by examining their osteoderms. Some isolated osteoderms have been claimed to belong to juvenile aetosaurs based on their size and shape but these hypotheses have often been questioned. For example, juvenile osteoderms of Calyptosuchus were later identified as those of the small-bodied pseudosuchian Revueltosaurus (which is not an aetosaur), and juvenile osteoderms of Desmatosuchus have been reinterpreted as those of aetosaur Acaenasuchus, which had a relatively small body size at maturity. Studies of the bone structure of paramedian osteoderms indicate that new bone was deposited along the edges of each plate over the course of an aetosaur's lifetime. This means that lines of arrested growth on the undersides of paramedian osteoderms can be used to determine an individual's age. Comparing the ages of individual specimens with their total body lengths indicates that aetosaurs increased in length at relatively constant rates, but increased in body mass at different rates depending on whether they had wide bodies like Typothorax or narrow bodies like Aetobarbakinoides. Aetosaurs also seem to have grown more slowly than modern crocodilians. Analysis of the limb bones of aetosaurs indicates that they grew quickly when young and more slowly when adults. This pattern of growth is seen in most other pseudosuchians. Biochronology Because species of aetosaurs typically have restricted fossil ranges and are abundant in the strata they are found in, they are useful in biochronology. Osteoderms are the most common remains associated with aetosaurs, so a single identifiable scute can accurately date the layer it is found in. One aetosaur, Typothorax coccinarum, has been used to define the Revueltian land vertebrate faunachron. A land vertebrate faunachron (LVF) is a time interval that is defined by the first appearance datum (FAD), or first occurrence, of a tetrapod index fossil and is commonly used to date Late Triassic and Early Jurassic terrestrial strata. Since the FAD of T. coccinarum is at the beginning of the Norian stage, the Revueltian LVF starts at the beginning of the Norian around 216 million years ago. The Revueltian ends with the next FAD, which happens to be that of the phytosaur Redondasaurus and the start of the Apachean LVF. Biochrons for aetosaur genera have been developed for dating strata in the Chinle Group of the southwestern United States. Up to 13 genera of aetosaurs are known from the Chinle Group, with most occurring in multiple localities and over short time spans. In 1996, paleontologists