Sunday, June 25, 2017

Dikelocephalus minnesotensis

Investigating the rocks of Saint Croix National Scenic Riverway (SACN) is a tougher nut than working in MNRRA. Most of the area where rocks are exposed in MNRRA is part of some kind of park (city, state, regional, county, etc.) and generally accessible to the public. Much of the area with outcrops on the St. Croix is private land, and many of the key localities in the literature are now overgrown, destroyed by construction, or are roadcuts next to busy highways. Determining where you are in the strat column is also more difficult. In MNRRA, it's hard to get mixed up if you can tell sandstone from limestone/dolomite and shale. In SACN, you're dealing with several quartz-rich medium to coarse sandstones that tend to look the same, with some intervening shaly, dolomitic, or finer-grained sandy formations, and in the literature practically every single investigator had their own preferred system of names right up until the 1960s. Finally, in MNRRA there are abundant and diverse fossils in the Platteville and Decorah, while in SACN the special of the day is the BLT (burrows, lophophorates [brachiopods and hyoliths], and trilobites) with a side order of mystery snails, and you have to work for everything but the B.

The sweet siren song of the Franconian trilobite.

Nevertheless, the St. Croix Valley contains about a dozen fossil-producing areas of Late Cambrian age that have received significant attention in the literature, from Afton up to St. Croix Falls, and it is due to the historic investigations that the uppermost Cambrian of North America is sometimes known as the Croixan or St. Croixan. Several dozen species, mostly brachiopods and trilobites, have been named from these sites. One of the best known of these is the trilobite Dikelocephalus minnesotensis. D. minnesotensis is among the first fossil species described from Minnesota, in the batch out of David Dale Owen's expedition (Owen 1852). By pride of page priority, it gets to be the type species of the trilobite Dikelocephalus. Owen did not designate type specimens, so the illustrated fossils have come to be regarded as the type material. They include a partial cranidium (USNM 447020) and pygidium (USNM 17863), which have fortunately survived all these years. According to Owen, these specimens came from near Stillwater: "This species was first found, and is most common, in a dark gray, argillo [clayey]-calcareous [limy] bed intercalated in member d of F. 1, ninety or one hundred feet below the base of the Lower Magnesian Limestone [Prairie du Chien Group], near the margin of Lake St. Croix, above Stillwater".

Dikelocephalus minnesotensis, Owen (1852), Table 1, figure 1. The original caption is: “Dikelocephalus Minnesotensis (N.S.) from the fifth Trilobite-bed of F.1, on the banks of the St. Croix, at Stillwater, Minnesota. Restored outline, in dim contour.” More modern reconstructions based on more complete material can be found at trilobites.info. I included a photo of one of the syntypes, USNM 17863, in my recent article for Park Paleontology.

Although the geology is reasonably precise for 1852, the geography is vague. Winchell (1888) noted that the horizon in which the first specimens were found was the St. Lawrence Formation. (This publication is also useful for documenting a mastodon [or mammoth] specimen from Brown's Creek in Stillwater.) Hughes (1993) and Labandeira and Hughes (1994) gave the type locality as Fairy Glen, just north of Stillwater. I am not sure if this is because they had some information I don't, or if it was just based on the fact that Fairy Glen is, historically, the place you would go to find the St. Lawrence Formation near Stillwater. Maybe it's in Labandeira 1983. (note, 2017/06/27: probably not; it's also in Nelson 1949, which also assigns type localities post hoc to several other species.) At any rate, you can visit Fairy Glen, because it's now a small park, but don't expect to be seeing hordes of trilobites; Hughes (1993) reported that the productive section was "no longer exposed", and having wandered around there for a while I don't doubt it. You can see plenty of the overlying Jordan Sandstone, though, and Fairy Falls.

Getting to this point takes some effort; the best way is partially overgrown this time of year.

