Natural Science Forum / Biology / Paleontology / July 2005

Not having seen the rocks it might be that most of the cobbles would look
like that if you broke one in half. The core would be unchanged while the
outer shell would have undergone chemical change.

Nothing there that jumps right out and seems to lock them in to a
specific time.  My impression is that coiled cephalopods first
appeared in the Ordovician, so probably not Cambrian.

As for the last picture, my guess, and it is a guess, is that you are
looking at a concretion that has fractured along a bedding surface.
There appears to be a lot of faintly preserved organic material on the
surface.  That's just my take.  If someone else says something
different, they are likely correct.