Scolecodonts and other signs of worms

Clinton R. Stauffer had one of the most convenient field areas it is possible for a geologist to have. He worked for the University of Minnesota from 1914 to 1944, producing a number of papers on paleontology and stratigraphy; they include a study of the Paleozoic of Minnesota (Stauffer and Thiel 1941), a list of Pleistocene mammal finds from the state (Stauffer 1945; an update would be greatly appreciated!), and several descriptions of microfossils (Stauffer 1930, 1933, 1935a, 1935b). He did a fair amount of collecting for the university, and a fair amount of collecting within the university. "Aha," you may say if you are familiar with the geography; "the campus is split by the river, with prominent and accessible bluffs." This is quite correct, but he did not limit his on-campus collecting to the bluffs. For example, during the construction of Northrop Auditorium in 1927, he obtained rocks from the excavation for the heating shaft (Stauffer 1930), and described a number of conodonts and other fossils from this material (Stauffer 1930, 1933, 1935a). If he wanted comparable material from other locations, it was only a matter of miles to southwestern St. Paul/southeastern Minneapolis, where he worked extensively in the Ford Plant/Fort Bridge/Minnehaha Creek/Lock & Dam 1 area (this was convenient both in terms of location and time; the dam, auto plant, and bridge were all completed during this time frame, so there was a lot of disturbed ground and excavated rock to pick through). It wasn't all roses and brachiopods, though; those bluffs on campus are not a place for anyone who have a healthy respect for gravity and large heavy rocks. The combination of a narrow footpath and overhanging rock makes the area about the most hazardous I have seen for Twin Cities geology.

The bluffs south of the Washington Avenue Bridge. Not recommended.

One of the types of fossils Stauffer is associated with is scolecodonts, jaw elements of predatory polychaetes, aka bristle worms. Worms don't often leap to mind as great candidates for fossils, as opposed to, say, snails and clams, which have the enormous advantage of durable shells. Generally, this impression is quite correct; most worm-type creatures ("vermiform", if you're in the market for another vocabulary word) lack substantial hard parts and are limited to providing burrows and other trace fossils. Burrows are common fossils throughout the Paleozoic rocks of the Twin Cities, ranging from mm-scale to the diameter of a finger. Of course, not all of them are of a wormy persuasion, but for many soft-bodied invertebrates, they are all we will ever know of their existence. Dokken (1987) provides a good overview of Ordovician burrow types in Minnesota.

A vertical burrow in the Platteville Formation in the vicinity of the Stone Arch Bridge, near the Steamplant.

The worm crawls in, the worm crawls out: a set of burrows from the Decorah of the Brickyard produced by an animal probing through the sediment multiple times. This type of fossil is called Licrophycus ottawaensis in old collection labels, more recently Phycodes, and is probably something else by now, trace fossil nomenclature being what it is.

Scolecodonts provide a more detailed glimpse of some of the worms. They are durable hollow chitinous structures, mm-scale and covered with pointy bits. Being microfossils, they are not the kind of thing that you usually spot just walking around. Instead, they are extracted from the host rock in a lab, by washing or using acids to dissolve the rock (Stauffer 1933). There are actually multiple scolecodont elements to a jaw; see for example Scolecodont.net for diagrams of some arrangements. Fossil scolecodonts look quite similar to modern bristle worm jaws, and the actual animals probably looked a lot like modern forms. I would avoid doing any image searches if you are bothered by worms, especially worms that can look like centipedes (remember the bristles), but scolecodonts are often ascribed to the group Eunicida, so something like the genus Eunice would be an appropriate point of comparison.

Stauffer (1933)'s scolecodonts came from several localities and formations in the Twin Cities, including the Washington Avenue Bridge bluffs (Glenwood Formation), the former Johnson Street Quarry (Platteville Formation), a well in St. Paul (Platteville), the Decorah Shale of the University of Minnesota campus, and the Ford Bridge bluffs (Decorah). He noted that the specimens were often water-worn and poorly preserved, and suggested that they had been exposed to erosion, with the original worms perhaps living in tidal flats or near the low water line, like some modern forms. Stauffer got a few dozen new species and genera out of his scolecodonts, probably too many, although parsing scolecodonts can be difficult (Eriksson 1999). More problematic is that Eriksson (1999) found that Stauffer's illustrations don't always correspond well to the actual fossils. As with so many other topics in paleontology, more research and more specimens would be welcome! More information (and images) concerning scolecodonts of a similar age can be found at Cincinnatian Fossils and Stratigraphy, Dry Dredgers, and Fossilid.info.

Local scolecodonts from Stauffer (1933)

On a personal note: I can't describe how surprised and happy I was to find that the previous post (Monoplacophorans) was picked up by Thomas R. Holtz, Jr. and Brian Switek on Twitter and Facebook. Of course, I had to figure out a way to outdo that post, so I went back through my list of potential topics to see what popped...


...and obviously, I got stuck and wrote about worms.


References:

Dokken, K. 1987. Trace fossils from Middle Ordovician Platteville Formation. Pages 191–196 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, MN. Report of Investigations 35.

Eriksson, M. 1999. Taxonomic discussion of the scolecodont genera Nereidavus Grinnell, 1877, and Protarabellites Stauffer, 1933 (Annelida, Polychaeta). Journal of Paleontology 73(3):403–406.

Stauffer, C. R. 1930. Conodonts from the Decorah Shale. Journal of Paleontology 4(2):121–128.

Stauffer, C. R. 1933. Middle Ordovician Polychaeta from Minnesota. Geological Society of America Bulletin 44(6):1173–1218.

Stauffer, C. R. 1935a. The conodont fauna of the Decorah Shale (Ordovician). Journal of Paleontology 9(7):596–620.

Stauffer, C. R. 1935b. Conodonts of the Glenwood beds. Geological Society of America Bulletin 46(1):125–168.

Stauffer, C. R. 1945. Some Pleistocene mammalian inhabitants of Minnesota. Minnesota Academy of Science Proceedings 13:20–43.

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