Concentric rings, most likely marine

[Fingerprint]

I found this fossil on a pebble beach in central Italy, Adriatic coast, Marche region. It shows concentric rings, with a pattern that looks vaguely like a "fingerprint". It could be a brachiopod, although the concentric rings pattern continues inside the rock, so it does not look like a thin shell (as you can see from the side and bottom view, the rock is broken right where the fossil is, with a flat side at a 90 degree angle). Maybe some kind of inarticulate brachiopod? The geologic maps indicate that the area should be mostly Pliocene/Pleistocene sediments, but on the same beach I also found another rock with a clear ammonite trace fossil, as well as various much more recent fossilized shellfish, and in general pebbles on the beach include very different kinds of rocks, so I would not be too sure of the dating.

 

If the dating is really Pliocene/Pleistocene, it could be Terebratula, but again it does not look much like a shell. It's incomplete though, so maybe eyes more expert than mine can tell.

Edited September 4 by Fingerprint

[Rockwood]

Calcareous algae? 

[Fingerprint]

Interesting hypothesis, thanks! I did think about some kind of marine plant or coral, but while there would be many common ones that would produce circles with radial patterns, I could not find many that would leave concentric patterns. This speaks more about the limits of my knowledge than the actual possibilities though.

[Mark Kmiecik]

Fragment of a conularid?

[Fingerprint]

Also a fascinating hypothesis, thanks!

 

My doubt though is that in most conularid fossil pictures I can find the arched pattern is rather “herringbone” shaped, and does not continue “inside” the fossil, in cross sections. In this specimen it appears that the pattern is present in both parallel faces of the rock, and also in the 90 degree section (although it’s hard to determine what the orientation is, and therefore if we actually see a cross section).

 

The fact that the ”rings” seem to occur on three sides of the rock (two parallel faces and a perpendicular section) is what I find most puzzling. I would expect it to be a thin shell layer in case of a brachiopod, or only present on the two parallel faces if the fossil was a section of a cylindrical or conical body made of concentric layers. I cannot seem to make up the shape of this creature.

[Fingerprint]

One possibility that would explain the three-dimensional pattern could be that the rings are actually thin dome shaped layers, so they appear as concentric rings or arches on both cross and longitudinal sections. It still does not suggest a clear identification to me though.

[supertramp]

Third hypothesis: fragment of a stromatoporoid sponge (ellipsactinia?)

[DPS Ammonite]

If a metal knife does not scratch it then it might be a banded chert nodule. It appears to be a very 3D object based on exposure on different sides.

 

 

[Fingerprint]

@supertramp that’s a great hypothesis, and in my opinion the most likely, also given the area where it was found. Ellipsactinia and Sphaeractinia both match the pattern very well and are found in other nearby areas. I didn’t know about this organism, great to learn, thanks so much!

 

@DPS Ammonite I did consider a non biological origin, although it seems unlikely as the arches are many, thin, and very regular. You make a good point though, and pseudo fossils are common. I tested with a knife, and it does seem to scratch, but I am not expert enough to completely rule out the possibility.

[Fingerprint]

@supertramp suggestion of a stromatoporoid, probably Ellipsactinia or Sphaeractinia, really seems like the most likely option to me, given the pattern and the location it was found. I read Ellipsactinia and Sphaeractinia lived in the Tethys sea, and is common in the Adriatic (see https://www.dst.uniroma1.it/Volumi/VOL 40/18 RUSSO.pdf). If that's correct, the dating is Mesozoic, most likely Late Jurassic.

 

I am more than happy to hear other hypotheses, but for my first post on this forum the responses already go way beyond my best expectations, I learned so much from you people. Thanks everybody!

[DPS Ammonite]

Welcome to the Forum.

 

If softer than a metal knife blade, does it fizz in acid? Also, do you have better photos that show the pores and pillars that stromatolites typically have? See paper that you linked to for photos that show those structures.

Edited September 5 by DPS Ammonite

[Fingerprint]

It does fizz when dropping vinegar on it, not only the (supposed) fossil but also the rock that it’s embedded in.

 

Regarding images of microstructures, I have to figure out a way to produce good ones, as at the moment I only have access to a poor quality magnifying lens. I hope I can post something soon.