 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 Spencer G. Lucas and Andrew B. Heckert recognized five biochrons based on the presence of aetosaurs throughout the Chinle Group . The number of biochrons grew to 11 in a 2007 study by Heckert and Lucas along with Adrian P. Hunt and Justin A. Spielmann . These biochrons occurred from the Otischalkian LVF to the Apachean LVF and included genera such as Longosuchus , Tecovasuchus , and Typothorax . 
