Hi Gail, I have not collected any fossils in the West so I am not very familiar with fossils in that area. My best guess to get the discussion going is that it is a bone of some kind. (maybe a hoof, but I am very much guessing.) If I found a similar fossil in a marine sediment I might be tempted to call it a coral, but I think it may be a bone. Where in South Dakota did you find it. Combining the location with a geological map may help you narrow down what kind of environment it was formed in. A simple map is found at http://www.sdgs.usd.edu/geologyofsd/geosd.html
Also look at websites about the fossils of South Dakota. https://www3.northern.edu/natsource/earth/Fossil1.htm
If it is a bone it does not look like it has much matrix to remove. Hopefully someone more knowledgeable of fossils in your area will respond too.