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I can not really find a definition, not even in a geologic dictionnary: What are articulate (resp. inarticulate) shells?

To be more specific, the term is related to tsunami deposits (tsunamite).

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If I remember correctly from my intro to paleo class, the terms articulate and inarticulate refer to a classification of brachiopods depending on the nature of their hinge. Articulate brachiopods have something like interlocking hinges (like a door hinge) made from serrated (or tooth like) parts of the shell. On the other hand, inarticulate brachiopods hold their shells together using only their muscles.

This means that the upper and lower shells of an inarticulate brachiopod will separate after it dies and its muscles decay. However, the shells on an articulate one will pretty much hold together unless a very strong mechanical force is applied. I don't know much about tsunami deposits, but this may have something to with that.

I'm not sure if this terminology extends to other phyla like molluscs, but I remember it from brachipods.

And here's a little picture because pictures make everything better.

enter image description here

Hope this helps!

-Antonio

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  • $\begingroup$ Thank you, I think I got it. But are you sure that it's not vice versa? I think the articulate shells do have a ligament; the organic band that decomposes 3-4 weeks after the shell's exitus. This would explain why articulate shells are used to identify tsunamites and tempestites (storm deposits): If the shells are imbricated closed, they were covered alive. Or am I wrong? $\endgroup$ – Arne Feb 28 '16 at 14:18
  • $\begingroup$ Uh oh... I hope my memory didn't betray me here. It's been a while. Let me go dig up my paleo book! brb! $\endgroup$ – Antonio Feb 28 '16 at 18:00
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    $\begingroup$ Well, according to good ol' Wikipedia: "Articulate ("jointed") brachiopods have a tooth and socket arrangement by which the pedicle and brachial valves hinge, locking the valves against lateral displacement. Inarticulate brachiopods have no matching teeth and sockets; their valves are held together only by muscles. (R. C. Moore, 1952)" - R.C.Moore, 1952; Brachiopods in Moore, Lalicher, and Fischer; Invertebrate Fossils, McGraw-Hill. $\endgroup$ – Antonio Feb 28 '16 at 18:08

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