Flint sea sponge cap remains?

[Thames Adventurer]

I collected these off the Thames foreshore. I have been lead to believe that these are the remains of sea sponge (that they once held them, or were them??) but I don't really understand. Can someone here help confirm and explain this please?

Thank you

[TqB]

Sponges (and other potential fossils) probably became enclosed in colloidal silica gel permeating the Chalk that later solidified to form flint. The silica gel itself is largely derived from sponge silica.

Once enclosed, various preservations occur. In your specimens, most of the original sponge has been lost and you have what is largely an external mould maybe with some external layers.
Before being broken open, there was probably more inside. Masses of spicules are frequently preserved as loose, powdery material which is good for microscopic study and identification. 

[Thames Adventurer]

3 hours ago, TqB said:

Sponges (and other potential fossils) probably became enclosed in colloidal silica gel permeating the Chalk that later solidified to form flint. The silica gel itself is largely derived from sponge silica.

Once enclosed, various preservations occur. In your specimens, most of the original sponge has been lost and you have what is largely an external mould maybe with some external layers.
Before being broken open, there was probably more inside. Masses of spicules are frequently preserved as loose, powdery material which is good for microscopic study and identification. 

 

Thank you very much, this is a really helpful explanation! 

 

Why do some of the broken shells have a small hole in them- were they once attached to something, or was this just part of the sponges design or fossilisation process? 

[TqB]

1 hour ago, Thames Adventurer said:

 

Thank you very much, this is a really helpful explanation! 

 

Why do some of the broken shells have a small hole in them- were they once attached to something, or was this just part of the sponges design or fossilisation process? 

Yes, they were often attached by a stem which leaves a hole. There's also a hole at the top anyway in many sponges which were hollow in life, like various shapes of vase.