More Cannonball and Fox Hills Formation (An Unusual Ammonite) 8/9/21

[Thomas.Dodson]

I lined up a couple more properties over the weekend so I made another trip out to exposures of the Cannonball Formation (Paleocene) and Fox Hills Formation (Upper Cretaceous).

I only planned for two properties but the first was so large (an entire section with many outcrops) that I figured this was fine. It was a nice morning and much of the haze from the Canadian wildfires has subsided.

Junipers often creeped over sandstone concretions on the cliffs.

A slump exposure of Cannonball Formation viewed from the top of another exposure.

The top of hills were often capped with hard sandstone. Loosely consolidated and worn sandstone in lower layers had abundant shell fragments but not much else. A couple more complete fragments might be identifiable.

Distinctly bedded sandstone.

A nice hike even if the fossils weren't good.

Another hour long drive and I was at the Fox Hills site. Fossils were loaded here. Some free-weathered (these are compressed Protocardia subquadrata)...

...and lots in concretionary layers. There's quite a bit I brought back to prepare. Species I've noticed just in the outer concretion include indet. fish scales, Mactra warrenana, Piestochilus feldmani, Protocardia subquadrata, Pteria nebrascana, Pteria linguaeformis, Cucullaea shumardi, and Euspira sp.,

A split Hoploscaphites (~6 cm). I brought back the entire concretion intact. To the left are Mactra warrenana and below is wood.

Some concretions were massive. This one was a little under a meter long.

And unfortunately contained a worthy specimen. Unable to remove the whole concretion I removed the layers containing the ammonite by chisel. Annoyingly the ammonite went through both the outer concretion layers (already separated in part) and the inner layers so the layers will have to be reconnected during prep.  It's about 8 cm long complete.

The gem of the day is this ammonite which I originally thought was a Placenticeras. Despite the preservation it is an excellent find. It did require multiple repairs as I found it in multiple pieces. A puzzle and some glue later and I have this.

 

It is difficult for me to imagine based on my experience but there are some reports of massive Sphenodiscus from the Fox Hills as well as Placenticeras being present although I haven't found any scholarly sources to back this up yet. Some sources claim they disappear before the Fox Hills. It seems I'll have to do some research when I get the time.

Outer concretionary layer removed (other side is solid sandstone).

Some "body" removed (outer shell adhering to other side).

Removed body.

Continuing up the hill there is another exposure, this time of the Timber Lake Member that represents brackish transition. Cattle trails were littered with Crassostrea subtrigonalis. No other species found.

 

Edited August 10, 2021 by Thomas.Dodson
Clarified/explained ID uncertainty

[FranzBernhard]

Great report, thanks for sharing! I like to follow your adventures in such an unusual landscape (compared to my area).

I have one question:

4 hours ago, Thomas.Dodson said:

A split Hoploscaphites (~6 cm). I brought back the entire concretion intact. To the left are Mactra warrenana and below is wood.

They appear to be in the same bed / bedding plane? I find wood and marine animals deposited together always fascinating.

I have yet to find such things in my upper Cretaceous area, but its mentioned in the literature, wood and ammonites together!

Franz Bernhard

[Manticocerasman]

12 minutes ago, FranzBernhard said:

Great report, thanks for sharing! I like to follow your adventures in such an unusual landscape (compared to my area).

I have one question:

They appear to be in the same bed / bedding plane? I find wood and marine animals deposited together always fascinating.

I have yet to find such things in my upper Cretaceous area, but its mentioned in the literature, wood and ammonites together!

Franz Bernhard

It is not uncommon to find ammonites in combination with driftwood.

I've seen quite a few of those combinations in the jurassic of Lyme regis.

And I've found some driftwood before in the gault clay deposits in france in the same beds as the ammonites.

 

[fossilhunter21]

Great report!

 

Thanks for sharing!

[Thomas.Dodson]

8 hours ago, FranzBernhard said:

Great report, thanks for sharing! I like to follow your adventures in such an unusual landscape (compared to my area).

I have one question:

They appear to be in the same bed / bedding plane? I find wood and marine animals deposited together always fascinating.

I have yet to find such things in my upper Cretaceous area, but its mentioned in the literature, wood and ammonites together!

Franz Bernhard

 

As Manticocerasman mentioned it isn't uncommon. In the Fox Hills of North Dakota I'd say it is pretty typical. One of the concretions I brought back has a large piece ~16 cm) of poorly mineralized wood that is splintering everywhere.

 

One of my favorite ammonite sites here is chock full of wood fragments. The one in my avatar actually has small pieces stuck in the aperture. I once threw some small pieces of the concretions from this site in hydrochloric acid to eat the sandstone and leave some of the soft/non mineralized wood. The end result is wood displayed in the position it was preserved.

 

 

 

[FranzBernhard]

9 hours ago, Manticocerasman said:

It is not uncommon to find ammonites in combination with driftwood.

I've seen quite a few of those combinations in the jurassic of Lyme regis.

And I've found some driftwood before in the gault clay deposits in france in the same beds as the ammonites.

 

1 hour ago, Thomas.Dodson said:

As Manticocerasman mentioned it isn't uncommon. In the Fox Hills of North Dakota I'd say it is pretty typical. One of the concretions I brought back has a large piece ~16 cm) of poorly mineralized wood that is splintering everywhere.

One of my favorite ammonite sites here is chock full of wood fragments. The one in my avatar actually has small pieces stuck in the aperture. I once threw some small pieces of the concretions from this site in hydrochloric acid to eat the sandstone and leave some of the soft/non mineralized wood. The end result is wood displayed in the position it was preserved.

Thanks to both of you! I find this still fascinating .

Franz Bernhard

[Thomas.Dodson]

No Placenticeras are listed in the Fox Hills from Ammonites and the Other Cephalopods of the Pierre Seaway for one reference. Under the genus description for Sphenodiscus the book mentions "The genus attains a size of 50 (!) cm in the Timber Lake Member of the Fox Hills Formation, but rarely gets larger than 15 cm in the Pierre Shale".

 

In North Dakota I've collected multiple complete specimens of Sphenodiscus lenticularis from the Trail City Member (which in North Dakota was originally treated as part of the Pierre) but only questionable fragments from the Timber Lake. The complete specimens of these are generally dwarfed by the more common Scaphitids. I don't recall reading about large sized individuals of this species. I know many species of ammonites show indeterminate growth but the difference is so drastic I can't help but speculate that this Timber Lake specimen is a different species. I'll need to dig for sources that cover individual Sphenodiscus species as I've rarely come across them in the past. Even Ammonites and the Other Cephalopods of the Pierre Seaway only list the genus.

 

[PFOOLEY]

@Thomas.Dodson, great photos...looks like those hills just keep rolling on...what a great playground.