Very Early Oyster History
A few remarks of a (lightheartedly) scientific nature about oysters Written by: Christian W. Gronau
What is the oyster's place in the scheme of things? (Aside from being the center of the universe for The Oysterman.) How long have they been around? (Quick answer: at least as long as the dinosaurs, and - fortunately for The Oysterman - they did not go extinct, when the great reptiles departed this world.)
Molluscan taxonomy is complicated and far from settled: different zoologists, over the years, have developed different systems for organizing the bewildering complexity of this group, and that includes the classification and naming of oysters (for instance: the Pacific Oyster Crassostrea gigas was known as Ostrea laperousiiin the 1950s).
Here is a recent scheme, placing this oyster into the larger context of animal life:
One of the significant differences between oysters and other bivalves (eg. clams) is that their shells are not mirror-images of each other. That is to say, as bivalves, they are not equivalved: their left shell is usually deeply cupped and often attached to the substrate (making it the "lower" valve), while the right valve is smaller and flat, acting like a lid on top of the lower one.
FIG. 2 - N. American Geography, 70 million yrs ago 2 The earliest larval stage of oysters still shows the original symmetry of "normal" bivalves (plus a number of other traditional features, which are lost in the adult stage). The early larvae "echo" the shapes of the oysters. ancient ancestry: at some point in the early Devonian (a good 400 million years ago (!) in the middle of the Palaeozoic Era) a group of simple bivalves decided to "lie down on the job": they lost their original bilateral symmetry and were henceforth obligated to recline on their left valves - just as modern oysters do today (see Fig. 1 for an early example). 
They also abandoned several other features most modern clams have retained. For instance: a toothed hingeline (still visible along the upper edge of the shell in Fig. 1B), a second adductor muscle, a mobile foot for digging in the substrate, siphons to enable breathing while buried.
The roots of the Ostreidae can be traced back into very deep time indeed, but the group really came into its own, with dozens of different species, towards the end of the Mezozoic Era, in the Cretaceous period (around 100 million years ago).
Alberta, to pick an example, had an extensive "Inland Sea" at that time (Fig. 2); the shoreline and river deltas are part of what became the famous Alberta Badlands with its countless dinosaur fossils. The Cretaceous ocean waters were home to a rich marine fauna, including many ammonites and, of course, countless oysters, creating veritable "oyster reefs" (Figs. 3 and 4).
 FIG. 3 - A handpiece of consolidated oyster reef 3 FIG. 6 - Kicking back at the oyster (sand) bar |
 FIG. 4 - Two oyster fragments from the Late Cretaceous of Alberta 4 |
 Dinosaurs roaming oyster-strewn intertidal beaches: now there's a picture! FIG. 5 - Exogyra Ponderosa, lower valve.5 It is entirely reasonable to speculate that among the first connoisseurs of oysters were the dinosaurs! Indeed, there was one Cretaceous species of oyster, worthy of being dinosaur food: Exogyra ponderosa (Fig. 5) had an extremely massive and heavy hemispherical shell. |
So, we might imagine that, just before the fateful asteroid struck and brought the Mesozoic Era and the rule of the dinosaurs to an abrupt end, the giant reptiles had a good time, gorging on oysters at the local sand bar. (Fig. 6)
As mentioned above, the evolutionary history of oysters begins with fully symmetrical bivalves, living upright in the substrate. Today, oysters are totally obligated to lie flat on the seabed (or be cemented to rocky substrate).
A whimsical parallel could be drawn to the emergence of flatfish (like flounder, sole and halibut): their origin too is with bilaterally symmetrical forms, swimming in an upright attitude. And in their case too, "ontogeny recapitulates phylogeny" to quote Haeckel's old (1906), somewhat discredited "biogenetic law": the young flatfish start out looking like "normal" fish (ie. like their ancestors) and then grow increasingly asymmetrical as they age (apparently repeating some of the stages of their evolutionary past). If we want to push this oyster/fish analogy to its limit, we might say that the large Japanese (Pacific) Oyster (Crassostrea gigas) is the "Halibut" of the oyster world.
Ancient aristocracy of the mollusk world: that's what oysters are ; and while it might be wild speculation to think of dinosaurian connoisseurs slurping oysters on the half-shell tens of millions of years ago, the practice is well established among the recent inheritors of the earth: Homo sapiens is partying on, enjoying the hedonistic pleasures of the present, with only fleeting thoughts given to the looming future and what shape or kind the "asteroid" will be, to terminate our party.
Footnotes
- Rhomboptriidae is a Devonian sister family to the Ostreidae (both belonging to the order Ostreioida). Many well preserved Australian fossils of this group illustrate the process of becoming asymmetrical in great detail.
- Adapted from Rahmani, 1983.
- Composed of many pecimens of Ostrea Glabra, from the Late Cretaceous of Alberta, not too far from Drumheller.
- Two oyster fragments, preserving the hinge and umbo area, just as one might find them on a modern beach today
- Upper Cretaceous, Mississippi.
About The Author
Written: June 7th, 2006.
Christian Gronau studied Palaeontology and Geology in Germany, before immigrating to Canada. He worked as a mine geologist in the North West Territories, where he met his wife Aileen. Both have been living - off the grid - on Cortes Island for nearly 30 years, where they operate a small, beach-only, oyster and clam farm ("Last Farm Oysters"), now in its 25th year.
