How to distinguish between tube worms and vermetid snails?

[Nadav]

I'm studying a coniacian site and one of the most abundant fossil there is some sort of calcareous tubes. Sometimes solitary but most of the specimens are a colony of many tubes tight together. Sometimes they're attached to objects and sometimes are not. I'm wondering if there's a way to determine if they're vermetid snails or tube worms (or something else).

[Al Dente]

8 hours ago, Nadav said:

I'm wondering if there's a way to determine if they're vermetid snails or tube worms

I believe Vermetid gastropods have an aragonitic shell. Aragonite is not as stable as calcite. There probably aren’t any aragonitic shells at your Coniacian site.

[Nadav]

1 hour ago, Al Dente said:

I believe Vermetid gastropods have an aragonitic shell. Aragonite is not as stable as calcite. There probably aren’t any aragonitic shells at your Coniacian site.

Thanks. So aragonite shells aren't common on fossil records?

[Al Dente]

34 minutes ago, Nadav said:

Thanks. So aragonite shells aren't common on fossil records?

They are fairly common in younger sediments (Pleistocene, Pliocene) but they get less common the older the sediment gets. Cretaceous sediments can have them but usually only in clayey deposits.

[Nadav]

5 hours ago, Al Dente said:

They are fairly common in younger sediments (Pleistocene, Pliocene) but they get less common the older the sediment gets. Cretaceous sediments can have them but usually only in clayey deposits.

Thanks again. But I'm still confused as ammonites shell were also aragonitic and are very common to find in Cretaceous sediments. Also, the site is marley limestone 

[Al Dente]

5 hours ago, Nadav said:

Thanks again. But I'm still confused as ammonites shell were also aragonitic and are very common to find in Cretaceous sediments. Also, the site is marley limestone 

Ammonites with actual shell are not that common compared with ammonites preserved as steinkerns, where the shell is missing. There are several well known sites where ammonites with preserved shell are found. These are frequently found for sale making it seem they are more common than they really are. Have you ever found an ammonite with shell in your area? I’m guessing you are finding only steinkerns.

[Ludwigia]

5 hours ago, Nadav said:

But I'm still confused as ammonites shell were also aragonitic and are very common to find in Cretaceous sediments.

 

To my understanding, ammonites originally had aragonitic shells, but their preservation up to the present time in this form is quite rare. Most of them have shells which have been converted to calcite over the millenia, which is much more stable than aragonite. Here is a thread I opened up a few years ago on the subject of aragonite vs. calcite in shells

[Nadav]

8 minutes ago, Al Dente said:

Ammonites with actual shell are not that common compared with ammonites preserved as steinkerns, where the shell is missing. There are several well known sites where ammonites with preserved shell are found. These are frequently found for sale making it seem they are more common than they really are. Have you ever found an ammonite with shell in your area? I’m guessing you are finding only steinkerns.

You are right. I haven't found ammonites with preserved shells in this site (I did in other sites) but there are other gastropods with nicely preserved shell in the site like this one:

[Al Dente]

23 minutes ago, Nadav said:

there are other gastropods with nicely preserved shell in the site like this one:

I’m guessing this gastropod shell was originally calcite or has been replaced with calcite. There are a few Cretaceous outcrops in the state I live where original aragonite shells are preserved. The shells are white and very soft. They will crumble easily if you aren’t careful. It is very difficult to remove them from the matrix without destroying the shell, even though the matrix is unlithified.