Saturday, July 4, 2015

The noble ostracode

One of the most diverse fossil groups in the Paleozoic rocks of Minnesota is also one of the least familiar to the layperson. This group is the Ostracoda, or "seed shrimp", a class of crustaceans that is still thriving. They owe their lack of familiarity to their diminutive size; they aren't called seed shrimp for nothing. Although found practically anywhere you can rub a couple of water molecules together, they are often more or less invisible to humans because most of them are on the order of 1 mm long. They owe their abundance in the fossil record to their durable shells, being equipped with a pair of valves not unlike a clam. Inside the valves is the tiny crustacean itself, with several pairs of appendages and distinct internal organs. With around 8,000 living species, there's a lot of diversity to go around. Ostracodes (also spelled ostracods) inhabit marine, freshwater, and moist terrestrial settings, are free-swimming or bottom dwellers, and include predators, herbivores, and detritivores. Definite ostracodes appeared during the Ordovician; there are some suspiciously ostracode-like creatures in the Cambrian, such as bradoriids, but they are not currently considered ostracodes. Fossil ostracodes are of particular interest for paleoenvironmental studies (depth, temperature, salinity, etc.) and biostratigraphy. There is one slight drawback to appreciating them as fossils, though: their size. Without a good microscope and the equipment to extract ostracodes, their charms will necessarily be vicarious for most people. Nothing against microfossils (I personally have a fondness for forams, because one of my first projects involved them), but those are just the practical breaks. There is, however, one exception in the rocks of the Twin Cities, a giant among ostracodes, the form commonly known as Eoleperditia fabulites or Leperditia fabulites. This species tops out around a centimeter long and resembles a bean. It can be both common and recognizable in the Platteville Formation.

This is the fabled giant ostracode Eoleperditia (or Leperditia) fabulites, from the University of Minnesota collections.

And here are examples in the wild, in the Platteville Formation (probably Mifflin Member).

Stauffer and Thiel (1941) have their usual list, featuring several dozen species, but it probably won't do much for you unless you have the requisite facilities or are just really good at spotting sesame-seed-sized fossils. Also, there are many publications specifically dedicated to the ostracodes of the Platteville and Decorah, and they're going to be more useful than an uncited faunal list. If you want the true Minnesota Ordovician ostracode experience, a selection of citations includes Ulrich (1890, 1892, 1897), Kay (1934, 1940), Hansen (1951), Cornell (1956), Swain et al. (1961), Swain (1987), Swain and Cornell (1987), and Johnson et al. (1991).

References cited:

Cornell, J. R. 1956. The Ostracoda zones of the Decorah Shale. Thesis. University of Minnesota, Minneapolis, Minnesota.

Hansen, D. L. 1951. Distribution of Ostracoda in the Decorah Shale Formation at St. Paul, Minnesota. Thesis. University of Minnesota, Minneapolis, Minnesota.

Johnson, J. D., L. V. Benolkin, and F. M. Swain. 1991. Ostracoda from the Glenwood Shale (Ordovician-middle Caradocian) of Minnesota. Revista Espanola de Micropaleontologia 23(2):141–152.

Kay, G. M. 1934. Mohawkian Ostracoda: species common to Trenton faunules from the Hull and Decorah Formations. Journal of Paleontology 8(3):328-343.

Kay, G. M. 1940. Ordovician Mohawkian Ostracoda: lower Trenton Decorah fauna. Journal of Paleontology 14(3):234-269.

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

Swain, F. M. 1987. Middle and Upper Ordovician Ostracoda of Minnesota and Iowa. Pages 99–101 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.

Swain, F. M., and J. R. Cornell. 1987. Ostracoda of the superfamilies Drepanellacea, Hollinacea, Leperditellacea, and Healdiacea from the Decorah Shale of Minnesota. Pages 102–130 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.

Swain, F. M., J. R. Cornell, and D. L. Hansen. 1961. Ostracoda of the families Aparchitidae, Aechminidae, Leperditellidae, Drepanellidae, Eurychilinidae and Punctaparchitidae from the Decorah Shale of Minnesota. Journal of Paleontology 35(2):345–372.

Ulrich, E. O. 1890. New and little known American Paleozoic Ostracoda. Journal of the Cincinnati Society of Natural History 13(3):104–137.

Ulrich, E. O. 1892. New Lower Silurian Ostracoda, no. 1. The American Geologist 10(5):263–270.

Ulrich, E. O. 1897. The Lower Silurian Ostracoda of Minnesota. Pages 629–693 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.

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