Fossil ID Help :)

[SCMDing]

Hey there Tnet'ers.

My wife has a student who asked if she knew anything about a "fossil" she had. My wife brought it home to me to figure out what it is. Of course, she thinks I am smart when it comes to these sorts of things, but in reality, you are my secret weapon!

At first glance, it almost looks like a poured mold, but looking closely, you can see that it is not. I also notice that there is a modern green dot on it which leads me to believe it may have been in some sort of rock/fossil store. I have seen where they have marked geodes this way. You can see it on the bottom of the second picture.

So here is a picture, let me know what you think. I am probably going to document how I found out what it was, and make it a teaching moment for this student so that in the future she may be able to research it on her own. Thanks in advance.

 

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[Harry Pristis]

So, it has perfect bilateral symmetry? It is made of what? Heavy/dense? Why isn't it a cast? Is there any evidence of vascularization? Where is it from? Where was it obtained by the student?

It seems very large for a Tilly bone. Steinkern of a clam leapt to mind, but you say it's not a cast.

Better images would be helpful.

 

[SCMDing]

For some reason photobucket shrunk the image on me. Is there a particular area/angle you would want to see? I am moving the image out to an old site, it should hold the original size now. Sometimes you need to refresh the page to see the image, which is the main reason I no longer use it

OK, now to your questions.

1) It has near perfect symmetry. There are a couple chipped spots.

2) It is heavy and dense. The area where the two points of the shell ( it looks like a big beetle shell to me ) has some big chips missing and you can see that is has a harder black stone look to it. I have a couple trilobite fossils and it reminds me of the actual trilobite stone material. Between the two points ( I really apologize for all the wrong terms here, I am no fossil expert ) the material has almost a glassy grey surface to it, where the rest of it is more rough.

3) I am not entirely sure what I need to look for for evidence of vascularzation. All of the lines/grooves on what I would call the shell appear a little odd to me and are what originally made me think it was cast.

4) And lastly, it was from a kindergarten student who knew nothing about it. My wife was just going to contact the parents right then and there to return it, but she wanted to know if it was real before she called them. The family is not very well off and if it happened to be worth something, she wanted to make them aware of what they were letting their child bring to school.

Hope that helps, I appreciate the quick response. Let me know if you want a picture of a certain area. I can also do a REALLY detailed scan of a section if that will help.

 

[Harry Pristis]

It appears to be the steinkern (internal cast) of a bivalve.

 

[Solius Symbiosus]

Yep, internal mold of a bivalve.

Harry, wouldn't a cast imply a neg internal, or external

 

[Harry Pristis]

Yep, internal mold of a bivalve.

Harry, wouldn't a cast imply a neg internal, or external

In this case, the valves were the mold, and the cast is created by the infiltrating sediment. In other words, a steinkern is the cast of the interior of the fossil clam. We don't know what the exterior of the valves looked like except by extrapolation.