The first species recognized from outside the Niobrara chalk, "Hesperornis altus", lived about 78 million years ago in Montana, and is known from a partial lower leg from the base of the freshwater Judith River Formation (or, possibly, the top of the underlying, marine Claggett Shale formation). While initially placed in the new genus "Coniornis" by Marsh, this was due mostly to his belief that "Hesperornis" existed only in Kansas, so any species from Montana should be placed in a different genus. Most later researchers disagreed with this, and have placed "Coniornis altus" in the same genus as "Hesperornis" as "H. altus". A second species from Montana has also been described from the Claggett Shale. "H. montana" was named by Shufeldt in 1915, and while its known material (a single dorsal vertebra) cannot be directly compared to "H. altus", Shufeldt and others have considered it distinct due to its apparently smaller size.