Spring in the Fox Hills Formation

[Thomas.Dodson]

Despite the shortest and most mild winter I've experienced in North Dakota (getting out this early is rare) it still feels like it has been an eternity since I got out. Thankfully I finally got a hold of enough landowners to warrant a trip to the Fox Hills Formation and celebrate the spring weather.

 

While most of the later sites I visited were a bust the first site of the morning was excellent and contained fauna not often found in the Fox Hills Formation in North Dakota. 3 new species for me in fact. This site represents a brackish transition area of the top of the Fox Hills Formation.

 

A view from the collecting hills. Note the water bodies are still frozen.

Some farmers were burning stubble in the distance.

One of the more significant exposures. Compared to the other 2 outcrops on the hills this one was poorly fossiliferous.

Anomia micronema and Crassostrea subtrigonalis (glabra) litter the surface of the worn outcrops.

The bedrock was basically an oyster bed hash of graywacke sandstone. This oyster laden sandstone isn't unusual but you usually don't find the additional fauna (see below).

In-situ Crassostrea in one of the unworn exposures.

Some of the nicest specimens. Moving clockwise from upper right is Pachymelania wyomingensis, Pachymelania insculpta, Corbicula cytheriformis, Crassostrea subtrigonalis, and Anomia micronema. Pardon the bad picture and for our international friends the coin is about 1.9 cm. I took this picture for the landowner since he was interested and I included a scale he'd know. I was also too tired to retake it with my metric scales. I plan to take better ones later.

Nice sculpture to the Anomia. Most were not so iron colored.

There are some additional things in pieces of the sandstone I brought back. I have an idea what this is but want to prep it to see for sure. This is larger, about 5 cm across.

On the way back to the main roads I found a farmer in the middle of nowhere with a sense of humor.

I also collected a couple smaller concretions of the more typical ammonite Fox Hills concretions from another site. I'm not expecting much based on their size but if there is anything interesting post preparation I'll post those as well.

 

[FranzBernhard]

Very nice, thanks for sharing, @Thomas.Dodson.

Do the nice gastros occur within the oyster bed or above/below the oyster bed? 

Thanks!
Franz Bernhard

[Tidgy's Dad]

Nice report. Loved the field photography.

And the fossils, of course. 

[Thomas.Dodson]

7 hours ago, FranzBernhard said:

Very nice, thanks for sharing, @Thomas.Dodson.

Do the nice gastros occur within the oyster bed or above/below the oyster bed? 

Thanks!
Franz Bernhard

They seem to occur within the oyster bed. I have some matrix pieces with a couple more Pachymelania present with the oysters.

 

For those in interested in paleoecology here's a statement on the saline conditions of the North Dakota Fox Hills oyster beds. From Bivalvia and paleoecology of the Fox Hills Formation (Upper Cretaceous) of North Dakota by Rodney M. Feldman, 1967.

 

"Corbicula typically inhabits fresh water; however, a few species seem to prefer brackish water (Sinclair and Isom, 1963, Pg 13) Corbicula cytheriformis seems to be one of the latter types in that its association with oysters precludes fresh water. Oysters have been reported in water varying in salinity from 2.5 to 33 parts per thousand dissolved solids; however, they probably cannot survive indefinitely at salinities less than 9 parts per thousand. (Churchill in Hopkins, 1934). The size of the oyster banks indicates the oysters existed for a long time so that a salinity higher than 9 parts per thousand might be inferred. The presence of Corbicula indicates that the salinity was not as high as for normal marine water. The two, genera associated with each other, then, offer an excellent indication of marginal, estuarine conditions. "

 

The Pachymelania likely prefer similar conditions since I haven't found them elsewhere. Erickson gave a good evaluation of Fox Hills Gastropods that includes Pachymelania. I have a copy here I'll take a look at later to see if he has any comments on their ecology and the locations they've been found.

[FossilNerd]

9 hours ago, Thomas.Dodson said:

Nice sculpture to the Anomia.

 

Nice report! Thanks for sharing, and congrats on being able to get out there so early. I love this little Anomia. 

[Thomas.Dodson]

4 hours ago, Thomas.Dodson said:

The Pachymelania likely prefer similar conditions since I haven't found them elsewhere. Erickson gave a good evaluation of Fox Hills Gastropods that includes Pachymelania. I have a copy here I'll take a look at later to see if he has any comments on their ecology and the locations they've been found.

