Pachyosteus is an extinct monospecific genus of 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.[1] The type species Pachyosteus bulla has a broad skull about 7 to 10 centimetres (3 to 4 in) long, a comparatively long median dorsal plate, and a short rostral plate that meets the pineal plate.
Pachyosteus is a member of the family Selenosteidae of the clade Aspinothoracidi, which belongs to the clade Pachyosteomorphi, one of the two major clades within Eubrachythoraci. The cladogram below shows the phylogeny of Pachyosteus:[2]
Coccosteomorphi
Rhachiosteus pterygiatus
Eastmanosteus calliaspis
Eastmanosteus pustulosus
Kiangyousteus yohii
Golshanichthys asiatica
Westralichthys uwagedensis
Dunkleosteus raveri
Dunkleosteus terrelli
Dunkleosteus amblyodoratus
Heterosteus ingens
Yinosteus major
Tapinosteus heintzi
Bullerichthys fascidens
Kendrickichthys cavernosus
Bruntonichthys multidens
Dinichthys herzeri
Hadrosteus rapax
Gorgonichthys clarki
Heintzichthys gouldii
Pachyosteus bulla
Gymnotrachelus hydei
Stenosteus angustopectus
Brachyosteus dietrichi
Melanosteus occitanus
Rhinosteus parvulus
In his cladogram, Rücklin (2011) regards Pachyosteus bulla as a basal selenosteid, being the sister taxon of the American genera, and the Kellwasserkalk genera of Germany and Morocco.[3]
Parabelosteus
Brachydeirus
Pachyosteus
Selenosteus
Stenosteus
Gymnotrachelus
Microsteus
Walterosteus
Draconichthys
Enseosteus
Melanosteus
Rhinosteus
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