How do you know when something is fossilized?

[Jess1313]

My question is how does one tell when something is fossilized, or is a fossil? I know some things just look like they are "fossilized", but what about things like clams or shells? I found myself wondering this while watching one of Paleo Cris' videos. I can't see any difference between a modern shell and the shells he finds in Florida. Maybe it's a touch type of thing. Any information would be appreciated!

[DPS Ammonite]

The only requirement for the remains of an organism to become a fossil is the passage of time. Usually remains become a fossil after about 10,000 years. No change of form such as replacement, silicification, mineralization etc. is needed. Loose shells on the beach older than 10k years might not be recognized as a fossil if they are still extant. You need to find shells in a formation that is older than 10k years for the shell to be recognized as a fossil. If a shell has been extinct for over 10k years than you know that it is a fossil even if it is not found in a datable formation. Sometimes knowing what species an organism is will tell you if it is a fossil. 

[RuMert]

As usual, the important is not how the object looks but where it comes from. If there is a described formation and the specimen is taken from the layer, it's a fossil from this formation. If it's not taken from the layer, but found on the shore, for example, it needs to be compared to the fossils from the layer and if matches, it's again considered a fossil from that formation. Sometimes (but rarely) the formation is not described from that exact site, but you see clear signs of it (typical Jurassic clay with Jurassic ammonites, for example). A specimen can have parts of matrix attached, than again it should be matched to the nearby fossil layer. Otherwise, with no formation known from that site, no signs of it , and no matrix and preservation characteristics of a given formation on the specimen its 99% not a fossil

[FossilNerd]

Great information from the folks above. The age of the shells and what formation they are found in will both help determine if the shell is a fossil or not.

 

To add to it, I would say another facet to the answer of your question is knowledge about the shells. Cris has studied these shells and knows the features and diagnostic characteristics that point to them as belonging to a certain extinct genus/species. These features, or combination of, will not be found on any other species. Sometimes knowing the species of a fossil will limit it to a certain time period or formation.

 

To the uninitiated these shells look just like any other, but to someone who has studied them, they look completely different as they know what features to look for that make it a different species. Some diagnostic features are very subtle. Especially when it comes to shells.