Erickson notes the same. From Revision of the Gastropoda of the Fox Hills Formation, Upper Cretaceous (Maestrichian) of North Dakota. (Erickson, 1974).

 

"Both P. wyomingensis and P. insculpta were brackish-water forms and are restricted to deposits in lagoonal facies of the upper Fox Hills Formation where they occur with Crassostrea. This Crassostrea-Pachymelania assemblage contains, in addition, such genera as Corbicula, Anomia, and Neritina, but there are never any truly fresh-water members present." I do find it interesting how the Crassostrea beds are common elsewhere but until now I haven't found the associated species elsewhere. This might be related to preservation bias.

 

 

[Thomas.Dodson]

Updates and more pictures as promised. I prepared the matrix blocks from the site as well.

 

The different bivalve was Tancredia americana. The outer shell was in a pretty fragile state.

More Anomia micronema.

The Crassostrea subtrigonalis.

Corbicula cytheriformis after some scrubbing.

Cleaned up the Pachymelania as well.

This awesome little internal mold is clear crystal.

This is an Anomia that has encrusted a Pachymelania wyomingensis. Note the tubercle underneath.

Anomia on a Crassostrea.

 

I also went through concretion pieces from the other site I mentioned and busted the whole concretions up. As I expected from the whole ones, just Gervillia recta and Pteria nebrascensis.

This piece was already open. I kept it because Gervillia subtortuosa are rarer at this site.

After bringing it back I discovered some partial Sphenodiscus lenticularis ammonites. If the top one was complete it would have been a monster Sphenodiscus!

I did collect another Gramattodon sulcatinus and Semitriton buccinoides from a busted concretion piece. These are rather rare here so I always welcome more specimens.

[Jeffrey P]

Love the wide open snow free spaces. Most of us are hung up on the Fox Hill ammonites, but you've shown us that the bivalves and gastropod specimens have plenty to offer too. I'm very into Devonian and Cretaceous bivalves so I see this in ideal terms. Very exquisite! And thanks for sharing them.

[Pagurus]

45 minutes ago, Thomas.Dodson said:

This awesome little internal mold is clear crystal.

 

Awesome, indeed! 

 

Those Anomia micronema  amaze me. It's hard to believe they're close to 70 million years old. I often stuff my pockets with their modern relatives, Anomia simplex. They're impossible to resist while strolling along the shore. Thanks for the landscape photos too.

[Thomas.Dodson]

26 minutes ago, Jeffrey P said:

Love the wide open snow free spaces. Most of us are hung up on the Fox Hill ammonites, but you've shown us that the bivalves and gastropod specimens have plenty to offer too. I'm very into Devonian and Cretaceous bivalves so I see this in ideal terms. Very exquisite! And thanks for sharing them.

Yes, I think some of the gastropods and bivalves are underappreciated considering the ornamentation and original shell preservation on many specimens. Lots of nice ammonites too, just not in these unnamed members. I've posted Fox Hills ammonites as well as gastropods and bivalves in the thread linked below if you haven't seen it and want to take a look.

 

30 minutes ago, Pagurus said:

 

Awesome, indeed! 

 

Those Anomia micronema  amaze me. It's hard to believe they're close to 70 million years old. I often stuff my pockets with their modern relatives, Anomia simplex. They're impossible to resist while strolling along the shore. Thanks for the landscape photos too.

I'm pretty impressed by the shine on them. I've collected Anomia in Texas and Georgia before but these have to be my favorite. I couldn't resist picking up any more complete ones I saw.

 

[FranzBernhard]

On 3/8/2021 at 6:50 PM, Thomas.Dodson said:

I do find it interesting how the Crassostrea beds are common elsewhere but until now I haven't found the associated species elsewhere. This might be related to preservation bias.

For sure! Crassostrea has calcitic shells, most other aragonitic shells.

Other point could be if the Crassostrea bed is strictly autochthonous or somewhat reworked, giving the additional chance of something being introduced from nearby.

You may find this Neogene example somewhat informative:

Miocene Crassostrea bed with extra fauna (link to research gate, free download of paper)

 

Franz Bernhard