their upper surfaces and smooth on their undersides. They have a heterogenous internal structure: the inner portion of each osteoderm is 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 a keratinous (horn) covering, like modern crocodilian scutes, which are another example of pseudosuchian osteoderms. Osteoderms are useful in diagnosing aetosaur taxa, and aetosaur species can often be identified from individual scutes based on their shape, structure, or ornamentation pattern. Aetosaurs have four rows of osteoderms running along their dorsal (back) side, forming a continuous plate often called the carapace. The inner two rows, which flank the midline of the spinal column, are known as paramedian osteoderms. These tend to be wider than long and strongly ornamented with radiating pits or grooves. Nearly all aetosaurs possess a small boss or raised surface, known as a dorsal eminence, on the upper surface of each plate. The dorsal eminence is often set posteriorly (backwards) or medially (inwards) on their respective paramedian osteoderm, though there are many exceptions within the group. The paramedian osteoderms almost always have raised or depressed anterior edges, where the plates are overlapped by 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. Although two paramedian osteoderm rows are common in early archosauriforms, few reptiles approach aetosaurs in the complexity of their osteoderms. The osteoderms of doswelliids, erpetosuchids, and certain crocodylomorphs are occasionally confused or compared with those of aetosaurs. Both an anterior bar and dorsal eminence occur in Acaenasuchus, a close relative of aetosaurs originally misidentified as a juvenile desmatosuchine. The outer two rows of osteoderms, which lie beside the paramedians, are known as lateral osteoderms. They parallel the paramedians over nearly the entire backside, though the first two paramedians behind the head (known as nuchal osteoderms) are solitary. Lateral plates generally are separated into two surfaces, or flanges, flexed between their dorsal eminence. The upper, or dorsal flange lies in the same plane as the paramedian osteoderms. The lower/outer, or lateral flange wraps down onto the side of the body. The dorsal eminence between these flanges often has the form of a low blade, knob, or spike. In the cervical lateral osteoderms, which are positioned on the