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What is Pebble Beach like?

What is this strange symmetric mark on a pebble found on the beach?

  • Found on the beach near ChaÅ‚upy, Poland. https://goo.gl/maps/mbRk5.

  • Answer:

    It almost certainly is a fossil - the structure is too regular and as you note asymmetric to be inorganic, and it looks like a calcareous fossil in a dark limestone. It would be nice to know a bit more about where it was found, and take a closer look at the shell structure, but my guess is that it is an orthoconic cephalopod.  These are related to ammonites and modern Nautilus, but with a straight shell. They occurred throughout the Palaeozoic and in rock shops you often see slabs of black limestone from Morocco filled with them. On this interpretation this is a cross-section through the shell with the curved lines being the plates (septa) between the chambers and the breaks are where the cross-section goes through the central tube. Here from wikipedia (Orthoceras page) is figure illustrating the morphology ad some more complete fossil specimens.

Jeremy Young at Quora Visit the source

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Ammonite would be my first guess as well.  I have some of these in the limestone  tiles of my bathroom floor.  There are lots of different ones found world  wide in limestone, shale and sandstone beds. Ammonites ruled the world for about 200 million years starting to appear in the fossil record in the late Silurian and early Devonian periods or about 400 million years ago.  Then all ammonites became extinct at the end of the Cretaceous, 65.5 million  years ago.  So this is a very old rock!

Viola Yee

It almost looks like a turritella cut in half. However the columella is absent. This is what a turritella looks like This is what a turritella looks like when it is cut right down the middle along its length (cross section)  The long column that separates the chambers and runs from the pointed end (apex) to the bottom (aperture) is called the columella. I think if the section plane did not pass through the middle, the columella will not be seen. I hope I have described it well enough for you to imagine it.

Surya Sankarasubramanian

That, my friend, seems to be part of a fossilized squid spine (sea-tongue). Modern squid backbones are usually given to parrots to chew on.

Daniel Viljoen

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