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small orthoconic nautiloids versus tentaculites? What should I look for?


SteveE

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Blair County, Pennsylvania (USA)

Silurian...

According to the map the likely guess is Clinton Formation, but my gut on site said "Wills Creek"

 

Anyway, what do I have here? Small straight nautiloids or Tentaculites?  How do you tell them apart?

 

 

 

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I'm pretty sure these are Tentaculites or at least something from the Tentaculitida, orthoconic nautiloids have septa and a siphuncle, Tentaculites does not and though these show little internal structure, i think enough to see that one or two of them appear 'hollow'. 

Also I don't think I've seen such tiny nautiloids in such number, they are usually somewhat bigger.  

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I think nautiloids this size would have thinner walls than these apparently do.  

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9 minutes ago, Rockwood said:

I think nautiloids this size would have thinner walls than these apparently do.  

All I see is surface..... what do you see that suggests wall thickness?

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1 hour ago, SteveE said:

All I see is surface..... what do you see that suggests wall thickness?

 

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Nice piece, most likely tentaculites, although I have seen accumulations of very small orthoconic nautiloids too. Your specimens appear to be aligned, could be a nice paleocurrent study.

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3 hours ago, westcoast said:

....Your specimens appear to be aligned, could be a nice paleocurrent study...

That's an interesting idea;  This piece was in the float, but there's more in situ upslope.

 

Rockwood, thanks for trying to show me that.  Maybe if we were live in person I'd see what you see

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5 hours ago, SteveE said:

Rockwood, thanks for trying to show me that.  Maybe if we were live in person I'd see what you see

I collected a rock this summer with roughly one hundred of them sectioned as little 'o's. ;)

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4 hours ago, Rockwood said:

I collected a rock this summer with roughly one hundred of them sectioned as little 'o's. ;)

Ach so!   In the field I thought I had mud traces.  At home, after a rinse and with good light and magnification I was so surprised to see these guys I never thought to look at the REST of the rock.  I would have written these holes off as chemical weathering etc, except that there are some holes right where the two faces of the sample meet, and can see the rest of the organism on the other face.  Thanks for getting me to look for little Os!   Another thing I like about this is that it apparently shows a change of habitat conditions, since there are two slightly different layers, and the little guys are in one but not the other.

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11 minutes ago, SteveE said:

habitat conditions

Habitat conditions don't necessarily equal deposition results. The formation I collected from is thought to be a marine river delta environment. I doubt this is a natural life position for them.

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Those are 100% Tentaculites. When you find those tiny elongated cone shapes with regular "rings" or ridges those are the identifying characteristic.  As for your sample, note how the fossils all tend to line up in one direction. As @westcoast said that indicates a current that oriented the shells before deposition. The fact that they are concentrated in one layer of your rock could be due to facies change or it could just be a storm deposit. The Wills Creek is a good formation to find Silurian fossils in. Hard to find good exposures though.

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