Aetosaurus was first named, with the description of the type species A. ferratus, in 1877 by German paleontologist Oscar Fraas.
At the time, Aetosaurus was known from 22 articulated skeletons that had been found in the Lower Stubensandtein of Germany.
Thirty years later, Fraas' son Eberhard described a second species, A. crassicauda, also from Germany.
A. crassicauda can be distinguished from A. ferratus by its larger size; A. crassicauda reached a maximum length of 150 centimetres (59 in) while A. ferratus reached a length of up to 90 centimetres (35 in).

In addition to the Stubensandtein in Germany, A. ferratus is also known from the Calcare di Zorzino Formation in Cene, Italy.
Specimens of Aetosaurus that have been recovered from the Fleming Fjord Formation in Greenland likely represent A. ferratus.
Some material from the Chinle Group in the southwestern United States probably represent A. ferratus, as well.

In 1998, the genus Stegomus was synonymized with Aetosaurus.
In 1896, paleontologist Othniel Charles Marsh named Stegomus arcuatus from a cast of an aetosaur known as YPM 1647 from the New Haven Formation in Fairfield, Connecticut.
This cast consisted of the dorsal carapace.
Several other casts preserving the surface of the skull and tail have been found in the Passaic Formation of Hunterdon and Somerset counties, New Jersey and an outcrop of the Lower Sanford Formation at  the Triangle Brick Co. Quarry in Durham County, North Carolina.
Stegomus arcuatus was found to be synonymous with Aetosaurus on the basis of several similarities, including a lack of spikes and a distinctive radial pattern of grooves on some of the caudal scutes.

A. arcuatus has paramedian scutes that are much wider than they are long, even in comparison to other species of Aetosaurus.
There is very little pitting on the surface of the scutes, although the porosity of the sandstone that makes up the casts has been mistaken for pitting.
The tail narrows significantly past the base.
The carapace is "waisted", meaning that it narrows in front of the pelvis.

The following cladogram simplified after an analysis presented by Julia B. Desojo, Martin D. Ezcurra and Edio E. Kischlat (2012).
Aetosaurus was first named, with the description of the type species A. ferratus, in 1877 by German paleontologist Oscar Fraas. At the time, Aetosaurus was known from 22 articulated skeletons that had been found in the Lower Stubensandtein of Germany. Thirty years later, Fraas' son Eberhard described a second species, A. crassicauda, also from Germany. A. crassicauda can be distinguished from A. ferratus by its larger size; A. crassicauda reached a maximum length of 150 centimetres (59 in) while A. ferratus reached a length of up to 90 centimetres (35 in).
In addition to the Stubensandtein in Germany, A. ferratus is also known from the Calcare di Zorzino Formation in Cene, Italy. Specimens of Aetosaurus that have been recovered from the Fleming Fjord Formation in Greenland likely represent A. ferratus. Some material from the Chinle Group in the southwestern United States probably represent A. ferratus, as well.
In 1998, the genus Stegomus was synonymized with Aetosaurus. In 1896, paleontologist Othniel Charles Marsh named Stegomus arcuatus from a cast of an aetosaur known as YPM 1647 from the New Haven Formation in Fairfield, Connecticut. This cast consisted of the dorsal carapace. Several other casts preserving the surface of the skull and tail have been found in the Passaic Formation of Hunterdon and Somerset counties, New Jersey and an outcrop of the Lower Sanford Formation at  the Triangle Brick Co. Quarry in Durham County, North Carolina. Stegomus arcuatus was found to be synonymous with Aetosaurus on the basis of several similarities, including a lack of spikes and a distinctive radial pattern of grooves on some of the caudal scutes.
A. arcuatus has paramedian scutes that are much wider than they are long, even in comparison to other species of Aetosaurus. There is very little pitting on the surface of the scutes, although the porosity of the sandstone that makes up the casts has been mistaken for pitting. The tail narrows significantly past the base. The carapace is "waisted", meaning that it narrows in front of the pelvis.
The following cladogram simplified after an analysis presented by Julia B. Desojo, Martin D. Ezcurra and Edio E. Kischlat (2012).
Aetosaurus