Sunday, October 22, 2017

Sunday morning Decorah fossils

I've recently made a small collection of Decorah Shale pieces from spoil piles at a construction site. Most of them will go to other people and groups for use in education, but while I've got them I'm certainly going to take the opportunity to photograph them. Incidentally, construction can be a good source of fossils in the Twin Cities, if you don't mind disruption of the original stratigraphic context (which tends to happen anyway with the Decorah around here). Of course, as always, you'll want to ask for permission, and it's advisable to make collections when someone is working there, so you aren't mistaken for a trespasser or other nefarious sort.

These pieces are mostly chunks of the Decorah's thin limestone beds, within the lower third or so of the formation, although there is one Platteville-type block and one interesting rounded fossiliferous cobble that could have been transported from a Paleozoic outcrop some distance away. Having seen my fair share of the lower third of the Decorah, I feel reasonably qualified to make a few observations:

The site is another point which has at least one thin pale gray bed, notable for its abundant snails, that also produces good trilobite pieces, and often has white equant to triangular smooth bits a millimeter or three across. Some of them look like small bivalves, others could be just about anything.

There are several different kinds of snails in this slab, and a cranidium is draped across one near the center.

In terms of bryozoans, this particular part of the Decorah seems most favorable to twiggy branching bryozoans, follow by flat branching. A couple of pieces have bifoliate bryozoans of the "saltine" variety, with relatively large holes. This is in contrast to the lowermost part of the shale, which I've seen produce abundant solid bifoliate bryos. In addition, I found one fairly large loose Prasopora, with an unusual hoof-like shape; maybe the colony was backed up to something and couldn't grow in the usual circular shape?

Here it is, upside-down. The original object that the colony grew around is visible in the center of the concentric circles and looks to be an inarticulate brachiopod. There's some wear on the back surface, but it doesn't look like it was originally a complete circle that later broke. You can also see some other shells and a bryozoan fragment that the colony enveloped.

Some of the hashes are almost pure twiggy and flat branching bryozoans, while others are dominated by large strophomenid brachiopods.

So many bryozoans... (there's also a small baggie of more pieces that were embedded in the clay but not attached to the block.)

Okay, so this one's not entirely strophs, but it's as good an excuse as any to show a few, and anyway stroph hash doesn't look as nice as a few more complete shells.

Once you get used to trilobites, their fragments start to show up in abundance. Because of the peculiarities of trilobite anatomy and growth plus the local geology, we don't usually find whole examples here. Instead, we find fragments of molts, usually the pygidium (tail end) or parts of the cephalon (head end). Hypostomes, the mouth parts, are also fairly common as far as these things go. One trick to finding them is that trilobite pieces have a different composition from most of the other local fossils, and often appear dark.

Several fragments here. Can you spot the heads?

Here's a pretty good hypostome; some trilobite is running around without its dentures!

This is the pygidium (I believe upside-down) of a Ceraurus-like trilobite.

Several of the pieces have chunks of crinoid stems, an inch long to several inches long. I have discovered an interesting relationship between stem length and positive feelings in situations where you usually only find single columnals. The solitary columnal is almost beneath notice. A chunk an inch long or so is a neat curiosity. A chunk greater than six inches long but with no calyx inspires the thought "No! Where's the rest of it?"

This is gonna take some work.

There is a fair amount of rusty iron staining in these rocks, infiltrated from the overlying sediments. I can get a lot of it off the matrix with a toothbrush, but it's more tenacious in the fossils themselves.

There had been a lot more red to this before. This is the classic BBC (brachs, bryos, and crinoids).

I've seen a lot of lower Decorah blocks, and as far as I'm concerned you might as well call horn corals "hens' teeth". I see more of the encrusting tabulate Lichenaria than horn corals. Maybe it's just me. On the other hand, I see horn corals fairly often in the Platteville.

I'm not entirely sure, but I think the bryozoan has grown over the coral, instead of the coral encrusting the bryozoan.

1 comment:

  1. I tend to pick up horn corals like crazy.Lichenaria tend to be rare for me. I found a big coral chick at lilydale a few years ago. I prob will put pics on fossil forum but not anytime soon. From books I had is as tetradium species. Obvious a very rare find.