Phragmolites

I could have titled this "That One Snail", because that's what Phragmolites seems to have become in my circle of acquaintances; for whatever reason, we remember having seen this distinctive fossil but can't remember what it's called. This is where a common name, something like "sculpted ring snail", might come in handy. If you haven't seen them, they could easily be mistaken for a tiny coiled cephalopod, or some kind of worm tube or crinoid columnal if the specimen is not exposed clearly. They range in size from less than a centimeter to about an inch across and are flat-coiled (planispiral), with the outer coils wider than the inner coils and partially covering them, and a distinct but small keel. There are strong markings on the coils that resemble the sutures of a nautiloid or ammonite.

These little fellas.

I've been calling Phragmolites a snail. To be precise, it's a bellerophont, and technically speaking it's not certain if bellerophonts were true gastropods or monoplacophorans (or just how meaningful the distinction is). This involves whether they had undergone torsion and is best left to the specialists to hash out. As with most of the local Ordovician snails and snail-like forms, most of the local species of Phragmolites were described in Ulrich and Scofield (1897), who created the genus name Conradella (after Timothy Abbott Conrad, the invertebrate paleontologist who described Phragmolites in 1838) for them because they thought it was more fitting than Phragmolites. (Phragmolites means "partitioned stone", because Conrad mistakenly concluded that the textured exterior reflected a chambered interior.) This is not regarded as an actionable reason to create a new genus these days, and the genus has long since reverted to Phragmolites, even in Minnesota (see for example usage in Stauffer and Thiel 1941).

Some nice (larger) specimens in the Science Museum collections; the upper left is in Decorah hash, and is quite a bit larger than I'm used to seeing in the Decorah. Note the old name "Conradella".

Ordovician snails and snail-like mollusks of the Upper Midwest have not attracted a great deal of comment since Ulrich and Scofield (1897). Sloan and Webers (1987) provided a stratigraphic chart of mollusks in which they recognized five species spread out through several formations: P. fimbriatus and P. triangularis in the Platteville, P. obliquus in the Decorah and Cummingsville, and P. compressus and P. dyeri picking up the baton and carrying it into the Prosser Limestone. (Which, I suppose, makes mine P. obliquus, but I haven't done the necessary heavy lifting to make a firm statement.)

University of Minnesota (formerly) R653, several specimens described by Ulrich and Scofield (1897) as "Conradella" fimbriata.

A close-up on one of the nicer pieces of this series.

Incidentally, the type specimens of these species are a bit of a headache. Ulrich and Scofield (1897) provided a great favor by reporting the catalog numbers of the specimens they attributed to each species. This was somewhat undercut by then neglecting to mention which, if any, they considered to be type specimens, in effect making the entire collection of a given species a cotype series. For example, "Conradella" fimbriata is represented by "about 22 specimens" at the University of Minnesota and six other specimens held by Ulrich, with museum register numbers 653, 5110, and 8724 (Ulrich and Scofield 1897). On one of my visits to the former collections, I did indeed see specimens from all of these numbers. These specimens were collected from the Vanuxemia bed of the "Stones River group" in Minneapolis and Dixon, Illinois. (Stratigraphically, this translates to basically the upper Platteville as now defined; i.e., the Mifflin Member, or Sardeson Bed #2.) In similar historic cases, later authors have generally designated illustrated specimens as lectotypes, on the grounds that the naming authors must have considered the illustrated specimens to be good examples. Ulrich and Scofield (1897) included illustrations of multiple specimens of this species and other "Conradella" species, which makes things murkier but at least there are illustrations. A more significant complication is that some of the specimens have ended up at the Smithsonian, putting us in a position where the University of Minnesota collection (now at Cincinnati) is reported as having the syntypes of "C." fimbriata (Rice 1990), while the USNM's online database reports it has seven syntypes under USNM 45755 and another under 706473 (and it's not like this is a recent phenomenon, either; Bassler 1915 reported 45755 as cotypes of the species). How the USNM ended up with the specimens is not stated, but I suspect Ulrich gave his specimens to them.

Most of Plate LVII from Ulrich and Scofield (1897), which in turn is mostly "Conradella". Under the spellings given when assigned to Phragmolites (due to grammatical gender), 1–6 are P. obliquus, 7–10 are P. fimbriatus, 11 is P. imbricata, 12–15 are P. elegans, 16–18 are P. grandis, 19–22 are P. triangularis, 23–26 are P. bellulus, and 27–33 are P. dyeri. Of this group, only P. fimbriatus, P. obliquus, P. triangularis, and P. dyeri are found locally.

References

Bassler, R. S. 1915. Bibliographic index of American Ordovician and Silurian fossils: Volume 2. Bulletin of the United States National Museum 92.

Rice, W. F. 1990. Catalog of paleontological type specimens in the Geological Museum, University of Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Information Circular 33.

Sloan, R. E., and G. F. Webers. 1987. Stratigraphic ranges of Middle and Late Ordovician Gastropoda and Monoplacophora of Minnesota. Pages 183–186 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., and W. H. Scofield. 1897. The Lower Silurian Gastropoda of Minnesota. Pages 813–1081 in E. Ulrich, W. Scofield, J. Clarke, 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.