"Hesperornis" was primarily marine, and lived in the waters of such contemporary shallow shelf seas as the Western Interior Seaway, the Turgai Strait, and the North Sea, which then were subtropical to tropical waters, much warmer than today. However, some of the youngest known specimens of "Hesperornis" have been found in inland freshwater deposits of the Foremost Formation, suggesting that some species of "Hesperornis" may have eventually moved, at least partially, away from a primarily marine habitat. Additionally, the species "H. altus" comes from the freshwater deposits at the base of the Judith River Formation.