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North Dakota Outing April 1 Cannonball Formation/Fox Hills Formation


Thomas.Dodson

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On the 1st I had the opportunity for another trip and made it out to Morton County. I was waiting until now to post the report because I wanted to finish preparing a crab to include in the report but I've been busy. I went to one Fox Hills Formation site but mostly I had sites lined up from the Paleocene Cannonball Formation and some Fort Union Group formations. Compared to Emmons County across the Missouri River there is less Fox Hills Formation and it is replaced mostly by the overlying Hell Creek Formation and Paleocene units.

 

Some scenery showing outcrops of the Cannonball Formation at one of the sites. Outcrops are common but fossils in the formation are rather slim pickings.

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Small crab bearing concretions have been reported from the Cannonball Formation once in the past but the sites where they were collected and the species described has since been destroyed. A shame considering the overall rarity of crabs from the interior seaways and North Dakota overall. The concretions were instantly recognizable at this site but were sparsely fossiliferous. Because of that I was splitting most of the concretions in the field.

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Persistence did pay off and as I was about to give up I did come across a crab.

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Later I glued the concretion back together to prep it.

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After preparation. I was hoping for a more complete and well preserved crab (something like Washington crabs) but considering the rarity I really can't complain. This is Camerocarcinus arnesoni.

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A carpus underneath the carapace. I also found a manus in the concretion.

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An internal mold of Arctica ovata from a nearby site. Only one other Arctica was found.

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There is one more crab bearing concretion to prep but it appears to be much more partial. There's also some concretions I was unable to split in the field so I brought back to exert more muscle on them. Hopefully there will be more stuff.

 

Edited by Thomas.Dodson
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The Fox Hills Formation site was beautiful. The lower level was clearly Fox Hills, the Tancredia-Ophiomorpha assemblage.

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Shell hash was composed of almost entirely Dosiniopsis deweyi. This is unique to this area as it's rare elsewhere in the Fox Hills.

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A Panopea occidentalis. About 10 cm long.

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A valve of Tancredia americana and some Dosiniopsis deweyi. There's also some Corbula monmouthensis and Syncyclonema halli. Syncyclonema halli is very rare overall but common in this area as researchers have noted before.

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A poorly preserved Tellinimera scitula.

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Some loose Dosiniopsis deweyi and a Tancredia americana.  I just grabbed these to show the landowners who were interested what was found here. I have excellent samples from another site so I didn't want to weight down my pack with hash. Luckily these were loose.

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The upper layers didn't contain anything but petrified wood. I'm not 100% sure this represents the overlying Hell Creek or the sparse upper Fox Hills yet.

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I did grab some nice quality agatized wood to play around with on the saws. There was a limit to how much I wanted to weigh down my pack with this.

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Thanks for sharing! Very educative!

So these crabs are really, really hard-core collecting, congratulations! Nothing to see, hit and miss and so on... :).

Personally, I prefer the second kind of experience - something already to see without hammering :).

(Well, well, was a lucky guy in the second way last Sunday, can not really complain,...:default_faint:)

Franz Bernhard

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Thank you for sharing.

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"On ne voit bien que par le coeur, l'essentiel est invisible pour les yeux." (Antoine de Saint-Exupéry)

"We only well see with the heart, the essential is invisible for the eyes."

 

In memory of Doren

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  I see the crab carapace but in the first crab pic I  see what looks like Dactyl?  Funny how rocks can throw curve balls at you.  Cool carapace.

 

RB

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6 hours ago, RJB said:

  I see the crab carapace but in the first crab pic I  see what looks like Dactyl?  Funny how rocks can throw curve balls at you.  Cool carapace.

 

RB

You're not wrong. The dactyl was tucked very close to the underside of the carapace. It was visible only in the cross section through the carapace before I put it back together. It's still under more matrix like shown here.

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This is another picture I snapped really quick before reassembling the concretion that shows how tucked it is. Note the outline of the carapace.

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  • 1 month later...

I finally got around to preparing that other crab bearing concretion. I assumed it was partial because cracked open it only exposed part of a cheliped(?) and there wasn't a carapace visible. In reality the entire carapace was towards the exterior of the concretion and rather complete. There was only a dactylus associated with the carapace though.

 

A strange white sandstone stuck to the carapace but it cleaned up quite well with light air abrasion. There are some parts missing from the carapace but again, I'm happy with this based on the rarity. I definitely need to go back to this site, I only explored a portion of the property.

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I took these right after air abrasion so there's a little stuck dolomite I still need to clean off.

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