Fox Hills Formation in North Dakota 3/18/21 Pictorial Trip Report

[Thomas.Dodson]

Went to attempt some new sites in Emmons County North Dakota yesterday. It was a pretty intensive day (out for 12-13 hours) for only a little bit of fossils but I thought some people might like to see the usual trip report photos and fossils I did find.

 

Extensive clayey shale exposures are always worth photographing. Some horses were curious what I was up to but eventually lost interest. I'm not 100% sure this is Fox Hills (the clayey shale exposures of the Trail City and Timber Lake members look a lot like Pierre Shale when not accompanied by sandstone) but it didn't produce so it doesn't matter.

A worn roadcut exposure with various bits of Ophiomorpha sp. Part of the Timber Lake Member most likely. For those not familiar, these trace burrows are incredibly abundant in the Timber Lake Member in North Dakota and become scarce in the Iron Lightning Member (also called the Bullhead and Colgate).

Here's some of the burrows in-situ at a different site. Sometimes they form complex connecting tunnels.

Speaking of Ophiomorpha, an isolated butte in a crop field produced some of the most detailed specimens of them I've seen. I thought I was long done collecting Ophiomorpha specimens but in this case I made an exception. Pictures don't do the details justice.

 

 

Petrified wood also occurred in the outcrop. Larger pieces are less common in the Fox Hills than the successive Hell Creek Formation and Paleocene formations.

The small butte.

To check other exposures on the property I had to traverse the fields.

Other exposures of the Fox Hills Formation on the property were numerous but didn't produce anything but more Ophiomorpha (with lower quality than the butte).

Bank swallow nests in the softer sandstone outcrops.

 

 

[Thomas.Dodson]

Not satisfied with just wood and Ophiomorpha, I decided to stop at a site I visited last year on the way back. This particular site has no exposure left; it appears to have been destroyed in 2018 by the "improving" of the road. I was aware of this site since before 2018 so I am rather disappointed I never visited it until last year. Still, Pocket Gophers have excavated shells in making their mounds and they appear in the dirt. In this case I decided to get down and dirty and look through the mounds. I would have sifted the soil if it weren't so wet. It produced much more variety than last time but is still nowhere near the original species richness reported from the site. Two new species of Neritid snails were also described from here before it was destroyed. Here's a link to a publication discussing one of the new species and the history of the site. LINK

Crassostrea subtrigonalis.

Tancredia americana, mostly small ones.

Pachyemelania insculpta (left and center) and Cryptorhytis flexicosta? (right and upper). 

Corbicula cytheriformis (right) and what might be a worn Corbicula moreauensis. The single specimen described in the original study of North Dakota Fox Hills Bivalvia is listed from this site. I'm not sure but it conforms even less to any other Fox Hills bivalve, including from the type area.

 

The only example of matrix from the former exposure.

Edited March 19, 2021 by Thomas.Dodson

[Tidgy's Dad]

Nice report and great photos.

Thanks for sharing. 

[jpc]

Looks like you are developing a nice collection of shells from the Fox Hills.

[FossilNerd]

Good report. At least the gophers are earning their keep! I see why you picked up those Ophiomorpha.  Very nice specimens. 

[Thomas.Dodson]

16 minutes ago, FossilNerd said:

Good report. At least the gophers are earning their keep! I see why you picked up those Ophiomorpha.  Very nice specimens. 

I'm certainly impressed by the gophers. Maybe if they keep digging and I keep going back I can find those species only found there.

 

There's a part of me that wants to start keeping some Ophiomorpha I find just to section them looking for decapods like the one Crawford found in an Ophiomorpha burrow. This seems like something I'd tire of pretty quickly though! Aside from Crawford's single specimen I've not heard of any others.

[Norki]

I continue to be surprised by how similar the Fox Hills fm. is to the Bearpaw fm., including those Ophiomorpha, which look essentially identical to the Bearpaw examples... I also wasn't aware that there had been a published example of a decapod occurring inside of one, that's really interesting to me.

