Sunday, October 25, 2015

Minnesota's dinosaurs

The stats for Equatorial Minnesota have recently shown visitors looking for "Minnesota dinosaur", no doubt because of the recent announcement of a ~90 million-year-old theropod claw from Hill Annex Mine State Park on the Iron Range. Minnesota, as you might know, is not noted for its dinosaur fossils. (I usually have to add the modifier "non-avian" or "classic" to "dinosaur", but I don't know of any bird fossils, either, unless you want to count refuse bones from archeological sites. Maybe the St. Peter Sandstone holds a buried Pleistocene-age cave in a filled river channel somewhere...) However, there are a handful of reports of Cretaceous dinosaurs from Minnesota.

Minnesota actually has a decent amount of Cretaceous rocks, particularly in the western half of the state, but the Iron Range also has Cretaceous rocks, and small outliers are present in the southeast (Sloan 1964, 2005; Mossler 1983). These rocks represent deposition on the eastern margin of the great Western Interior Seaway, which you may be familiar with as the home of Pteranodon, Elasmosaurus, giant sea turtle Archelon, the toothed seabirds Hesperornis (flightless) and Ichthyornis, and various mosasaurs, ammonites, and clams. There is a mix of marine and nonmarine rocks. The problem we have in this state is we can't get at most of the rocks, due to an inconveniently thick blanket of glacial drift. Outcrops are largely confined to the Minnesota River Valley, with a few areas peeking out in the mining districts up north and in areas southeast of the Twin Cities (Morey 1982). Because of the inadequate exposures, it's been difficult to make correlations or name the rocks. In the north and northeast, there is a marine formation known as the Coleraine Formation, and in the southeast there is a nonmarine formation known as the Windrow Formation, but otherwise we have to make due with informal units. In southwestern Minnesota, the rocks are mostly known by bed number, and are thought to correlate to most of the Western Interior Seaway cycles from the Dakota Sandstone to the Pierre Shale (see for example Setterholm 1990).

Minnesota's Cretaceous rocks have produced a variety of microfossils (forams and pollen, particularly), leaves, bivalves, snails, ammonites, sharks, and even a few non-dinosaurian reptiles (the crocodylomorph Teleorhinus mesabiensis, now considered a synonym of Terminonaris robusta, was named from Minnesota), but precious few dinosaurs. A few reports of Minnesota dinosaurs have escaped into the wild. A recent Minneapolis Star Tribune article (Sawyer 2015) cited three: a vertebra, a serrated tooth, and a claw. In order:

  • What appears to be the first dinosaur fossil recovered from Minnesota is a centrum (the thick round part of a vertebra) of a dinosaurian tail vertebra, found in Crow Wing County (Erickson 2003). The specimen is reposited at the Science Museum of Minnesota as SMM P74.28.1, indicating it showed up in 1974, but beyond that there is not much more that can be gleaned. Erickson (2003) reported it as being between 70 and 65 million years old.
  • I suspect that the serrated tooth is one of the "dinosaurian elements" mentioned in Hanks et al. (2011), from either the Carlile or Coleraine formations. The abstract doesn't make the localities explicit, but I recall seeing the poster and I think it was mentioned there. The "serration" description indicates a carnivorous dinosaur.
  • The claw, of course, is the newest member of the dinosaurian assemblage. It comes from a carnivorous dinosaur, is largely complete, and is of modest size (the MPR article has a photo).

There are also a couple of reports that require further documentation. A post on the Dinosaur Mailing List suggests that Robert Sloan had found dinosaur vertebrae in Cretaceous rocks at Springfield, Minnesota, but Sloan (1964, 2005) discussed the Springfield finds at some length and did not mention any dinosaur vertebrae. He did report shark vertebrae, but the closest thing to dinosaurs was a croc. Witzke (2001) alluded to hadrosaur bones from the Dakota Formation in Minnesota. This would be of some interest beyond just local pride because of the general rarity of dinosaur bones in the Dakota. (I assume, based on age, that any "hadrosaur" in the Dakota would much more likely be a non-hadrosaurid hadrosauroid iguanodont, which might be small beer to you, but you never want to get these confused under the wrong circumstances.)

Although Minnesota does not have much for dinosaurs to date, the state certainly has the rocks for better material. The main issues are the lack of exposure and the preponderance of marine rocks. Given what we know of dinosaurs found in the Western Interior Seaway, the best bet for a nice specimen in Minnesota, as opposed to the current crop of leftovers, is a "bloat-and-float" armored dinosaur or ornithopod.

References:

Erickson, B. R. 2003. Dinosaurs of the Science Museum of Minnesota. Science Museum of Minnesota, St. Paul, Minnesota.

Hanks, H. D., B. R. Erickson, and S. A. Haire. 2011. Vertebrate remains from the Late Cretaceous of South Dakota and Minnesota. Abstracts with Programs—Geological Society of America 43(5):264.

Morey, G. B. 1982. Geologic map of Minnesota, bedrock outcrops. Minnesota Geological Survey, St. Paul, Minnesota. State Map 10. Scale 1:3,168,000.

Mossler, J. H. 1983. Bedrock topography and isopachs of Cretaceous and Quaternary strata, east-central and southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Map 52. Scale 1:500,000.

Sawyer, L. 2015-10-8. Fossil adds to evidence of dinosaurs in Minnesota. Minneapolis Star Tribune.

Setterholm, D. R. 1990. Geologic maps of the Late Cretaceous rocks, southwestern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Map 69. Scale 1:750,000.

Shurr, G. W., J. P. Gilbertson, R. H. Hammond, D. R. Setterholm, and P. M. Whelan. 1987. Cretaceous rocks on the eastern margin of the Western Interior Seaway: a field guide for western Minnesota and eastern South Dakota. Pages 47–84 in N. H. Balaban, editor. Field trip guidebook for Quaternary and Cretaceous geology of west-central Minnesota and adjoining South Dakota. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook 16.

Sloan, R. E. 1964. The Cretaceous System in Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 5.

Sloan, R. E. 2005. Minnesota fossils and fossiliferous rocks. Privately published, Winona, Minnesota. Available from the Minnesota Geological Survey.

Witzke, B. J. 2001. The Age of Dinosaurs in Iowa. Iowa Geology 26:2–7.

2 comments:

  1. From a June 6, 1997 Minnesota Daily story about Sloan's impending retirement:

    - Sloan also uncovered the only dinosaur fossil ever found in Minnesota, a bone from a duck-billed dinosaur.
    "It was a crummy dinosaur tail bone, the least important fossil I ever found," Sloan said. "It came from the Oak Clay Pit in Springfield, Minnesota, which is where all the brick on the Minneapolis campus comes from."

    http://www.mndaily.com/1997/06/06/dinosaur-professor-ends-long-career-u

    Unless Sloan had a memory lapse or was misquoted, it would mean two dinosaur tail verts have been found in Minnesota.



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    1. Thanks for the link! The thing I wonder about is why he didn't include it in his 2005 volume. A couple of scenarios leap to mind: he changed his mind about the identity of the specimen between then and 2005; or he forgot to add it. A lot of the section relating to Springfield was reworked from his 1964 publication, and reusing canned text is an easy way to miss updating something.

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