This part of the Palaeontology of SW Germany series has arrived at a deep low in the number of vertebrates shown, and it is time I fight my way through the rest of the spineless stuff to the finish line and move on into the Middle and Upper Jurassic. Or not, as vertebrate remains are very scarce there, compared to the Posidonia Shale. Well, maybe I can simply switch museums and bring you some awesome verts from Stuttgart soon 😉
So today we will have clams for an appetizer. No Surf & Turf, though, because there is no realistic chance of getting a good steak from any animal preserved in the Posidonia Shale. There is a sauropod, but that’s one rare freak occurrence, so I won’t count it.
Bositra buchi. Sorry for the out-of-focus shot. My hands were getting tired by the time I got around to this all-important fossil.
What? You don’t see the importance of a clam called Bositra buchi for the Posidonia Shale? Well, google it!
Oh, no useful hits. Nothing on wikipedia, for instance. Hmmmmmm…..
OK, it is time to let the cat out of the bag: the previous name of this clam was Posidonia bronni! Now that is guaranteed to have a wikipedia….. oops, it doesn’t. Hmmmmmm!!! That is, in fact, a surprise to me! Every two-bone dinosaur find that gets a new name in some obscure West African or Chinese journal has its own wikipedia or facebook page, but the clam that gave the name to one of the most important Lagerstätte in Europe, a Lagerstätte that has a tremendous historical importance, too – that clam doesn’t even get a page for the genus! Even the German wikipedia, which one would suspect to be choke full of pages on the Holzmaden fossils (and indeed it has much more to offer in some respects than the English version), is pretty bereft of entries even on many of the marine reptiles of exquisite beauty. If only I had time – and had a beginning of a clue about them!
So, to end a long lament, here’s the deal: Bositra buchi, then known under the name of Posidonia bronni, is a clam so very common in the Lias ε that it gave its name to the entire rock formation. Despite the extreme abundance of the little cepahlopod in some layers, it is not called the Dactylioceras shale. It is not called the Fischsaurier Shale, although the ichthyosaurs are very impressive and rather common. No, it is called the Posidonia Shale. Which makes it a real pity that some guy realized that Posidonia bronni should rather be called Bositra buchi. Yuck, taxonomy! Here, it did its worst.
To get over this ugliness let’s have a look at some other clams – much rarer than Posidonias.
For once I have left the writing in the photo, whereas normally I crop to rock-only. But here you can nicely see, based on the unobtrusive but well-readable label, that these shells are quite large. How come we find such big clams in the anoxic Posidonia Shale?
For one thing, not all we find comes form the bottom of the sea, and only the bottom water was anoxic and thus hostile to life. Think of the crinoid colonies that grew on floating logs. They did quite well in the oxygenated surface water, but obviously died when the log finally absorbed enough water to sink.
Then, as I mentioned the Posidonia Shale has a bunch of layers that saw, at least part of the time, enough oxygen on the bottom of the sea to have crabs crawl around. A dead giveawy for these layers is the high carbonate content, because the water was less acidic. And if you take a careful look at the rock slab above you can see 3D shell negatives on the left, cut through. The calcite is gone, but the clams are still there – and this is the point where, over a beer, I would offer you a wager: I bet that the shells of Lima and close relatives are mostly constructed from aragonite, not calcite. Remember the aptychus preservation issue? Yep, same thing again here: the shell is preserved because it is a tiny bit less solvable than those of other organisms. And the shell lived there at a time with unusually high oxygen content. In an extreme case – we can imagine such a scenario – it a small organism may have lived on top of a rotting ichthyosaur carcass, only a few inches above the bottom mud – and have gotten enough oxygen there, while all around the bottom-most water layer was low enough in oxygen to kill all similar organisms. But in the case of these Plagiostomas the explanation is likely simpler: Lima shells are filter feeders that are free-swimming – the clam may simply have swum into the Posidonia Sea on its own – only when it left the free water column did it die.
OK, on to the next one: Pseudomytilus dubius.
If you’ve ever seen a Mytilus – the Blue, the California or any other species – you will know why that fossil above has that name. Much like the extant genus, Pseudomytilus uses byssus to attach itself to stuff, including floating logs and crinoid colonies on floating logs and all kinds of other marine life attached to floating logs – you get the drift. So, again, a highly oxygen dependant species in a anoxic sediment.
The remaining few clams on show are very small. Let’s first look at Oxytoma inequivalvis.
Finally, two specimens of Chlamys:
Chalmys sp. You can see the toolmarks stemming from preparation on the sediment in the lower photo, which tells you how tiny these specimens are. I found a specimen that is beautifully pyritized, and measures a whopping 11 mm across. Which fits nicely with the overall oxygen-deprived model of the Posidonia Shale sea, in which such clams should be very rare indeed.
Next up, the “fishes” (Yay, vertebrates!), and then I can finally get around to the finishing posts, one of which will discuss the life reconstructions. Phew – the end is in sight after all!