Arachnophyllum or Acrocyathus?

[LisaL]

Hello all,

 

I found this chunk of silicified colonial rugose coral in a creek near the Grand River in Grand Ledge, MI (south central MI). The creek cuts through Pennsylvanian bedrock and is full of fossil plant material. 

 

To me, it looks a lot like photos of Arachnophyllum pentagonum I've seen on blog entries about the Louisville limestone, which I understand correlates with the Cordell Dolomite formation of the Manistique group of northern MI.

 

I'm guessing it wouldn't be too unexpected to find a Silurian coral glacially transported to this area of south central MI? 

 

But maybe a Mississippian coral like Acrocyathus would make more sense, given that the Mississippian Bayport limestone is closer to this area.

 

Can anyone recognize the genus? 

 

Many thanks!

Lisa

 

 

 

 

 

[minnbuckeye]

How about Hexogona

 

'Petoskey Stone': Michigan’s State Stone

 

August 17, 2011 |

By Joseph J. "PaleoJoe" Kchodl

The Hexagonaria coral (Hexagonaria percarinata) is commonly and incorrectly referred to as the “Petoskey stone.”

[TqB]

That's a hard one to call without sections. 

You need to work out if it's cerioid (for Acrocyathus) or astreoid/aphroid (for Arachnophyllum). Does wetting it help to see what the septa are up to?

Also, the dissepimentarium structure would help - there seem to be some lonsdaleoid dissepiments in at least some corallites. If patchy, that would support Arachnophyllum while entire would fit Acrocyathus. (Either rules out Hexagonaria, and the corallites seem too large.)

 

I think the septa extending right to the axial column fits Arachnophyllum.

 

But I expect you've worked out all that already! We need a section.

[LisaL]

Thank you so much @TqB and @minnbuckeye!

 

Tarquin, you give me something new and exciting to learn every time, thank you! I think I finally worked out that dissepiments are lonsdaleoid if they form a zone that septa do not pass through, although I'm still studying on this. I'll take more time to see if I can discern any dissepimentarium and septa structure. Thanks again so much!

[TqB]

3 hours ago, LisaL said:

I think I finally worked out that dissepiments are lonsdaleoid if they form a zone that septa do not pass through, although I'm still studying on this. I'll take more time to see if I can discern any dissepimentarium and septa structure. 

That's right, they tend to be big and bubbly too - here are two Carboniferous Actinocyathus (formerly Lonsdaleia) specimens showing the archetypal form.

 

 

[LisaL]

These are breathtaking examples. I can see the lonsdaleoid structure very well. Thanks so much!

[LisaL]

Update! Since learning more about the Bayport Limestone and preservational and lithological differences between Mississippian and Silurian material from Michigan, along with the proximity of this Grand River tributary to possible sources of Bayport Limestone, I’m now thinking this specimen is likely Acrocyathus floriformis instrlead of Arachnophyllum. :)

[TqB]

44 minutes ago, LisaL said:

Update! Since learning more about the Bayport Limestone and preservational and lithological differences between Mississippian and Silurian material from Michigan, along with the proximity of this Grand River tributary to possible sources of Bayport Limestone, I’m now thinking this specimen is likely Acrocyathus floriformis instrlead of Arachnophyllum.

 

Good thinking. Now it would be nice to see a bit of it ground to check that it's cerioid.

[LisaL]

1 hour ago, TqB said:

 

Good thinking. Now it would be nice to see a bit of it ground to check that it's cerioid.

Ha, I hadn’t thought to try! I’ll give it a try by hand, like I did with the roemeriid. It’s quite hard, though!

[kgbudge]

I'm afraid rugose coral is about as specific as I can guess. But a very nice specimen.

[TqB]

1 hour ago, LisaL said:

Ha, I hadn’t thought to try! I’ll give it a try by hand, like I did with the roemeriid. It’s quite hard, though!

You may not have to grind very far of course, just enough to see the outer edges and junctions of the corallites.

[LisaL]

@TqB: The results of sandpaper scrubbing so far have been pretty disappointing! It’s made the edges whiter, but I still can’t see any structure, can you? I thought I just needed to get the edges flush with the corallite centers to see more structure, but the quartz seems too botryoidal and beady. Do you think it’s worth continuing? 
 

 

 

 

 

 

 

 

[TqB]

52 minutes ago, LisaL said:

The results of sandpaper scrubbing so far have been pretty disappointing! It’s made the edges whiter, but I still can’t see any structure, can you? I thought I just needed to get the edges flush with the corallite centers to see more structure, but the quartz seems too botryoidal and beady. Do you think it’s worth continuing? 

 

I agree that the mineralisation seems to have obscured the structure which certainly happens sometimes - there's a lot of beekite there. A shame but hand grinding probably isn't going to reveal much more.

I can see some dissepiments in the side view (fourth photo) so perhaps a slice would show some structure in parts of it. 

[LisaL]

1 hour ago, TqB said:

 

I agree that the mineralisation seems to have obscured the structure which certainly happens sometimes - there's a lot of beekite there. A shame but hand grinding probably isn't going to reveal much more.

I can see some dissepiments in the side view (fourth photo) so perhaps a slice would show some structure in parts of it.

 

Thank you! “Beekite”  was the word I couldn’t think of.  I’m sure I could find a local lapidarist to slice it for me. Will post back if I have any luck!