The USNM 4753 holotype was found in the layers of the Lakota Formation, which dates back to the Barrem . It consists of parts of the pelvis, two iliac bones and sacral vertebrae. The length of the ilium is estimated to be 45 centimeters, indicating a total body length of Osmakasaurus depressus from 4 to 5 meters.
In 1896, geologist Nelson Horatio Darton at the Buffalo Gap, near Hot Springs in South Dakota, came across several Euornithopoda bones. The remains were described in 1909 by Charles Whitney Gilmore as a new species of Camptosaurus depressus . In 2008, Kenneth Carpenter and Yvonne Wilson decided that this species belongs to the genus Planicoxa and was named Planicoxa depressa .
In 2011, Andrew MacDonald published a study on the relationships of all forms, which at different times were attributed to Camptosaurus . He concluded that Planicoxa depressa did not actually have a kinship relationship with the species Planicoxa venenica , and in this regard identified a new genus, Osmakasaurus depressus . The name of the genus comes from the language of the Lakota tribe “osmaka” - a ravine, referring to Gap Buffalo - buffalo canyon. MacDonald classified the new genus as a member of the Styracosterna clade [3] .