Paleontology of SW Germany 3.1.4: still stuck in the Lias

and still stuck at the urweltmuseum and quarry Fischer. As mentioned, the museum has a large hall with many nice fossils. It also has a mural.

Wall Painting_small

click through for larger version; full sized 17MB version available via email.

It is a pretty cool painting, showing life reconstruction of the specimens on exhibit. It doesn’t try to be a great landscape reconstruction, which is great – less atmosphere, more animals :)

Here’s a close-up of the most common kind of animal in the Posidonia Shale.

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An ammonite is, simply stated, a cephalopod shoved into a calcium carbonate shell, which typically is coiled more or less tightly. The shell has a big chamber containing the animal’s body, from which the head and tentacles stick out. As the animal grows, it periodically creates a wall partitioning off the posterior part of the chamber. This results in a series of ever larger chambers with walls between them, and it is by these walls and how they attach to the outer shell’s inside (creating a pattern called sutures)  that ammonites are classified. The outside of the shell is covered by an organic layer, the periostracum, which is involved in the secretion of the mineral shell. What’s so cool about the Posidonia Shale is that the mechanically tough anorganic shell is gone, but the soft, organic periostracum is still there!

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I posted this Harpoceras before – it is a perfect example of the typical Posidonia Shale preservation. You can see the gold-ish colored fossilized periostracum nearly flat on the rock, and the white line is the remains of the membrane covering the siphuncle.  So why oh why is the shell gone? And, if we look closely, why is the shell not really fully gone: there are places where the fossil is not perfectly flat, but shows breakage and shell-like curving fragments. Especially visible on this Phylloceras:

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Here, you can see that some of these fragments have moved, as if the shell was crushed by sediment compaction – but why did this not happen in the Harpoceras above, which has a perfectly smooth outline?

The secret is the anoxic (oxygen-depleted) conditions that prevailed at the bottom of the Lias-Sea, and tiny differences in shell morphology between the two ammonite genera. When the dead animal settled on the sea bottom the regular decay process of soft tissues, including the periostracum, could not take place. Rather, the softer soft tissues putrefied away happily, whereas the tough periostracum stayed practically untouched. The high levels of humic acids, created by the partial decay of organic matter, started dissolving the shells. At the same time, the shell was getting covered by sediment. At some time, the combination of an ever thinning shell and increasing vertical force crushed the shell. Depending on the thickness of the shell and sedimentation rate, this could take very long, long enough in fact for the shell to be nearly completely gone, in which case the periostracum must have been a kind of balloon-like sac. I have seen fossils where this was the case, which you can tell by their absolute flatness and them often being somewhat egg-shaped: the periostracum offered no resistance to being deformed. If thicker parts of the shell were present, such as ribs, these often remained and left some relief in the sediment, which is why we can see very nice strong ribs on, e.g., Dactylioceras fossils. Or why the Harpoceras above retained all the thin fragments of the shell within its oval outline: it had a strong keel running along the outer edge, and this keel contained them. The Phylloceras, in contrast, got crushed the way an egg would, and the thinned shell fragments could move a bit more. What’s really cool is that usually, the living chamber is not really fully flat, but has a thin sediment infill.

But now we hit a further puzzle: the calcium carbonate forming the shell got dissolved – why do we see the wonderful aptychi, the calcitic dark plates we can see on the Phylloceras? Calcite is calcite, whey were they not dissolved?

Well, note how carefully I phrased this: the calcite aptychi are still there, but the shells also made from calcium carbonate are (mostly) gone. CaCO3 is a mean bastard, it occurs in several polymorphs. Calcite is one of them, aragonite another. The ammonite shell consists of calcite, the aptchi of aragonite. And the two polymorphs have slightly different solubility: Aragonite is a tad tougher than calcite! The difference is about 7%, not much – but in a place where a lot of calcite is present to get dissolved (and thus alter the pH-value of the solution) it will usually not get dissolved. Which is why there are aptychi but not shells in many sediments of shallow seas.

OK, enough taphonomy and chemistry, let’s see some more fossils and shit and get out of Fischer’s!

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This is not an ammonite – no worries, they will be back in force once we cross the road and get into Hauff. This is an ichthyosaur skull (Leptopterygius burgundiae), and a special one (the sign claims) at that:the largest ever found! The animal was probably between 12 and 15 m long!

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Here’s a Stenopterygius - booooooooooooring if you studied geology at Tübingen. A dime a dozen….

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and another Steneosaurus bollensis

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a pretty and tiny Seiocrinus colony – wish I had that on my living room wall! It is about a meter square only.

And finally, two models: a plesiosaur and a pteryosaur. At the Fischer museum, these not-so-great models look a bit out of place – let#s compare to those at Hauff later :)

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Hauff up next, and I promise it will be a blast!

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About Heinrich Mallison

I'm a dinosaur biomech guy working at the Museum für Naturkunde Berlin.
This entry was posted in Holzmaden, ichthyosaur, lower vertebrates, Pterosauria, Travels. Bookmark the permalink.

10 Responses to Paleontology of SW Germany 3.1.4: still stuck in the Lias

  1. tsjok45 says:

    Well …. I really have to visit ( and see , at least once in my lifetime ) that museum I guess
    p.s. My very best an sincere wishes for the new year

  2. Pingback: Paleontology of SW Germany 3.1.5: Hauff! (finally) | dinosaurpalaeo

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