 Primitive genera , like the widespread Norian genus Aetosaurus and the Carnian Coahomasuchus , tended to be small , about a metre ( 3 @.@ 2 ft ) in length . However , more advanced forms were larger - about 3 metres ( 9 @.@ 8 ft ) in length - with some taxa , such as Typothorax and Paratypothorax , possessing broad turtle @-@ like bodies , and others , like Desmatosuchus , a narrow @-@ bodied genus at least 4 metres ( 13 ft ) long , equipped with large spines over the shoulders , which added to the animal 's defensive armament . 
