The poorly understood osteostracan genus Auchenaspis, from the Upper Silurian of England, belongs to the thyestidians (Thyestiida), a controversial and important group that is key to the understanding of the evolution of the Osteostraci and paired pectoral fins. The two under-studied species of this genus are comprehensively reviewed using anatomical and novel statistical approaches. No reliable anatomical characters can be used to distinguish between the two species, yet discriminant analysis finds significant separation with respect to size. Potential causes of this conflict are discussed. The findings enable Auchenaspis to be considered in terms of wider thyestidian evolution and contradict previous interpretations of the genus as representing a paraphyletic taxon, whose morphology is intermediate between the general osteostracan condition and the highly derived tremataspids.