“…Sacabambaspis and Arandaspis have large singular dorsal and ventral headshields and a trunk region covered in elongate rod shaped scales (Ritchie & Gilbert-tomlinson, 1977; Gagnier, Blieck, & Rodrigos, 1986; Gagnier, 1995). Astraspis and Eriptychius are less completely preserved then Arandaspida taxa due to their headshields being composed of non-fused platelets or tesserae (Denison, 1967; Elliott, 1987; Sansom et al ., 1995; Sansom & Smith, 2005). Unlike the heterostracans, the Ordovician pteraspidimorphs appear to have more than one gill or branchial opening, but histologically are very similar to heterostracans, possessing a three-layered dermal skeleton comprising a superficial layer of dentine tubercles capped with enameloid, a middle vascular layer composed of tissue similar to aspidin and a compact lamellar basal layer (Denison 1967, Sansom, Smith, & Smith, 1997; Sansom, Donoghue, & Albanesi, 2005, Lemierre & Germain 2019).…”