What is "pachyosteus"

Pachyosteus bulla is a medium-sized selenosteid arthrodire placoderm known from the Upper Frasnian Kellwasserkalk facies of Late Devonian Bad Wildungen, Germany and from the Famennian portions of the Holy Cross Mountains of Poland. P. bulla has a broad skull about 7 to 10 centimeters long, a comparatively long median dorsal plate, and a short rostral plate that meets the pineal plate. In his cladogram, Rücklin (2011) regards P. bulla as a basal selenosteid, being the sister taxon of the American genera, and the Kellwasserkalk genera of Germany and Morocco.