Monday, March 27, 2023

Prasopora, with a comment on biostratigraphy

Prasopora, the "gumdrop bryozoan", is one of the most recognizable Ordovician fossils in Minnesota. Museum collections from Minnesota have boxes of the little darlings rattling around together. And I—I hardly ever see the dang things in the field.

Yes, one of these things; a bit more "chocolate kiss" than "gumdrop" but well within morphological variation.

While my competence in many fields is questionable at best, in this case you can be assured I would recognize a Prasopora if I saw one, even if it was years before I learned the stress goes on the second syllable rather than the third. (I have an unerring instinct for putting the stress on the wrong syllable for scientific names I've read but never heard.) No, the real issue here is one of biostratigraphy. My usual stomping grounds cap in the lower third of the Decorah, and Prasopora doesn't really kick in until the middle–upper Decorah. This has been recognized since the days of "The Geology of Minnesota" (Ulrich 1895; Winchell and Ulrich 1897). At that time eight species were recognized (P. affinis, P. conoidea, P. contigua, P. insularis, P. lenticularis, P. oculata, P. selwyni, and P. simulatrix), all of which were restricted to a range extending from the "Fucoid and Phylloporina beds" of the "Black River Group" (roughly Sardeson bed 5, middle–upper Decorah) to the "Fusispira and Nematopora beds" of the "Trenton Group" (as high as Sardeson bed 8, in the Prosser Limestone) (Winchell and Ulrich 1897; approximate correlations after Sloan 1987). None of them are listed in equivalents to Sardeson beds 3 and 4, in the lower Decorah, and only P. conoidea, P. contigua, P. lenticularis, and P. simulatrix were reported from the closest "Fucoid and Phylloporina beds".

Same specimen as above, which has a convenient break showing a partial cross-section; it's not solid all the way through.

Forty years later Stauffer and Thiel (1941) were not quite as dainty in their stratigraphic divisions, simply having a Decorah Shale Member of the Galena Formation and a Spechts Ferry Member of the Platteville Formation (approximately the Carimona Member of the Decorah). The "Spechts Ferry" gets Prasopora grandis, which had been Monticulipora grandis back in 1897, when it had been reported from the Stictoporella bed (lower Sardeson bed 3). P. grandis also appears in S&T's Decorah Shale Member list along with the four "Fucoid and Phylloporina beds" species, Stauffer presumably having stratigraphically higher specimens of P. grandis than W&U. Whether or not grandis pertains to Prasopora has been a matter of some dispute, and it seems to have wandered back to the metaphorical arms of Monticulipora. More importantly, it doesn't look like classic gumdrop Prasopora, instead being "irregularly massive, often tending to become lobate or subramose" (Ulrich 1895). In other words, it's not the kind of thing the typical fossil enthusiast would associate with the genus.

Wee little discoidal Prasopora.

My personal experience with Prasopora is limited to a few pieces in the Valentine box that appear to represent P. conoidea and a small discoidal species, a couple of small discoidal specimens that blur the line between early-stage Prasopora and "less famous bryozoan encrusting the external surface of an inarticulate brachiopod in an aesthetically pleasing Prasopora-like way", and one great honking lopsided hoof of a colony I found a few years ago at a basement excavation. I don't generally attempt to assign species to bryozoan fossils, but P. simulatrix is the only species described by Ulrich (1895) to attain dimensions even vaguely like it, so I'll go with that.

A top view of an unfortunately resolutely three-dimensional object.

And the underside, showing the distinctive layering and a few bits of other things that became part of the structure.

Bonus news: for those of you who've had your fill of gumdrop bryozoans, the spring 2023 edition of the NPS Park Paleontology newsletter is now available.


Sloan, R. E. 1987. History of study of the Middle and Late Ordovician rocks of the Upper Mississippi Valley. Pages 3–6 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

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 L. Lesquereux, 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.

Winchell, N. H. and E. O. Ulrich. 1897. The lower Silurian deposits of the Upper Mississippi Province: a correlation of the strata with those in the Cincinnati, Tennessee, New York and Canadian provinces, and the stratigraphic and geographic distribution of the fossils. Pages lxxxiii–cxxix in L. Lesquereux, 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(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

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