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ID help on two fossiliferous rocks, please? Georgian Bay Formation / Mimico Creek, Toronto (Upper Ordovician)


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FossilDAWG

#1 is a cross section through a strophemenid brachiopod, and #2 is a section through a siphuncle of a nautiloid, likely an actinocerid nautiloid.

 

Don

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Rogue Embryo

Wow, I never would have guessed, in either case. You really have to know the animal from every different angle, including cross-sections. This is fascinating. Thanks for your experienced eyes!

 

Camille

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Rogue Embryo

So in #2, what are those long, curved black and greyish lines that seem to conform somewhat to the siphuncle's shape?

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FossilDAWG

I don't think they have anything to do with the siphuncle.  I suspect they are just sections through some largish brachiopod shells.  The brachiopod in #1 has both valves articulated and the interior is filled with calcite, which produces the hook-like shape.  In # 2, the shells are not articulated and so they have no calcite filling, so you just see the dark line of the shell.  BTW there is a second nautiloid at the top of the photo in # 2.  It is only a small piece, but different from the siphuncle as it seems to have straight sides, you can see a few possible chambers (camerae), and it has a narrow siphuncle in the middle. 

 

Don

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Rogue Embryo
Posted (edited)

This is helping me to visualize the animals more clearly. Lots of information packed into that paragraph, and lots to ponder. Ex.: the hook in the brachiopod's articulated shell (#1), and calcite filling for articulated brachiopods. Also, I hadn't noticed the nautiloid with siphuncle at the top of #2.

 

Thank you, Don!

 

Camille

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Mark Kmiecik
19 hours ago, Rogue Embryo said:

This is helping me to visualize the animals more clearly. Lots of information packed into that paragraph, and lots to ponder. Ex.: the hook in the brachiopod's articulated shell (#1), and calcite filling for articulated brachiopods. Also, I hadn't noticed the nautiloid with siphuncle at the top of #2.

 

Thank you, Don!

 

Camille

 

You become adept at recognizing cross-sections of shells of these animals at assorted angles once you've seen thousands of them. Hang in -- you'll get there.

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