Fossils have been found from the Dockum Group and lower Chinle Formation outcropping in Texas, New Mexico, and Arizona, United States, and date back to the Carnian stage of the Late Triassic.

It was not until the publication of a 1995 paper on tetrapods of southwestern United States that these species were recognized as belonging to the genus Leptosuchus.

However, because of the distinctive size of the rostral crest in L. gregorii, it was assigned to its own genus, Smilosuchus, in that same paper.

[2] Despite this, L. gregorii has recently been seen as belonging to Leptosuchus, as it is believed that the large, complete crest was independently developed in this particular species.

Long and Murry (1995) considered it to be a nomen nudum, as there was never full formal documentation of this specimen.