Sunday, September 21, 2014


Bryozoans make up one of the "B"s of the Decorah BBC (brachiopods, bryozoans, and crinoids), and may be the most abundant; I have seen rocks that are more or less bryozoan hash. Bryozoans are known colloquially as "moss animals", which is fair enough in terms of some of them encrusting things in the manner of carpets of moss, and also being animals. They are often compared to corals, but are quite different. A coral animal is a sack; food goes in and waste goes out the same passage. Bryozoans have a one-way flow with two holes. Coral animals are much larger than bryozoan animals; for example, for the Ordovician fossils of the Twin Cities, coral apertures typically measure several mm across, while the apertures in bryozoan colonies are sub-mm in scale. Finally, bryozoans have never gone in for the algal symbiont trick like corals have done. Not having symbionts, bryozoans must rely on filter-feeding, which the individual bryozoan animals do via a structure called a lophophore, a sort of ring of tentacles around the mouth. This structure is also found in some soft-bodied marine critters with very poor fossil records and the other "B" of the BBC, the brachiopods, showing that they were related, making up a group called the Lophophorata. Various extinct groups known from "worm tubes", like our old friends the cornulitids, may also have been lophophorates.

Some brief anatomical terminology: first of all, when you look at a fossil bryozoan, you are looking at the skeleton of a colony, which can also be called a zoarium. The individual animals would be called zooids, and the section of the colony surrounding a single zooid is its zooecium. All of the zooids of a colony are genetically identical, having budded from the same original zooid, although in practice they don't necessarily all do the same things; some members of a colony may be specialized to perform various tasks. To see the individual habitations, break out your hand lens or use the power of nearsightedness to look at the colony at close range. You should see that the surface is covered by tiny pores, smaller than a millimeter; these each held a zooid.

As noted, bryozoans are one of the Big Three for the Decorah, along with brachiopods and crinoids. They are also by far the most variable in shape. While brachiopods tend to look like symmetric clam shells, and recognizable crinoid fossils are almost invariably solitary columnals (the "Cheerios") or a couple of articulated columnals, bryozoans take on a number of different shapes. It doesn't take much observing or collecting to come up with variants on all of the following:
  • Cylindrical twiggy or branching forms;
  • Branching forms with flat cross-sections;
  • Unfurled flags or potato chips;
  • Perforated crackers;
  • Nets;
  • Chocolate "kisses" (Prasopora);
  • Vine- or strand-like (Vinella);
  • Encrusting (any darn form they please, often close to the form of the thing they're encrusting)
To put it a bit more scientifically, with the help of the Digital Atlas of Ordovician Life:
  • Twiggy or cylindrical branching forms are ramose or dendroid;
  • Flat branchers are frondose;
  • Flags and potato chips are bifoliate;
  • Crackers are fenestrate;
  • Nets can be fenestrate or anastomosing;
  • Chocolate kisses are hemispherical;
  • Vines are a type of encrusting bryozoan;
  • Encrusters need no further explanation;
  • And anything without an obvious form is amorphous
Bryozoan structures tend to be mutable, and most forms can only be distinguished by making thin sections or acetate peels. Given these technical issues, the basic structural categories are probably of more immediate use than more traditional taxonomy. In practical terms, when you go out looking for bryozoans from a formation like the Decorah where the fossils are harder than most of the rocks and weather out readily, you will readily find chunks of the more durable forms, like ramose and hemispherical. The more fragile frondose, bifoliate, fenestrate, and anastomosing forms tend to be confined to slabs.

This slab, from one of the thin limestone beds of the Decorah Shale, is mostly bryozoan hash, although there are a few larger fragments, including a large ramose bryozoan in the upper left and two large fenestrate pieces (the grayish perforated pieces), as well as scattered pieces of frondose bryozoans.

A fairly typical group of loose bryozoan fragments from the Decorah, dominated by ramose bryozoans. The large dark piece is frondose, and you might just be able to make out the fine stippling of the pores without clicking. The upper round piece is an encrusted brachiopod shell.

Stauffer and Thiel (1941) listed 179 species and varieties of bryozoans from the relevant formations. I'm sure you just love bryozoans, but loving 179 species' worth can be a lot to ask out of an audience, particularly when you'd need dedicated equipment to distinguish a fair portion of them, and many of them have never been re-evaluated since being named in the late 19th century (much of it is straight out of Ulrich [1895]). We can probably get by all right with just the genera at this point, which leaves us with 55, still quite a bit but much more manageable. As before, Gl is the Glenwood, Pl is the Platteville, Ca is the Carimona Member of the Decorah Shale, De is the shaly part of the Decorah, and Gp is a combination of the Cummingsville and Prosser formations. Also, as before, this is state-of-the-science for, basically, pre-World War II. I went through to find the obvious changes, but there's going to be things missing.

