Maybe the simplest answer is the correct one. Clams are bivalves. The clam opened up (dead for whatever reason) and the ammonite swam or got trapped inside it?
Whatever the reason (and I agree that a chance falling of a dead ammonite on a dead shell is the likely one) you must henceforth think of this specimen as your "clammonite."
Im thinking along the lines, where the clam died and the clam opened up because its muscles no longer kept the clam shut (the reason that you rarely find both clam valves or shells together on the beach or as fossils). Then the ammonite swam in perhaps scavenging a meal.
Then, my thinking falls apart, because the ammonite is surrounded by both clam valves. That would mean that somehow the two valves shut, trapping the ammonite. How would this have happened, when the clam was already dead?
Also, I like the "clammonite" theory.
Most likely the clam died, the shell opened up (as dead clams do because the muscle along the hinge is the last tissue to rot away) and the dead ammonite was tumbled inside by the current. It may have been immobilized once inside (the current wasn't strong enough to push it back out or it was uni-directional). Eventually sediment surrounded and covered it, the weight closing the clam shell back together. That's my hypothesis and I'll sit by it. (I'm in a chair, so I can't stand by it!).
I have found all sorts of critters washed inside Ordovician orthocone cephalopods - Isotelus pygidia, Rafinesquina and other brachiopods, small crinoid calices, scolecdont teeth, etc. I am sure empty shells were sought for shelter as they are today, but random mixing of dead things also produces good results.
It could've happened as a result of a flip of a fin near the two doomed critters. What seems certain to me is once the ammonite was inside the clam a quick covering of sediment would've been needed to preserve the two fossils together. There's something romantic about this thread.
Most clams cannot open that wide when they are alive. (Some scallops do, but that wasn't a scallop.) They feed through siphons which are surrounded by muscles, below them are the internal organs and the retracted foot muscle. Only dead clams will open wide enough to hold another organism like an ammonite.
The only time I went clam digging was for "soft shelled clams" Mya arenaria on a mudflat on the Siuslaw River on the central Oregon coast...I followed a spurt of water with my shovel and turned up half a dozen clams, so in the bucket they went; until I limited out with the allowed 20 or 40...or whatever.
I get them home and it turns out I have...like 4 or 5 clams...and a bucketful of sand and clamshells...Whaaaaaaaa?
So the ammonite died at sea and washed into the stinking gray muck where the clams do their thing...Five years later a little clam dies in his tunnel, putrifies in his relaxed open shell and the ammonite's shell is worked into the clam's gape by the same process of infiltration that filled my empty clams.
Im thinking the deaths were reversed. At least initially. The ammonite died, fell to the bottom, and happened to hit an open clam. The clams natural response to such a touch would be to close. I dont know how much ability a clam has to disgorge such an object once it gets trapped within its shell. If it couldnt be dislodged, the process of decomposing may have been sufficient to kill the clam, sort of like sepsis.
Not so complex. I suspect the clam in question was a hard-shelled variety. It probably died at or near the surface and ended up on the sea floor, not in a tunnel. It was briefly moved around by the currents before the ammonite got wedged inside.
When I was in Florida this summer, I found tellins and venus clams with their shells wide open, but still connected. When I picked some of them up, a few had sediment which could have contained smaller snails and bivalves stuck in the fine sand. Statistically speaking, large particles (e.g. shells) will wash in occasionally and a few of those will be buried and preserved. The odds of finding a clam with an ammonite inside is still a one-in-a-million! That makes it a great fossil and would be ideal for a museum exhibit on fossil preservation
Perhaps this is early ceph hiding behavior not unlike the choices an octopus makes today when squeezing into extremely cramped chambers. OR some hermit crab-like strategy carrying about an extra bit of armor.
I've found a bed of Cretaceous oysters which several of the valves have grown around a baculites like structure. I doubt this is wood attachment owing to the consistent shape of the indentation. By indentation I mean a inch deep side-- 90* turn over a similar distance and then back up the other side.. I can't account for how the valve grew to enclose a portion of whatever it attached to yet remained sync'd with the other valve's symmetry. Did the oyster settle on a dead baculite, or a live one? Was it parasitic or symbiotic? Sounds like a doctorate thesis in waiting.
I have found some Cretaceous inoceramid bivalves in Colorado (Niobrara Formation) filled with several fish that apparently lived inside the shell of the live bivalve, which were quite large. I believe one or more papers have been published about those.
While this find may be rare (but do we really know that?--I mean, how many shells of this fossil clam were opened to see?), the ammonites may have lived in them, or used them for some purpose, whenever they could get inside. Being octopus relatives, using suckered arms they might have been able to pry the shell a bit to squeeze in. Maybe they used the shell as a brood chamber.
My inclinations. Some inoceramids, I believe exceeded 6 ft across and entire schools of small fish seemed to have sheltered within. I have found parts of large ones but never a whole one. I believe Kansas has yielded large ones with both vales intact and some with fish inside.
If we could solve this puzzle I am sure it would be insightful into the origins of some ceph behaviors--unfortunately this is the only case I have ever heard of and if it were an established "behavior" we would expect to have found more examples thus far. It remains a true enigma and this type of find I relish more over for the untold story within.
This has certainly been a grand discussion and I have enjoyed seeing the group working as a whole to seek an answer. One final question, has anyone made an ID on the two species?
Click Here to Go-Back