Platteville follies: a crushed giant rodent from Hidden Falls

I'd like to start off with a couple of photographs, to illustrate some facts of life concerning our old friend, the Platteville Formation:

An ever-so-slightly hazardous ledge along the road into Crosby Farm Regional Park.

Hidden Falls Regional Park: when good rocks go bad.

The Platteville Formation just loves to ledge out, particularly the lower part, which rests on the recessive Glenwood Formation, and once you get heavy rocks ledged out far enough... About ten thousand years ago, not very far from the second photo, there was a ledge of the Platteville Formation left jutting out in the figurative "wake" of the erosion of the Mississippi River system. At some point, a dog-sized rodent opted to go underneath it. Maybe this was its usual shelter, or maybe it was driven there by some circumstance; we'll never know, short of time travel. Unfortunately for the rodent, the slab failed, with predictable results. It would be covered by the debris until July of 1938, when a Works Progress Administration crew uncovered the site while widening what is now the north entrance road into Hidden Falls Park. Its skeleton, crushed and coated with calcite leached out of the slab, was recovered and eventually prepared, reconstructed, put on display at the Science Museum of Minnesota, and described scientifically as an example of the extinct mammal known as Castoroides ohioensis, a.k.a. the giant beaver.

From Powell (1948): the WPA project that led to the discovery (site with the superimposed white circled cross).

"Well," you say, "that was refreshingly concise." Ah, but there's always more to say.

The Hidden Falls giant beaver is one of a handful of examples of this extinct rodent found in Minnesota. Minnesota in general is not noted for its extensive Pleistocene fauna, what with all of the glacial events, but it does have its mammoths and mastodons and a few other "so forths". Surveys appear in Hay (1924), Stauffer (1945), and Swanson (1945), all of which predate the Korean War, so an update might not be a bad idea, but I digress. I know of three possible examples of Castoroides ohioensis from the state. The first consists of a partial lower jaw found during excavation for a cistern at the corner of Washington Avenue and 16th Avenue North in Minneapolis, back in 1879. It was found 8 ft (almost 2.5 m) down, along with freshwater clam shells (Winchell 1880), and currently answers (as much as it can) to University of Minnesota No. 3278 (Erickson 1962). The second specimen is the current topic of discussion. The third is unconfirmed, and consists of part of an incisor found while digging for a sump pit near Wells in Freeborn County (Erickson 1962). The Hidden Falls specimen is of particular interest because it includes much of the skeleton.

From Winchell (1880): the Minneapolis jaw.

If you want a great deal of detail on what was found with the specimen, Erickson (1962) is your friend. Unsurprisingly, crushing a critter can lead to lost pieces. The skull is missing a fair amount of material, and most of the vertebrae beyond the tail are lost or crushed. The ribs are fragmentary and the shoulder and pelvic bones are represented by small bits. The limbs are fairly complete, and help to illustrate the hands and feet. With other specimens for reference, a reconstructed skeleton was prepared and is now on exhibit at the Science Museum of Minnesota, on the lobby level near the mammoth bones (the catalog number is SMM 62-2001).

SMM 62-2001

The Hidden Falls Castoroides is a young adult, not fully grown (the Minneapolis specimen is somewhat larger). Incidentally, this species is sometimes described as "black bear-sized", but this is an overestimate; a more realistic range is 130 to 220 lb (60 to 100 kg) (Reynolds 2002), which you will probably agree is still worthy of calling it "giant". Powell (1948), the first description of the Hidden Falls specimen, devoted a great deal of space to discussing the possible lifestyle. He interpreted the species as more like a giant "swamp rat" in habits, with a proportionally longer and less broad tail compared to the modern decidedly non-giant beaver, and incisors with more rounded tips. Their teeth did not form a straight, flat chisel-like surface, but a more v-shaped gouge, probably better for clamping and tearing vegetation than cutting down trees.

Our Hidden Falls friend had some company in the afterlife. Bones of snakes, bats, rabbits, shrews, and modern-type beavers were found scattered in the talus. These probably do not represent animals that were smashed with it, but animals that lived later on and made homes in the open spaces of the debris. Shells of freshwater snails and clams may belong to mollusks that lived there when the river was cutting through, or from prey items brought to the pile to eat (Powell 1948). The bones of SMM 62-2001 have been dated to 10,320 ± 250 radiocarbon years before present (12,685 to 11,310 calendar years before present), essentially the very end of the Pleistocene, which provides a constraint on when the Mississippi River eroded through the area (Erickson 1967).

One other note: back in 1988, Castoroides ohioensis was proposed as the state fossil of Minnesota. This was never passed, despite what some sources would suggest. In hindsight, it's probably a good thing that the state fossil of Minnesota is not a giant beaver named for Ohio.

References:

Hay, O. P. 1924. The Pleistocene of the middle region of North America and its vertebrated animals. Carnegie Institute of Washington Publication 322A.

Erickson, B. R. 1962. A description of Castoroides ohioensis from Minnesota. Proceedings of the Minnesota Academy of Science 30(1):6–13.

Erickson, B. R. 1967. Paleontological evidence concerning some post glacial features of the Mississippi River valley. Scientific Publications of the Science Museum, New Series 1(2).

Powell, L. H. 1948. The giant beaver Castoroides in Minnesota. Science Bulletin [Science Museum of Minnesota] 2.

Reynolds, P. S. 2002. How big is a giant? The importance of methods in estimating body size of extinct mammals. Journal of Mammalogy 83(2):321–332.

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

Swanson, G. 1945. Minnesota’s fossil mammals. Minnesota Conservation Volunteer 8(45):22–25.

Winchell, N. H. 1880. Castoroides ohioensis, Foster. Minnesota Geological and Natural History Survey Annual Report 8:181–183.