Dikelocephalus as a genus had a fairly quiet existence in the literature (perhaps because nobody could spell it) until the early 20th century. Charles Walcott, as part of his work on the Cambrian, added some species and put others in new genera in 1914. Matters rested for a few years, then got weird. What happened was that Dikelocephalus fell under the attention of Edward Oscar Ulrich and Charles Resser, who were, how shall we put it, prolific multipliers of species (Sundberg 2007). Their natural tendencies, when combined with what is now known to have been a highly variable trilobite, produced wonders (Ulrich and Resser 1930). Nobody expressed their dismay at the situation quite like Gilbert Raasch, who wrote "By 1933, Ulrich and Resser had multiplied this...to the astounding total of 123 species and varieties. They simultaneously succeeded in rendering the Dikelocephalidae useless for purposes either of biostratigraphy or phylogeny, and this important fossil group has subsequently been shunned by paleontologists and stratigraphers" (Raasch 1951). He then devoted several pages to documenting their sins and finished by chopping 123 to 41. Some parts are difficult to follow, though, because of some lithological/biostratigraphic issues as detailed in Nelson (1953). Raasch was in a position to complain, having collected trilobites for Ulrich and Resser and being a leading expert on the rocks in question (Mikulic and Kluessendorf 2001). The problem wasn't completely solved until the work of Nigel Hughes and Conrad Labandeira (Labandeira 1983; Hughes 1991, 1993, 1994; Labandeira and Hughes 1994; Hughes and Labandeira 1995). In these works, which came to incorporate more than 2750 specimens (Hughes 1994), they came to the conclusion that except for a possibly distinct species in the Tunnel City Group, Dikelocephalus was just D. minnesotensis, and it was super-variable (Hughes 1994). This is thought to be an example of Rosa's rule, which documents the tendency for more primitive members of a lineage to be much more variable than more advanced members.

Having thought about the subject of a state fossil for Minnesota occasionally since I posted about it last year, I think that Dikelocephalus minnesotensis would make a good candidate. It's definitely Minnesotan (says so right there in the species name), it's got a historical hook, it's reasonably well-known, it's not teeny-tiny like most of Minnesota's fossil invertebrates (D. minnesotensis is downright big for a trilobite, topping out longer than 12 in/30 cm), and trilobites are pretty charismatic as fossils. Another point of interest: although a couple of other states have trilobites as state fossils, D. minnesotensis would be the only Cambrian state fossil.

References:

Hughes, N. C. 1991. Morphological plasticity and genetic flexibility in a Cambrian trilobite. Geology 19(9):913–916.

Hughes, N. C. 1993. Distribution, taphonomy, and functional morphology of the Upper Cambrian trilobite Dikelocephalus. Milwaukee Public Museum Contributions in Biology and Geology 84.

Hughes, N. C. 1994. Ontogeny, intraspecific variation and systematics of the Late Cambrian trilobite Dikelocephalus. Smithsonian Contributions to Paleobiology 79.

Hughes, N. C., and C. C. Labandeira. 1995. The stability of species in taxonomy. Paleobiology 21(4):401–403.

Labandeira, C. 1983. Paleobiology of the Dikelocephalidae (Trilobita, Upper Cambrian) and systematic revision of the genus Dikelocephalus (Owen), with special reference to changing species concepts in American paleontologic thought. Thesis. University of Wisconsin-Milwaukee, Milwaukee, Wisconsin.

Labandeira, C. C., and N. Hughes. 1994. Biometry of the Late Cambrian trilobite genus Dikelocephalus and its implications for trilobite systematics. Journal of Paleontology 68(3):492–517.

Mikulic, D. G., and J. Kluessendorf. 2001. Gilbert O. Raasch, student of Wisconsin's ancient past. Geoscience Wisconsin 18:75–93.

Nelson, C. A. 1949. Cambrian stratigraphy of the St. Croix Valley. Dissertation. University of Minnesota, Minneapolis.

Nelson, C. A. 1953. Revision of Croixan dikelocephalids—a comment. Journal of Paleontology 27(5):734–736.

Owen, D. D. 1852. Report of a geological survey of Wisconsin, Iowa, and Minnesota; and incidentally of a portion of Nebraska Territory. Lippincott, Grambo & Co., Philadelphia, Pennsylvania. Available at http://archive.org/details/mobot31753000174885 (plates not included), https://archive.org/details/reportofgeologi00owen (full plates) or http://books.google.com/books?id=Y_ZYAAAAYAAJ.

Raasch, G. O. 1951. Revision of Croixan dikelocephalids. Transactions of the Illinois Academy of Science 44:137–151.

Sundberg, F. A. 2007. Nightmare on Resser Street—dealing with Resser's trilobite taxonomy. Pages 213–224 in D. G. Mikulic, E. Landing, and J. Kluessendorf, editors. Fabulous fossils—300 years of worldwide research on trilobites. New York State Education Department, Albany, New York.

Ulrich, E. O., and C. E. Resser. 1930. The Cambrian of the Upper Mississippi Valley. Part I, Trilobita; Dikelocephalinae and Osceolinae. Bulletin of the Public Museum of the City of Milwaukee 12(1):1-122.

Walcott, C. D. 1914. Cambrian geology and paleontology II. No. 13. Dikelocephalus and other genera of the Dikelocephalinae. Smithsonian Miscellaneous Collections 57(13).

Winchell, N. H. 1888. The geology of Washington County. Pages 375–398 in N. H. Winchell and W. Upham. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 2. Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

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