Algae and not-algae

The last of the fossils to be covered to complete the Upper Ordovician Twin Cities set (barring some microfossils and real rarities) are a nebulous group of oft-times enigmatic organisms that either are algae, or have been classified as algae. "Algae" is a much more problematic a term than you might suspect. To put it simply, "algae" is more of a state of mind than a formal classification. At its broadest, "algae" covers basically anything that does photosynthesis and doesn't have the obvious distinct tissues of derived plants, like leaves and roots. This would include anything from diverse microbes to seaweeds. While this has some utility for back-of-the-envelope things, it is not the most useful term for serious classification. It should not be surprising that untold numbers of fossils have been been classified as algae, and that many of these "algae" belong to disparate groups, some of which still have unsettled classifications. About a half-dozen taxa from the Platteville, Decorah, and basal Cummingsville formations have fallen into the "algae" bin at one time or another. They include the following:

Rhodophyta (red algae)

Rhodophytes, the red algae, are marine algae, generally multicellular, with a long and distinguished history. Some rhodophytes secrete calcium carbonate and form large stony structures. Modern varieties that do this are classified as coralline algae. They appeared in the Mesozoic, but their antecedents go back well into the Paleozoic. In the Decorah, there is a species formerly identified as the tabulate coral Tetradium fimbriatum (e.g., Stauffer and Thiel 1941), which has been reidentified as a red algae. As red algae, it is known as Prismostylus columnaris (Steele-Petrovich 2011).

Chlorophyta (green algae)

Chlorophytes are some of the few "algae" that are closely related to land plants. We have one chlorophyte in the Platteville, the incredibly delicate Chaetocladus sardesoni. This species was described by Ruedemann in 1909 from specimens previously thought to be graptolites. There's nothing else quite like these feather-like or filament-like fossils in the local rocks (well, other than some graptolites, of course).

This is Plate 2 from Ruedemann (1909). Chaetocladus sardesoni is all but figures 1 and 2 (which are of Callithamnopsis delicatula).

See that line across the lower left part of the rock, running from top to bottom in the photo, that looks kind of like a faint dark stitch? That fine feature is a Chaetocladus sardesoni (University of Minnesota collections).

Receptaculitidae ("sunflower corals")

The receptaculitids are an enigmatic group of fossil organisms, known informally as "sunflower corals" because they look like the centers of sunflowers and they're kind of coral-like in gross appearance. The living thing, though, was apparently a bulbous, inflated structure, and so somewhat less sunflower-like in appearance. Although "coral" has stuck in the vernacular, it's not generally in play as an actual classification. The main debate over the years has been between "sponge" (sometimes allied to the Cambrian archaeocyathan sponges) and "dasycladacean algae" (another type of chlorophyte). Nitecki et al. (1999), a recent summary, came to the conclusion that it wasn't clear what these fossils were, and like many other organisms known from striking fossils, they have inconveniently become extinct. Be that as it may, receptaculitids make a useful biostratigraphic reference in Minnesota, because they are particularly common at the top of the Decorah and the lower part of the overlying Cummingsville Formation. In the Twin Cities, that combination limits you to Lilydale and a few neighboring areas. Stauffer and Thiel (1941) listed three species we would consider receptaculitids. All three came from the Decorah and "Prosser Member" of the Galena:
Fisherites reticulatus (listed under Spongiae as Receptaculites oweni)
Ischadites iowensis (listed under Spongiae)
Pasceolus globosus (listed under Cystoidea)

"Algae"

We've got one more oddball, traveling under the name Solenopora compacta. Solenoporaceans have historically been considered red algae, but the type species of Solenopora itself appears to actually be a chaetetid sponge, and other Ordovician relatives may be receptaculitids, which leaves our Solenopora in the lurch. Winchell and Schuchert (1895) thought it was a hydrozoan, among the relatives of corals and jellyfish. Stauffer and Thiel (1941), who reported from the Decorah, listed it as plain ol' Algae. Time will tell.

This is Plate F from The Geology of Minnesota, volume 3, to illustrate Winchell and Schuchert (1895). Figures 1 to 4 are Receptaculites oweni (Fisherites reticulatus). Figures 5 and 6 are of Ischadites iowensis. Figures 21 to 23 are of Solenopora compacta. (for reference, the rest: Figures 7 to 10 are of Ischadites koenigii. Figures 11 and 12 are Lepidolites dickhauti. Figures 13 to 15 are Anomalospongia reticulata. Figures 16 to 20 are species of our old friend Rauffella).

References

Nitecki, M. H., H. Mutvei, and D. V. Nitecki. 1999. Receptaculitids: a phylogenetic debate on a problematic fossil taxon. Springer Science+Business Media, LLC, New York, New York.

Ruedemann, R. 1909. Some marine algae from the Trenton Limestone of New York. New York State Museum Bulletin 133:194–216.

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

Steeler-Petrovich, H. M. 2011. Replacement name for Tetradium Dana, 1846 [it's not free, but because it's a short article you can see almost the whole thing in the preview]. Journal of Paleontology 85(4):802–803.

Winchell, N. H., and C. Schuchert. 1895. Sponges, graptolites, and corals from the Lower Silurian in Minnesota. Pages 55–95 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, MN.