 Aetosaurs were very heavily armored (most certainly as a defense against predators), with large quadrangular, interlocking bony plates, or osteoderms, protecting the back and sides, belly, and tail. Most osteoderms are heavily pitted on their upper surfaces and smooth on their undersides. Their centers are made of cancellous or spongy bone (also called diploë) and their outer portions are made up of compact bone. In life, these plates were probably covered in horn. Dorsal osteoderms, which are found on the backs of aetosaurs, are often ornamented with radial grooves. Dorsal paramedians, those found along the midline of the animal, are often wide and quadrangular with a small boss called a dorsal eminence on the dorsal surface of each plate. In aetosaurs, paramedian plates often have raised or depressed anterior edges where the plates articulate with the ones in front of them. If the anterior edge is raised, the area is called an anterior bar, while if it is depressed, the area is called an anterior lamina. In lateral plates, which are positioned on either side of the paramedian plates, the dorsal eminence is often enlarged into a prominent spike. This spike is especially noticeable in desmatosuchines such as Longosuchus and Desmatosuchus. Osteoderms are useful in diagnosing aetosaur taxa, and aetosaur species can often be identified from individual scutes based on their ornamentation pattern.
