 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 <unk> ) 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 <unk> 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 . 
