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In page Belemnitida:

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The guard–also known as the rostrum, scabbard, gaine, and sheath–[4] is the part of the animal most likely to be fossilized.[5][6] Guards are difficult to distinguish at the species level, and, consequently, synonyms are common and inflate the group's apparent diversity.[7] Preserved hooks can be used to distinguish belemnite species as each species has unique hook shapes. However, scolecodont segmented worm fossils have been mistaken for belemnite hooks and vice versa.[8] Preserved fossil guards are used to measure the ancient isotopic signature of the waters the individual inhabited in life, which gives information on the climate, habitat, and the carbon cycle.[9][10]