Fossil?

[BLT]

I found this rock in a middle Tennessee creek. (Mississippian, St. Louis Limestone & Warsaw Limestone) What is the area circled in red? It almost has the shape of a large tooth, but not the texture. Is it fossilized coral?

[Auspex]

This is an interesting mystery!
It seems to be a well-differentiated mass, with signs of an organized, repeating infrastructure.
I do think it is organic in origin; maybe a sponge?

[Fossildude19]

I'm thinking a worn bivalve steinkern.   

[Auspex]

Precedent

[BLT]

23 minutes ago, Auspex said:

maybe a sponge?

Hmm. Thanks! I’ve wondered if that was a possibility. 

11 minutes ago, Fossildude19 said:

I'm thinking a worn bivalve steinkern.   

I hadn’t considered that possibility. Thanks! 

[BLT]

1 minute ago, Auspex said:

Precedent

Hmm. I can see a slight similarity in the two. Yet, I still would not have thought this one to be a concretion. 

[BLT]

[BLT]

[ynot]

Interesting piece.

It could be a sponge, but I lean more to concretion.

[BLT]

3 minutes ago, ynot said:

Interesting piece.

It could be a sponge, but I lean more to concretion.

Thanks! It seems that each time I believe I can confidently identify a concretion, one comes along to stump me. 

[ynot]

Concretions can take many strange forms.

[Herb]

I am in the sponge camp

[BLT]

36 minutes ago, Herb said:

I am in the sponge camp

Thanks!

[JimB88]

the second is a worn rugose coral

[BLT]

1 minute ago, JimB88 said:

the second is a worn rugose coral

Thanks!

[JimB88]

np