[louie6324 (OP)]

Hey! My son found this in the garden last week and I have no idea what he has found! He is sure its something exciting but I'm having trouble helping him to identify exactly what it is. It was in one piece when he found it but like everything other 9yr old little boy, couldn't resist giving it a bash!!

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More pics to follow - by the way, we live in Buckinghamshire in England

Thanks in anticipation

[louie6324 (OP)]

More pics (don't know how I ended up with 2 lots of pics!)

[louie6324 (OP)]

nearly there......

[louie6324 (OP)]

All done!

[RD]

Sorry it's not a fossil egg ... http://www.fallsoftheohio.org/Egg-LikePseudofossils.html

[JimBob]

This is a fossil. It was originally a bi-valve, like a mussel or clam. The red arrow points to the place where the hinge of the clam was. The interior of the shell was filled with mud.

Later, the shell was replaced by the gray (grey for you antiquarians) chert - blue arrow.

[RD]

We are unanimous on chert, but I'm sticking with pseudofossil :
if it's a shell it has unbelievably thick walls for its size.

Chert or flint nodules in limestone can often take forms that resemble fossils.

http://en.wikipedia.org/wiki/Pseudofossil

[JimBob]

OK - step by step.

1. Clam shell gets buried in mud.
2. The insides rot out
3. The interior gets filled in with something a little harder, probably carbonate muds with a fair amount of clays (silicates)
4. After only a slightly longer period of time the aragonite of the shell - more easily dissolved - is dissolved away and replaced by the chert, the mud remains as it has much more silica in it than the shell did.
5. The chert keeps being deposited, well beyond the original limits of the shell and the chert nodule eventually stops growwing irrgulalry around the interior mold (the cast.)

Once you have seen a clam shell interior mold - which is what this - you will never mistake it for anything else. The area I live in was a shallow sea during the late Cretaceous. I have seen these things since I was a child.

SEE - Casts and Molds on Wiki

See http://en.wikipedia.org/wiki/Fossil#Casts_and_molds

[JimBob]

A good candidate for what the original bi-valve was could be a type similar to these:

http://fossilsofnj.com/invertebrates/exogyra.htm

The Chamidae are good candidates for the real progenitor of this fossil.

[frethack]

Ill second the fossil interpretation.  The shape is pretty uncanny, and you have both the cast and the mold.  It is possible that the chert deposited around the shell and then the shell was dissolved, allowing more chert to deposit.  Explains why the cast and mold are both present.  Id have to see a photomicrograph of the chert, but I would bet its a spiculite (formed from sponge spicules) and that it was buried in a peri-reef environment (possibly at the edges of a carbonate mound).

[JimBob]

Ill second the fossil interpretation.  The shape is pretty uncanny, and you have both the cast and the mold.  It is possible that the chert deposited around the shell and then the shell was dissolved, allowing more chert to deposit.  Explains why the cast and mold are both present.  Id have to see a photomicrograph of the chert, but I would bet its a spiculite (formed from sponge spicules) and that it was buried in a peri-reef environment (possibly at the edges of a carbonate mound).

Now you are reaching a bit there, frethac And I don't have a hand lens that doubles as camera for photomicrographs.

[frethack]

Reaching?  For the cast and the mold to both be present, the shell would have to have been there during at least the beginnings of chert formation.  If the shell were not present during silica deposition it would be more likely that the interior mold would be fused to the silica.  Here they are separate.

If youre referring to the chert being a spiculite, that is speculation (or is it spiculation?), as can be inferred from my post.

See if I ever agree with YOU again.

[JimBob]

Reaching?  For the cast and the mold to both be present, the shell would have to have been there during at least the beginnings of chert formation. 

I think I said that. RE: "aragonite of the shell ...."  "around the interior mold (the cast.)"

See if I ever agree with YOU again.

On politics you never have. I am accustomed to your dark side.

[Geezer]

Now looky here!

If you two rock-huggers can't get along, you'll leave me no choice but to lock this thread.

[JimBob]

What good would that do?

I can just unlock it, post my views, then lock it again. Frethac would need endure seeing the right answers ALL the time, not just part of the time when he posts something incorrect.

[frethack]

Dont make me dig through your posts JimBo! 

[JimBob]

Hey Geezer, this guy is threatening moderators. Should we ban him?

[yor_on]

Don't worry.

I'm here too, and I  w i l l  lock it..