Several gastropods from Galena Formation near Decorah Iowa

[tombk]

I was recently near Decorah, which is in northern Iowa and near the border with Minnesota. The predominant bedrock there is the Galena Formation. It is chiefly limestone with some interbedded shale. I was there only one full day and had other things to do (other than look for fossils, that is, if you can believe it). I did visit one road-cut along highway 52.

I am not geologically informed enough to know what member of the formation was exposed at the road cut, so the closest I can get to dating it is to say that it’s upper Ordovician. 
 

The road cut was not a place I’ll visit again. Few fossils. But I did come across a few gastropods that I want to show here. And I have one question in the middle about clockwise vs counterclockwise growth in gastropods. 
 

I think 3 are Liospira species: 

 

 

 

Another looks similar but spirals from the umbilicus outward in a clockwise direction out from the rock (that is, toward the viewer looking at the rock) rather than into the rock as the others do. Any thoughts on this reversal of direction? Can it occur in the same genera or species? 

 

A possible Sublulites species:

 

And an internal cast of a Hormotoma species (the first cast of such I’ve found where the whorls don’t touch each other).

 

thanks for reading my mini-adventure,

 

Tom

 

[connorp]

Nice finds. I would wager you were collecting the Dubuque Formation. It's fauna is similar to the underlying Stewartville/Wise Lake Member of the Galena Formation, but the gastropods are often not as large, and at least in the area you collected the preservation is much different. The area around Decorah is a great place to collect. There are a number of large roadcuts that combined expose the Decorah Formation up through the lower Maquoketa.

[Coco]

40 minutes ago, tombk said:

 

I feel like we can see the bottom of the gastropod.

 

Coco

[Notidanodon]

21 hours ago, Coco said:

I feel like we can see the bottom of the gastropod.

 

Coco

Look up Maclurites  this looks very similar 

[tombk]

26 minutes ago, will stevenson said:

Look up Maclurites  this looks very similar 

I think you may be on to something! Maclurites is described as having a “depressed spire instead of an elevated one” in “Invertebrate Fossils” by Moore, Lalicker, and Fishcher. This would explain why it looks different from the Liospira. (And why I thought it’s growth spiraled in a direction opposite to the Liospira.) I know Maclurites have a flat base, but I can’t check whether this specimen does because it’s buried in matrix. Thanks for the lead!

Tom

[Notidanodon]

41 minutes ago, tombk said:

I think you may be on to something! Maclurites is described as having a “depressed spire instead of an elevated one” in “Invertebrate Fossils” by Moore, Lalicker, and Fishcher. This would explain why it looks different from the Liospira. (And why I thought it’s growth spiraled in a direction opposite to the Liospira.) I know Maclurites have a flat base, but I can’t check whether this specimen does because it’s buried in matrix. Thanks for the lead!

Tom

No problem  

[minnbuckeye]

@tombk, I downplay the Maclurites id. Here is one that I found today and it was likely found within a mile of your specimen's location. Notice the "pretty" face does spiral opposite of a liospira. But the face is virtually flat. The opposite side is domed and unappealing. I suspect your unknown is truly a liospira. Often, an imprint of the liospira tricks you into thinking it is the original shell. Look at the lower edge where there is a defect. There doesn't appear to have depth to the gastropod, hence my opinion of it being just an imprint. Nice finds though!!

 

Edited October 4, 2021 by minnbuckeye

[tombk]

22 hours ago, minnbuckeye said:

I downplay the Maclurites id. Here is one that I found today and it was likely found within a mile of your specimen's location. Notice the "pretty" face does spiral opposite of a liospira. But the face is virtually flat. The opposite side is domed and unappealing. I suspect your unknown is truly a liospira. Often, an imprint of the liospira tricks you into thinking it is the original shell. Look at the lower edge where there is a defect. There doesn't appear to have depth to the gastropod, hence my opinion of it being just an imprint

You may be right. I see what you mean about the lack of depth suggesting that this is an imprint. It’s cool that you were collecting close to this spot. I was about ¾ of a mile south of the intersection of I 52 with College Road. 
 

(I actually kind of like the domed side of Maclurites myself.)

[minnbuckeye]

I surmised that was where you were hunting. The maclurites are found at the VERY TOP of the hill. Look for "holey" limestone and search in that type of matrix. Not much there in that spot though. However, Liospiras are found throughout most of that rock cut.