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Mazon Creek Best Of The Best Achistrum Sp


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I have been fortunate to hunt Mazon Creek fossils for nearly 40 years. I have collected Many tens of thousands of concretions. I have also purchased premium specimens from other collectors.

In the past, I have posted many of these specimens on the forum.

I have decided to start posting more in depth descriptions of some of the amazing animals that can be found in the MC deposit.

All specimens that I will post are from my personal collection.

The first animal that I will highlight is the holothurian or sea cucumber Achistrum sp.

Sea cucumbers are a common animal in today’s oceans but quite rare in the fossil record.

The Marine (Essex) portion of the Mazon Creek deposit is one of the few places in the world where complete body fossils of these animals can be collected.

These worm like animals are actually a type of echinoderm and show 5 radial body elements that run the length of the animal.

Well preserved specimens will show a sac like body and an oral ring preserving approximately 15 calcareous plates. Occasionally the intestinal tract and other internal features will be preserved.

Just like modern sea cucumbers, Achistrum sp had a leathery body covered in “J” shaped sclerites or Sigmoid hooks.

Often times, detecting these under a microscope is the best way to identity poorly preserved specimens.

As the animal dried out, the skin would crack and these cracks were eventually replaced by calcite. This gives the body of Achistrum sp a septarian like appearance.
While modern sea cucumbers have retractable tentacles surrounding the mouth, none have been observed in Achistrum sp.

The animal can reach length of over 15 centimeters however most found average under 10 centimeters.

Achistrum is relatively abundant and are occasionally found in masses of multiple individuals.

Despite many thousands of specimens collected, Achistrum has never been formally described. At one time it was believed that there may be as many as a dozen different species of sea cucumber found in the Mazon Creek deposit. This has been reduced to one or possibly two different types.

This first image shows an exquisite specimen that I collected at Pit 11 in 2017.

it is a complete animal and preserves evidence of some unusual muscular structure in the esophagus area that I have not seen before.

 

 

 

 

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The next specimen is a more typical preservation of a smaller individual (approximately 5 cm).

Note the ring of mouth plates.

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This is a well preserved multiple preserving one adult and one (possibly two) juveniles.

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Here is an interesting example of a highly inflated esophageal region along with a close up of the leathery skin and hook like structures.

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This is a grouping of at least five individuals.

 

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This specimen appears to show internal structures that were expelled from the body due to decay.

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Or fright. 

Some sea cucumbers can expel part of their respiratory system as a defensive mechanism. 

Fascinating thread, i love the  Mazon Creek flora and fauna and have a few myself, though nothing as spectacular as this.

Amazing specimens. 

I look forward to the next in the series with eager anticipation.:popcorn:

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8 minutes ago, Tidgy's Dad said:

Or fright. 

Some sea cucumbers can expel part of their respiratory system as a defensive mechanism. 

Fascinating thread, i love the  Mazon Creek flora and fauna and have a few myself, though nothing as spectacular as this.

Amazing specimens. 

I look forward to the next in the series with eager anticipation.:popcorn:

Thanks for the additional information.

I have had several other examples over the years that preserve eviscera protruding from the body.

I will try and post a few specimens a week as time permits.

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Wow! Spectacular specimens. Can’t wait to see more. :popcorn:
 

Are there any known ammonoids from the Mazon Creek deposit?

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1 hour ago, Heteromorph said:

Wow! Spectacular specimens. Can’t wait to see more. :popcorn:
 

Are there any known ammonoids from the Mazon Creek deposit?

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From "Middle Pennsylvanian (Desmoinesean) Cephalopoda of the Mazon Creek Fauna, Northeastern Illinois," Saunders 1979.

 

Cephalopods are very rare from Mazon Creek, but the species present are quite diverse.

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3 hours ago, Heteromorph said:

Wow! Spectacular specimens. Can’t wait to see more. :popcorn:
 

Are there any known ammonoids from the Mazon Creek deposit?

 

3 hours ago, Heteromorph said:

Wow! Spectacular specimens. Can’t wait to see more. :popcorn:
 

Are there any known ammonoids from the Mazon Creek deposit?

 

1 hour ago, connorp said:

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From "Middle Pennsylvanian (Desmoinesean) Cephalopoda of the Mazon Creek Fauna, Northeastern Illinois," Saunders 1979.

 

Cephalopods are very rare from Mazon Creek, but the species present are quite diverse.

I will post a few cephalopods in a future thread.

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Thank you for sharing your sea cucumbers! Those are very well preserved. I wanted to ask- how much magnification is required to see the sclerites? I have 5 Achistrum, but I don't think I have ever been able to see the hooks on any of them.  

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Wow, thanks so much for posting this, @RCFossils! Absolutely gorgeous specimens. And I love the chance to get in-depth with one creature at a time. I learned a lot from your post.

 

I have several specimens that I believe are Achistrum, but all poorly preserved and partial pieces. And none has the mouth ring. That's something I would be super excited to find sometime - I'll keep looking! And now I also need to look at my partials more closely to check if I can see any sclerites - fascinating!

 

Chris

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On 11/7/2019 at 8:34 PM, deutscheben said:

Thank you for sharing your sea cucumbers! Those are very well preserved. I wanted to ask- how much magnification is required to see the sclerites? I have 5 Achistrum, but I don't think I have ever been able to see the hooks on any of them.  

You should not need more the 10X to clearly see the J shaped hooks.

For whatever reason, sometimes they do not preserve.

If they are poorly preserved and without oral rings, it is also possible that they may be examples of Coprinoscolex. It is a spoon worm commonly found and can have similar preservation to Achistrum.

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Thanks! I will take a second look under the microscope. They do all have the oral ring present, at least. 

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So I looked at all the specimens I have that I thought were Achistrum, and now I don't think I actually have any. No oral rings, and no J shaped hooks. So perhaps I have some fragmentary spoon worms, instead! Chris

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