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Showing results for tags 'upper decorah formation'.
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From the album: Brachiopods of Platteville/Decorah Twin Cities Minnesota
Zygospira sp. Apparently there are a mystery with this genus when it comes to Twin Cities. I first noticed it only from Middle Platteville Formation - 100% nonexistent from upper Platteville Formation. Much more finer costae compare to Rostricellula minnesotensis and generally smaller. Belongs in Spiriferid family despite it strong resemble to Rhyconellid family. Then after a very long absent either the same species or a very similar species popped up again in upper Decorah formation and became very abundant. I suspects the reason for this seemly weird pattern is they may be much more specialized into the type of sediments that laid on the ocean floor. The middle platteville had layers of high carbonate shale mixed in, which does not exists in upper and lower Platteville formation. Then upper Decorah formation had a harder type of shale - middle and lower Decorah formation have shale that breaks into pieces very easily. Plus from my experiences with modern Florida gastropods there are two endemic species of gastropods that had very narrow ranges. One I don't have specimens of is Sanbriel Island turret snail which is endemic to southwest mainland Florida. Not found anywhere else. The other snail species, Shiny Atlantic auger, Hastula hastata is only found generally around Jupiter Island on Atlantic side. At first it may seem like the two snail species have nothing in common but I find it interesting that they occurs in a small zone in between the carbonate sediments of far southern Florida to the keys and the sandy rubble to the north.-
- brachiopod
- middle platteville formation
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From the album: Platteville to Decorah formation Ordovician Twin Cities
Another interesting thing about Upper Decorah Formation is numerous of unidentified Zygospira species along with orthids of different sizes. For some unknown reasons Zygospira species are largely missing from lower to middle Decorah Formation even though they're abundant in Middle Platteville as well too in Twin Cities.-
- brachiopod
- minnesota
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From the album: Platteville to Decorah formation Ordovician Twin Cities
I think the two large brachiopod, one in NW corner and one SE corner is maybe new to me. The muscle ridges are very different with two parallel to each other extending from hinge line. Also seem like Upper Decorah Formation tend to have more varieties of Strophomenid brachiopods that is rarer in Middle and Lower Decorah Formation. One new thing I just learned today is Sowerbyella minnesotensis tend to be very variable in # from site to site. It is very characterized of Upper Decorah Formation as well too, being entirely absent from Middle and Lower Decorah Formation. One site had Sowerbyella minnesotensis making up vast majority of shell hash while yesterday site where I collected most of the fossils have Sowerbyella minnesotensis in low densities.-
- brachiopod
- strophomenid
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From the album: Platteville to Decorah formation Ordovician Twin Cities
Prasopora conoidea with unidentified bryozoan. You can also see rust stains. Rust stain can be very prevailing in some layers or spots on rocks. Mainly because glacial deposits tend to be very iron rich from Norrthern Minnesota and then they get dissolved in groundwater and redeposit on the fossil shell hash because they're much harder for water to pass through. Another thing to know: Prasopora strongly prefers muddy sediments and thus wont appear or rarely in carbonate rich environment. . -
From the album: Platteville to Decorah formation Ordovician Twin Cities
Some orthid brachiopods, mainly Cincinnetina and Dinorthis along with several lookalikes tend to be found only in Upper Decorah Formation. Upper Decorah tend to overlaps with Galena Formation and Richmondian formation. That's why you can find some of the same orthid brachiopod species from Minnesota to Ohio to Kentucky.-
- brachiopod
- minnesota
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From the album: Platteville to Decorah formation Ordovician Twin Cities
First Vinlandostrophia brachiopod find of the year! I will have to compare it to the other two species later. Hash is much more fragmentary - I call it bryozoan hash.-
- brachiopod
- minnesota
- (and 3 more)
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From the album: Platteville to Decorah formation Ordovician Twin Cities
Larger picture of the same trilobite. Just to show an example of upper Decorah twin cities hash plates. Certain crinoid species are very strongly represent compare to middle and lower Decorah Formation. The largest crinoid species is often in single pieces or sizable stem pieces but head and suction portion are unusually absent. In Middle and Lower Decorah formation, the density of crinoid fossils tend to be lower.