It is known from a single incomplete specimen discovered in the late 19th century in Lower Jurassic rocks of south-central Massachusetts, United States.

It was originally thought to be a species of Stegomus, an aetosaur (a type of armored herbivorous reptile), but was eventually shown to be related to Protosuchus and thus closer to the ancestry of crocodilians.

[5][6] While thought to be Triassic when Stegomosuchus was originally described,[2] the Portland Formation is now known to date to the Early Jurassic, including the Hettangian and Sinemurian stages[7] (approximately 200 to 190 million years ago).

[11] Its classification was further reassessed by Alick Walker over forty-five years later, who reinterpreted Stegomosuchus as a close relative of Protosuchus, a crocodile-like reptile of similar age.

[13] Whetstone and Whybrow (1983) agreed that a protosuchian identity was probable, but found AM 900 too poorly preserved and lacking too many important parts of the body to classify further.