Saturday, October 31, 2015

The "fossils" of Pipestone National Monument

This is being posted on Halloween, which is fitting for a topic that lingers like a ghost in the literature. Even today, you can still find stray references to the fossils of Pipestone National Monument. "Occasional small trilobites - Lingula, Paradoxides" sounds promising, doesn't it? If you're reasonably familiar with the fossils of Minnesota, you may be wondering why you never heard of this before. There's a pretty good reason: these forgotten fossils are not trilobites or brachiopods, and odds are they are not fossils at all.

Pipestone National Monument exists because of the presence of a red metamorphosed mudstone known as catlinite, prized by many Native American groups as a material for making pipes, hence "pipestone". Catlinite is part of the Sioux Quartzite, which is one of Minnesota's Proterozoic rock formations. "Proterozoic" means "it's pretty darn old, but not super-old", occupying the time from about 2,500 to 541 million years ago. Jirsa et al. (2013) put the age of the Sioux Quartzite as roughly 1,730 to 1,630 million years old, in the late Paleoproterozoic. While this is older than a lot of rocks, it's worth remembering that something like at least 2,800 million years had already elapsed on Earth, putting us somewhere on Side 3 of the White Album. The late Paleoproterozoic *is* older than complex animals with hard body parts, and brachiopods and trilobites certainly fit those qualifications, so you can see how this becomes interesting.

The waterfall at Pipestone National Monument; photo supplied by Jerome Tweet.

While visiting Pipestone in October 1884 to obtain specimens for the New Orleans World Cotton Centennial Exposition, Newton Horace Winchell (as documented in Winchell 1885) noticed what appeared to be shells in catlinite: rounded objects about 6 mm in diameter and 0.5 mm thick, with white "scale" that was found to include calcium carbonate and traces of calcium phosphate. He interpreted these as the fossils of thin-shelled inarticulate brachiopods, which he named Lingula calumet based on a number of examples on a slab (UMPC 5559). A Mr. A. W. Barber of Yankton found an additional object which Winchell interpreted as the middle part of a trilobite, and named Paradoxides barberi (holotype UMPC 5555). This specimen was found loose, and had been weathered and apparently burned. At the time, there of course had been no numeric estimation of the age of the Sioux Quartzite, but it was considered Precambrian ("Huronian", to be precise), and Winchell was clearly anticipating controversy, because he reprinted letters supportive of his conclusions, provided by S. W. Ford and James Dwight Dana. And then...

These are Winchell (1885)'s figures of Lingula calumet (Fig. 6) and Paradoxides barberi (Fig. 7, apparently natural size in the original).

Nothing much happened. Go ahead, spend a little time running "lingula calumet" and "paradoxides barberi" through your search engine of choice and see what you come up with. Pay close attention to the dates. Winchell's two Pipestone species more or less disappeared from the literature after about 1899. Few people seem to have been much impressed with Paradoxides barberi. To put it bluntly, "Specimen #5555 looks no more like a trilobite than Winchell's illustration of it" (Darby 1972). The author of that statement interpreted it as probably a current feature or load structure. Lingula calumet  has been treated somewhat more kindly, in that some late 19th century researchers besides Winchell thought it might be a brachiopod, but again it too has fallen by the wayside. Darby (1972), apparently the only researcher to spend much time considering these specimens since the turn of the 20th century, raised the possibility that Lingula calumet represents compressed mud clasts formed by wave or current action, or some kind of unknown organic feature. The Sioux Quartzite of the Pipestone area is interpreted as primarily fluvial in origin, deposited by braided streams flowing south to south-southeast (Morey 1984). (Note that rivers in the Proterozoic were not quite like modern rivers, because without vegetation to anchor banks, river courses were very unstable.)

A photo of UMPC 5559 from Darby (1972). Mud clots? Flattened concretions? Poorly understood Precambrian lifeforms?

So, is it possible that Lingula calumet represents something organic? In the 19th century, there was little known about the diversity of Precambrian life. Now, of course, we know of all sorts of odd and exotic lifeforms that had their day before about 541 million years ago, but in 1885 Winchell was winging it, and not surprisingly he interpreted the objects in terms of fossils he knew. After all, the concept of "stromatolites", A.K.A. "Cryptozoon", had just entered the marketplace of ideas (and were still on pretty shaky ground). Coincidentally, the original description of Chuaria circularis, one of the first macroscopic Precambrian fossils, is not unlike that of Lingula calumet: disc-like objects about 2 to 5 mm across, composed of what seemed to be a very thin shell covered with a dark material that when removed revealed the apparent shell to have a phosphatic appearance (Walcott 1899). Another interesting fossil for our purposes is Horodyskia, the "string of beads" fossil. Horodyskia is known from rocks which are somewhat younger than the Sioux Quartzite, about 1,500 million years old, but is reasonably comparable to Lingula calumet in scale and gross morphology (compare the image above with the figures in the linked article). Could Lingula calumet be something similar to either of these, or another type of (non-brachiopod) fossil? I honestly don't know. What I know about Precambrian paleontology is rather generously outweighed by what I don't know, and the descriptions and figures in the literature aren't much to go on. It couldn't hurt to look, with the benefit of four decades of research on Precambrian fossils since Darby (1972). We can say, though, with regard to brachiopods and trilobites at Pipestone, it ain't so.

References:

Darby, D. G. 1972. Evidences of Precambrian life in Minnesota. Pages 264–271 in P. K. Sims and G. B. Morey, editors. Geology of Minnesota: a centennial volume. Minnesota Geological Survey, St. Paul, Minnesota.

Jirsa, M. A., T. J. Boerboom, and V. W. Chandler. 2013. Geologic map of Minnesota, Precambrian bedrock geology. Minnesota Geological Survey, St. Paul, Minnesota. State Map Series 22. Scale 1:500,000.

Morey, G. B. 1984. Sedimentology of the Sioux Quartzite in the Fulda basin, Pipestone County, southwestern Minnesota. Pages 59–74 in D. L. Southwick, editor. Shorter contributions to the geology of the Sioux Quartzite (Early Proterozoic), southwestern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 32.

Walcott, C. D. 1899. Pre-Cambrian fossiliferous formations. Geological Society of America Bulletin 10:199–244.

Winchell, N. H. 1885. Fossils from the Red Quartzyte at Pipestone. Minnesota Geological and Natural History Survey Annual Report 13, for the Year 1884:65–72.

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