Aptychi: the myth of the ammonite operculum

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Abstract
Re-examination of ammonite specimens with the aptychus apparently in place closing the aperture has shown that these are accidents of preservation and have been misinterpreted as indicating that aptychi functioned as opercula. There remain no convincing grounds for doubting that aptychi functioned only as lower jaws of ammonites, not as opercula. Chance preservation of exceptional specimens can sometimes be misleading.

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