Data:
Hello,
I was searching the Internet to see if I could find help identifying the fossil pictured below (and pictures attached in case the inserted pictures don't show). I am not a collector just happened receive this from the estate of friend who passed away recently and thought it would be nice to know what it is.

My best guess is Jurassic Ammonite but I really don't have a clue other than general circular shape. Any chance you can help?

It measures 10 inches across at the widest point and he colors below are darker in the picture than actual, but hoped the detail might help.

I hope you can advise or direct me to help figure this out.

Sincerely,

Mark T.

Send Ideas to: Mark T.

Identified:

Discussions:

• It certainly is an ammonite. You can see the typical suture lines on it. Your mention of an "internal cast" brings to mind a question I never thought of before. Ammonite fossils are almost always "solid", that is, the interior of the shell appears to be completely filled with solid material. However, were not ammonites compartmentalized internally, just like modern Nautilus shells? If so, how could sediment get into the innermost compartments of the shell? ... Paul M.
  

• 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 T.