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AlexT

Mystery circular fossil with radial ridges and grooves

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AlexT

Mystery circular, ridge and groove fossil.

 

This fossil is a slightly oblong to circular fossil ranging from 5mm in diameter to 12mm in diameter.  It is composed of a series of ridges and grooves radiating from the center to the edges.  At the center, the ridges do not connect to a central point (at least in no example that I’ve found).  Very few examples show the central feature, but in the best preserved of them, which appears to be complete (see photo) there is very small central circular depression and the radial ridges and groves terminate at the rim of the depression.  That example may also show remnants of a stem fragment or other object in the central depression, but it is not clear.  From the better preserved specimens it is easy to see that the fossils in question are three dimensional, rising from flat rock surface at the edge to the raised central depression.  However, most examples are missing the top third or half of the specimen (see second photo). In those cases, the fossil presents as a narrow ring of grooves and ridges with a large flat top and no obvious internal structure.  The number of ridges varies from 35 or so to over 60, but they are hard to count because what starts out as a single ridge at the top (near the depression) sometimes branches into 2 ridges at the bottom near the edge.  All of the specimens that I’ve found seem to be flat and secure to the rock on the bottom.  I haven’t tried to separate it from the rock to see if there are any details underneath.

 

These are common fossils at a site in Hedgesville, WV.  Jasper Burns, in his book Fossil Collecting in the Mid-Atlantic States, dates these rocks to the Devonian, specifically the Mahantango formation which I understand was laid down about 392 and 385mya.  In the same rocks, I’ve found plentiful shell fossils of numerous species that I haven’t identified yet, including, in one instance, a very small shell fossil trace embedded on the mystery fossil above.

 

Burns’s book doesn’t appear to discuss this fossil in his chapter on the same locality, and I haven’t had any success even using Index Fossils of North America.  From some photos online, my best guess is that it may be a crinoid segment, although they all seem to lay perfectly flat, and I wonder if it may be a holdfast of some sort.

 

Any help would be appreciated.

 

Thanks, Alex

IMG_8710 (2).JPG

IMG_8719 (2).JPG

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ynot

The first one is a crinoid columnal print, the second looks like a horn coral.

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Rockwood
37 minutes ago, ynot said:

The first one is a crinoid columnal print, the second looks like a horn coral.

I see what you mean, but I think it may be another columnal that's just not been fully exposed.

Notice how thin the outer rim is on the more obvious one.

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ynot
42 minutes ago, AlexT said:

Thanks, Alex

Hey Alex,

Welcome to TFF!

Can You post a picture of the second item from the side?

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Fossildude19

I agree - both are crinoid columnal imprints.

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TqB

I think they're all columnal impressions too.

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Peat Burns

One thing I noticed is that the ridges on the first specimen seem complete from edge to center.  On the second specimen, especially on the left, they seem to alternate between complete ridges and ridges that begin at the edge and end 1/3 of the distance to the center (kind of like septa of some horn corals in cross section).  I'm not sure if there are taxa of crinoid columnals that have patterns like this, though.  Just an observation.

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ynot

The second piece also appears to be conical rather than flat. I have not seen a cone shaped column segment. (why I asked for a side view, to see if it is an optical illusion of the picture.)

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Fossildude19
12 minutes ago, Peat Burns said:

One thing I noticed is that the ridges on the first specimen seem complete from edge to center.  On the second specimen, especially on the left, they seem to alternate between complete ridges and ridges that begin at the edge and end 1/3 of the distance to the center (kind of like septa of some horn corals in cross section).  I'm not sure if there are taxa of crinoid columnals that have patterns like this, though.  Just an observation.

 

I think that is an artifact of preservation. I've seen this kind of breakage in this type of sandstone. 

I'm still in the crinoid columnal imprint camp. 

Either the imprint is not fully exposed, or, has broken beneath the surface of where the imprint would have been. 

 

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TqB
21 minutes ago, Peat Burns said:

One thing I noticed is that the ridges on the first specimen seem complete from edge to center.  On the second specimen, especially on the left, they seem to alternate between complete ridges and ridges that begin at the edge and end 1/3 of the distance to the center (kind of like septa of some horn corals in cross section).  I'm not sure if there are taxa of crinoid columnals that have patterns like this, though.  Just an observation.

 

Yes, there are crinoid columnals with ridges like that, alternating like first and second order coral septa.  

