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ewolff

Concentric Rings

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ewolff

Can anyone ID this fossil by chance? I found it near the town of Llano, TX. It has several concentric rings, but no apparent radial lines. I'm admittedly a bit of a novice, so thank you in advance.

post-14417-0-05911300-1391920962_thumb.jpg

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Fossildude19

Looks like a concretion to me, rather than a fossil. :unsure:

This from the wiki link supplied above:

"Depending on the environmental conditions present at the time of their formation, concretions can be created by either concentric or pervasive growth (Mozley, 1996; Raiswell and Fisher, 2000). In concentric growth, the concretion grows as successive layers of mineral accrete to its surface. This process results in the radius of the concretion growing with time."

Regards,

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jgcox

without a sharper in focus photo my best guess would be a split concretion also.

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Herb

:popcorn:

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ewolff

Thank you all for the information! Apologies on the initial image quality; I've included another photo that I think is slightly better. It's roughly the size of a dime. Looks a bit different from most of the images I've seen of concretions, but again, I am a novice at identifying such things. So with the better image, would it still appear to be a concretion?

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Xiphactinus

Probably concretion, but possibly the end of a fish vertebra? [edit - based on last photo...definitely a concretion.]

Edited by Xiphactinus

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Ludwigia

I agree with everyone to date. Here's a baryte concretion which I found in Oligocene layers to give you an idea.

post-2384-0-09148900-1391966758_thumb.jpg

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ewolff

Thank you so much for all the input. I appreciate the confirmation of it being a concretion. I suppose I'll have to work a bit more on being able to tell the difference, haha. ;) To my untrained eye (and from a cursory search) it looked a bit like the base of some ferns from the Ediacaran Period. :D Thank you, again! I look forward to reading this board and learning more about the world of fossils!

Edited by ewolff

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ewolff

Btw, Ludwigia - I really appreciate the photo reference. I'm still a bit confused though because yours looks smooth and like its part of the rock. Mine has a definite surface, which rises up from the rock. Is this a common feature in concretions?

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Auspex

Btw, Ludwigia - I really appreciate the photo reference. I'm still a bit confused though because yours looks smooth and like its part of the rock. Mine has a definite surface, which rises up from the rock. Is this a common feature in concretions?

Yours has weathered some; the raised rings were harder/more resistant.

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ewolff

Hmm, that's really interesting. The surface is different from the rest of the rock though..? It's like an additional layer that's stuck to it. Forgive all my inquiries, I'm just finding this really interesting and am enthusiastic to learn more. :)

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Auspex

...The surface is different from the rest of the rock though..?...

This concretion never escaped from the matrix that gave it birth.

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ewolff

Ok, that makes sense. Thank you so much! Just for personal reference in the future, how would I tell the difference between what I found and something like this (aside from the obvious plant imprint next to it): http://www.discussfossils.com/forum/uploads/1040/Primochandelabrum_Leicester_Museum1.jpg ? I realize I'm highly unlikely to encounter one, but just so I don't make the same mistake again.

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Fossildude19

I've made some circles on your pic, that illustrate the harder layers of the concretion, that were a bit more resistant to wear than the inner, softer (relatively) layers. Concretions, at least the ones that are concentrically built, add layer after layer of sediment, and sometimes different minerals form and in some layers and not in others.

So the raised portions of your circles are the harder layers. :)

post-2806-0-13657600-1391991228_thumb.jp

And, as Auspex mentioned, the concretion is still within the hardened sediments that it originated in.

Hope this helps.

Regards,

EDIT: to answer your question, the picture you posted, you can see the matrix (rock) is all the same - no difference in the rings, whereas your's has definite layers.

Edited by Fossildude19

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ewolff

Wow! Thank you Fossildude!! I believe you have put all my questions to rest. I thought it might be something along those lines, but not having the background and having found it in the Llano uplift, I still was just wondering a little. Thank you for taking the time to lay things out for me. You have definitely gone above and beyond.

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Fossildude19

;)

You're more than welcome.

Regards,

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