Your fossil appears to be the internal casting of a ? possible
ammonite. The shells fill up with sediment and solidify and the
shell can get worn away just leaving a beautiful casting such as
yours. ... Avril B.
While the fossil is an internal mold, it is not an ammonite (no
chambers). It reminds me of a freshwater snail like a ram's horn....John
The coiling is not correct for a planorbid (ramshorn) gastropod,
which is sinistral-hyperstrophic, giving the misleading appearance
of being dextral.
The rocks there are too old to be an ammonite, which appeared in
the Triassic. The oldest ammonoids, of which the ammonites are the
youngest group, appeared in the Devonian.
This shell is an internal mold or steinkern, possibly a nautiloid
in which the septa have not been preserved (rather unlikely) or
miore probably a monoplacophoran or a bellerophontoid gastropod,
both of which have bilaterally-symmetric shells. ...Andrew G.
it surely looks like a bellerophontid. Especially since Andrew
said the rocks are too old for ammonites....
Dickson County has lower Carboniferous (Mississippian) rocks for
the surface. The lack of chambers strongly suggests a bellerophont
"monoplacophoran". Bellerophonts were planisirally coiled
shells, probably including some early gastropods, some relatives
of other living groups, and some extinct groups. ... Dr. David C.