. Much like today, oysters were sessile. They never moved about after the larval stage, at least not voluntarily. However, many other mollusks did, as they still do, and left trails and burrows which also turned into fascinating fossils. They are called "trace fossils" or "Ichnofossils". At times, they are very difficult or even impossible to qualify in terms of what mollusk or other organism actually left them behind, particularly since a variety of animals may have produced identical evidence.
Fossilized pearls, a rarity, have also been found. Although a pearl-like luster is occasionally found on fossils, the luster of the mother of pearl layer, the smooth inside shell layer that touches the actual mollusk (the so called hypostracum), usually does not survive in the process of petrification. Nonetheless, no other fossilized animal group can match the representative quality of shelled mollusk fossils.
There is some discussion among paleontologists as to when certain mollusks started emerging as what could actually be considered "an oyster". For instance, today's oysters attach themselves to a rock or some other type of substrate with the left shell half ("left valve"). Some primordial oysters did not necessarily always attach themselves to some substrate with the left valve. There is some speculation that all the oysters may have originally attached themselves with the right valve, and only later, for some unknown reason, switched to the left valve.
On the left we see five oyster fossils of the species Texigryphaea mucronata. On the right, five tiny fossils of the species Ilymatogyra arietina. All of the fossils pictured were found in Texas. The ruler I used as a size reference measures in cm. Incidentally, the metric system is preferred by seashell enthusiasts the world over. Some of the oyster fossils are tiny and others quite impressive in size. Some, like the "Devil's toenails" and so called "Ram horns" are quite common. They'll cost a collector about a buck each at a fossil or rock hound show. Other oyster fossils are extremely rare and, depending on their condition, can cost hundreds of dollars.
This is a rather large oyster fossil. Although I'm not a fossil expert, I believe this to be a fossilized Eastern oyster (Crassostrea virginica). It was found in Florida (St. Johns River, Duval County).