John C. W. Cope, David T. C. Sole; Ammonite jaw apparatuses from the Sinemurian (Lower Jurassic) of Dorset and their taphonomic relevance. Journal of the Geological Society ; 157 (1): 201–205. doi: https://doi.org/10.1144/jgs.157.1.201
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The occurrence of ammonite jaw apparatuses in situ within concretions from the Sinemurian (Lower Jurassic) of Black Ven Cliff, Charmouth, Dorset, provides evidence of burial of the ammonites with soft parts intact and this is of importance in considering the origin of the concretions. The lower jaws (anaptychi) of arietitid and eoderoceratid ammonites are preserved and three examples of upper jaws are recorded in the species Asteroceras stellare. Although usually accepted as being entirely of organic composition, the anaptychi of A. stellare have an outer calcareous layer.