The Heterostraci are a subclass of armored jawless vertebrates that were widespread in marginal marine environments around the Old Red Sandstone continent during the late Silurian to Middle Devonian. Although a number of clades have long been recognized. further analysis has been limited by lack of morphological information beyond that afforded by the armor. thus impeding understandin... https://www.dunebarcelona.com/product-category/service-manual/