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ynot
4 minutes ago, Fossildude19 said:

 

I think that is an artifact of preservation. I've seen this kind of breakage in this type of sandstone. 

I'm still in the crinoid columnal imprint camp. 

Either the imprint is not fully exposed, or, has broken beneath the surface of where the imprint would have been. 

 

 

2 minutes ago, TqB said:

 

Yes, there are crinoid columnals like that. 

What about the conical shape?

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AlexT

It is definitely conical -- well, mildly.  On the more complete specimens, the cone is visible.  The other ones (that appeared to be rings) are the same thing with most of the "cone" sheared off.  I don't know if that's just where the rock broke or whether it was sheared off at the time of burial/death.  Hopefully this view shows more of the cone aspect.  The lip of the rock blocks some of the view.

 

I appreciate what everyone is saying about crinoid columns imprints, but how would that work?  Wouldn't the most likely imprint of a column be side-on, giving a tubular impression?  These are all circular or slightly oblong.  I would guess that the odds would be against all of the column imprints being perfectly top only.  But it is very possible that I don't understand crinoid anatomy.

 

Thanks, Alex

IMG_8713 (2).JPG

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Ludwigia

I'm pretty sure they're both crinoids.

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TqB
Just now, ynot said:

 

What about the conical shape?

 

That's OK I think, I can't match that one exactly but I've got specimens that are saucer shaped.

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TqB
12 minutes ago, AlexT said:

It is definitely conical -- well, mildly.  On the more complete specimens, the cone is visible.  The other ones (that appeared to be rings) are the same thing with most of the "cone" sheared off.  I don't know if that's just where the rock broke or whether it was sheared off at the time of burial/death.  Hopefully this view shows more of the cone aspect.  The lip of the rock blocks some of the view.

 

I appreciate what everyone is saying about crinoid columns imprints, but how would that work?  Wouldn't the most likely imprint of a column be side-on, giving a tubular impression?  These are all circular or slightly oblong.  I would guess that the odds would be against all of the column imprints being perfectly top only.  But it is very possible that I don't understand crinoid anatomy.

 

Thanks, Alex

IMG_8713 (2).JPG

 

Crinoid stems often break into individual columnals so a bed with a lot of horizontal ones is quite common.

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ynot
On 3/14/2017 at 2:14 PM, AlexT said:

crinoid columns imprints, but how would that work?  Wouldn't the most likely imprint of a column be side-on, giving a tubular impression?

With the new picture I have to say I was wrong and it is a column segment imprint.

The segments are held together with soft body parts that decompose rapidly after death and the segments are separated easily.

The print is left when the segment dissolves and leaves a hole.

 

PS I was taking a closer look and the new picture is of the wrong piece. Can You post a picture of the other piece from the same angle as that?

 

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AlexT

Here is another view.  In fairness, this is the most "conical" of the several dozen specimens I found.  A few others show noticeable three-dimensionality, but most of them are flat, including some examples that have not been sheared off (i.e. that have msotly traceable ridges from edge to center).  So it may be an artifact of the rock, or burial, but nothing seems to be deformed as far as I can tell.

 

-- Alex

20170314_172339_HDR (2).jpg

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AlexT

I see.  I didn't realize that.  That makes sense now.  

 

ynot, all of the pictures have been of the same one except for the second in my first post that had two smaller sheared ones.  Is that what you want to see?  Thanks, Alex

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AlexT

By the way, thanks for the feedback, everyone.  This a great forum.  

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ynot
34 minutes ago, AlexT said:

two smaller sheared ones.  Is that what you want to see?

Yes. But I have lost confidence in My first opinion, and tend to think the others have it with crinoid segment print.

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AlexT

Thanks everyone.  Is there any way to identify it to a specific genus or species of crinoid?

 

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ynot

Probably not.

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Fossildude19

It's incredibly difficult to identify crinoids from individual columnals or imprints. Usually the calyx is needed to make that determination.

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AlexT
16 hours ago, abyssunder said:

I think this image could tell a story. :)

 

post-17588-0-11392900-1468410907.jpg.a9ba7430ac573f9f76e2d1fc9f241341.jpg

 

Thanks for the photo.  Without seeing the individual segments, it is hard to tell, but the parts that are visible look like my fossils and provide a good visualization of how it might separate.

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