I found this in gravel. Could anyone kindly tell me whether it's a plant or a creature - assuming it's actually a fossil!?
I think it is just a lump of rock, possibly flint.
I see the circular mark, but even that could be inorganic (flint sometimes grows over itself or incorporates other bits of flint).
To be a fossil, we'd expect to see some evidence of internal structure and/or surface texture and/or symmetry.
To be a cast or mould of a fossil, we'd still expect to see a recognizable shape.
To be a coprolite (fossil faeces), we'd expect a somewhat irregular shape, but evidence of food remains and/or a certain appearance!
Yes, a sponge is a possibilty, though going mainly on the overall shape rather than something more definitive.
Here's one that was discussed here in 2011
Thanks for mentioning the idea.
Dan, Mike, Tabfish,
Many thanks to you all for your thoughts. See why you thought of the earlier post too, Mike. I will take a look at the piece and see whether there's any evidence of the internal cellular structure pictured in your link.
Being a complete beginner in all this only adds to my admiration and gratitude!