 

I wonder if you've ever encountered small, elongate sandstone concretions with delicate decapods preserved inside of them though? These are abundant in the Bearpaw sandstones, and while I don't have any publications handy at the moment, they have been theorised to result from decapod burrows being filled with sediment during storms, which entombed their residents. I imagine that example of the decapod-bearing Ophiomorpha must have resulted from a similar event, which makes me wonder if the inclusion of decapods within these burrows is more common than we think.

 

Here's a photo of one of the Bearpaw burrow casts with decapod inside - I'm really curious to know if you've encountered anything similar to this in the Fox Hills:

 

 

Photo courtesy of the RSM.

Edited March 20, 2021 by Norki

[Thomas.Dodson]

2 hours ago, Norki said:

I wonder if you've ever encountered small, elongate sandstone concretions with delicate decapods preserved inside of them though?

I have not. The only decapod fossils out of the Fox Hills I've collected here were in the larger typical concretions of the marine facies. In the part of the Timber Lake Member where Ophiomorpha are common other fossils are quite rare and poorly preserved or crushed. With the exception of the upper brackish parts, this also holds true for most Fox Hills fossils in North Dakota found outside of concretions. The few ammonites I've collected outside the concretions are crushed flat.

 

It is possible that the Ophiomorpha burrows protect some of the fragile fossils from compaction and the like prior to fossilization. I'd still bet it's rather rare because of the general scarcity of decapods outside the concretions in the Fox Hills. Of all of Crawford's analyzed specimen only the Ophiomorpha one occurred outside concretions. Here's the paper by the way. DOWNLOAD LINK I guess Wage actually collected that specimen inside the Ophiomorpha, I just remembered it as Crawford because of the paper.

 

 

2 hours ago, Norki said:

while I don't have any publications handy at the moment, they have been theorised to result from decapod burrows being filled with sediment during storms, which entombed their residents.

I do know about this publication by Feldman where he examined the formation of Bearpaw decapod concretions in Montana. He suggests the evidence is against them being burrows himself. https://www.jstor.org/stable/23362141

 

[hemipristis]

I love the photo with the abandoned auto. It has that 'rugged American Wild West' feel

[RuMert]

The fossilized car is not bad either

Edited March 20, 2021 by RuMert

[RJB]

That one shrimp burrow is purty nice.   Ive never heard of those little sandstone concretions containing shrimps.  thats purty cool!   Ive seen a couple examples of very small lobster like critters from the Timber Lake but they were poorly preserved.  I  do have one that I quit workiing on awhile ago cause it was in so many pieces.  I may have to revisite that again.

 

RB

[Norki]

13 hours ago, Thomas.Dodson said:

I do know about this publication by Feldman where he examined the formation of Bearpaw decapod concretions in Montana. He suggests the evidence is against them being burrows himself. https://www.jstor.org/stable/23362141

 

 

Oh, yeah. That's one of them. I was especially thinking about this previously published paper by Cameron Tsujita, in which the hypothesis was first outlined (also I just love the title). I think he presents some pretty good evidence.

 

Anyway, I won't derail your thread further with this particular topic. I'm just especially interested in the ways that these two formations are so similar and how they intersect with one another.

Edited March 20, 2021 by Norki

[Thomas.Dodson]

4 hours ago, Norki said:

Anyway, I won't derail your thread further with this particular topic. I'm just especially interested in the ways that these two formations are so similar and how they intersect with one another.

Not a problem, I enjoy discussing the Fox Hills Formation. It's my most visited area of the state for a reason.

 

I know the Bearpaw of Montana is succeeded by the Fox Hills Formation but in North Dakota the Bearpaw is equivalent to the upper Pierre Shale. I wonder how much the Bearpaw of Canada overlaps in the ammonite zones of the Bearpaw in Montana. Incidentally, for the most part I've never heard of crab bearing concretions in the Pierre of North Dakota. Outside of the Cooperstown site I'm not aware of decapod fossils at all.

[Jeffrey P]

Interesting report and wonderful pictures. Definitely wide open spaces out there. IMPRESSIVE! The burrows very closely resemble ones we find regularly in the Upper Cretaceous Wenonah Formation in New Jersey that are attributed to ghost shrimp. I also saw similar ones at Chaco Canyon in New Mexico. I am impressed with the bivalve fossils you've been finding.