Anolotichia (Ca, De)
Arthroclema (Ca, De, Gp)
Arthropora (Ca, De, Gp) [=Graptodictya]
Arthrostylus (Ca, De)
Aspidopora (Ca, Gp)
Atactoporella (Ca, De)
Batostoma (Pl, Ca, De, Gp)
Berenicea (Ca, De) [genus not recognizable, local species apparently assigned to Proboscina]
Bythopora (Ca, De)
Callopora (De)
Ceramophylla (De)
Ceramoporella (Ca, De, Gp
Chasmatopora (Pl, De, Gp) [including C. corticosa, now assigned to Sardesonina]
Coeloclema (De, Gp) [local species now assigned to Diamesopora]
Constellaria (Gp)
Corynotrypa (Ca, De, Gp)
Crepipora (Pl, De, Gp)
Cyphotrypa (Ca, De, Gp)
Dekayella (Pl, Ca, De, Gp) [=Heterotrypa]
Dianulites (Gp)
Diastoporina (Gp)
Diplotrypa (Gp)
Eridotrypa (De, Gp)
Escharopora (Pl, Ca, De, Gp)
Eurydictya (De, Gp)
Favositella (Pl, De, Gp)
Graptodictya (Gp)
Hallopora (Pl?, Ca, De, Gp)
Halloporina (De, Gp)
Helopora (Ca, Gp)
Hemiphragma (Ca, De, Gp)
Homotrypa (Pl, Ca, De, Gp)
Homotrypella (Ca, Gp) [=Peronopora]
Leptotrypa (Pl)
Mesotrypa (Ca, Gp)
Mitoclema? (Gp)
Monotrypa (Pl, Gp)
Monticulipora (Ca, De, Gp)
Nematopora (Gp)
Nicholsonella (Pl, Ca)
Orbignyella (Pl)
Pachydictya (Ca, De, Gp)
Phaenopora (Gp)
Phyllodictya (Ca)
Prasopora (Ca, De, Gp)
Proboscina (De)
Rhinidictya (Pl, Ca, De, Gp) [=Stictopora]
Spatiopora (Ca)
Stictoporella (Ca, De) [including a species now in Stictoporellina (S. cribrosa), our common fenestrate form]
Stigmatella (Ca)
Stomatopora (Ca)
Stromatotrypa (Pl, Ca) [=Batostoma]
Trematopora (Ca)
Trigonodictya (De)
Vinella (Gl, De, Gp)

An interesting point about the distribution of genera and species is that this is definitely a Decorah show, especially if you remember to include the Carimona. There are some in the "Prosser", a few oddballs in the Platteville, and a solitary representative in the Glenwood (Vinella), as well as one indeterminate form in the St. Peter reported by Sardeson (1896) but omitted by Stauffer and Thiel, but they pale in comparison to the combined might of the Carimona and Decorah (especially if you're looking at the species lists, and not just genera). If you're used to the Decorah and its abundant bryozoans, the underlying Platteville and its wall-to-wall brachiopods will come as a surprise. This difference would seem to be saying something very loudly about the substrate and other environmental conditions.

If you'd like to do further research on the genera, here are several links you can start with:
Baltoscandian Fossils
Digital Atlas of Ordovician Life
Fossils and Strata of the Type Cincinnatian (under Bryozoa)
You can also see what the University of California Museum of Paleontology and Wikipedia (quite an article, although unfortunately the primary editor has passed away) have to say about bryozoans.


Sardeson, F. W. 1896. The Saint Peter Sandstone. Bulletin of the Minnesota Academy of Natural Sciences 4(1):64–88.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.

Ulrich, E. O. 1895. On Lower Silurian Bryozoa of Minnesota. Pages 96–332 in Lesquereux, L., C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(1). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.


  1. Is the middle round fossil in the Decorah photo above a Plasapora? I found a bunch of these in Decorah layers outside Decorah Iowa, and have not had a definitive ID yet, but internet searches indicate the Plasapora bryozoan. Any help?

  2. Figuring out how to get notified if someone answers my question above...

    1. Hello, Joan;

      Thank you for stopping by! Would you mean "Prasopora"? The round fossil in the photo is actually a disc-like object, with little height, and I suspect the bryozoan has encrusted a round shell (or it was just a really lazy Prasopora). Prasopora bryozoans are indeed common, and they do look round from above, but in profile they typically range from domes, to gumdrops, to "Hershey kisses".

    2. Yes, "prasopora"... Love those typos. So the one in your photo isn't domed. Is there a way to get a photo to you to check?

    3. If you've got a digital image or a scanner, you can send it to thescelosaurus [at]

  3. I have the image, but the email address above didn't work. Help?

    1. I turned @ into [at] in a likely futile effort to keep spambots from picking it up as an email address. You should be able to type it in like any gmail address.

  4. I got that...maybe misspelled the rest. Sent again this AM. Thanks.