Some field observations. 

 

The interior of nautiloid shells fill in by sediment sifting in through the interior canal that the animal used to balance gas within its chambers in life. Additionally holes in the shell itself - such as those created by sulphur sponges are a common pathway for sediments. As the original shell is often dissolved, evidence of such perforations are lost.  

 

In my expereince collecting Eocene fossils in NC it is not ucommon to recover nautilods where the innermost chambers (as well as nuclear whorls in large gastropods) are not filled in with any sediment, thus their molds are often missing the upper most chambers or whorls. In large nautiloids the absence of those preserved chambers may not be evident as they are obscured by subsequent chambers. There is often evidence of sulphur sponge borings on the molds of gastrobods including nautiloids. The challenge is to find that rare fossil where sediments accumulated all the way to the nuclear whorls before the shell became buried but before the shell disintegrated as well.

 

Many ammonites I have seen sectioned are hollow but were filling in with crystaline minerals at the time they were found.

 

I think about how much sand one can dump out of long dead helmet shells found along North Carolina beaches to imagine the process in action.

John Timmerman

Wilmington, North Carolina  

 

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