Rhachiosteus

Rhachiosteus is an extinct monospecific genus of arthrodire placoderm from the Middle to Late Devonian of Germany. It is known only from a single specimen, which may be a larval or juvenile form, as the skull of said specimen is only 19 millimetres long.^[1]

Rhachiosteus Temporal range: Devonian: Givetian - Frasnian, 387.7–372.2 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Artist's reconstruction of Rhachiosteus (with Trematosteus in the background)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	†Placodermi
Order:	†Arthrodira
Suborder:	†Brachythoraci
Clade:	†Eubrachythoraci
Clade:	†Pachyosteomorphi
Genus:	†Rhachiosteus Gross, 1938
Species
Rhachiosteus pterygiatus Gross, 1938 (type)

Phylogeny

Rhachiosteus is a basal member of the clade Pachyosteomorphi, the sister taxon to Coccosteomorphi, which together are the two main sub-clades of Eubrachythoraci. The cladogram below shows the phylogeny of Rhachiosteus:^[2]

Eubrachythoraci

Coccosteomorphi Coccosteoidea Coccosteidae Millerosteus minor Coccosteus cuspidatus Dickosteus threiplandi Watsonosteus fletti Protitanichthys rockportensis Panxiosteidae Plourdosteus canadensis Panxiosteus ocullus Janiosteus timanicus Incisoscutoidea Harrytoombsia elegans Torosteus tuberculatus Torosteus pulchellus Mcnamaraspis kaprios Compagopiscis croucheri Trematosteus fontanellus Camuropiscidae Incisoscutum ritchiei Incisoscutum sarahae Rolfosteus canningensis Tubonasus lennardensis Fallacosteus turneri Camuropiscis laidlawi Latocamurus coulthardi Pachyosteomorphi Rhachiosteus pterygiatus Dunkleosteoidea Eastmanosteus calliaspis Eastmanosteus pustulosus Kiangyousteus yohii Golshanichthys asiatica Westralichthys uwagedensis Dunkleosteus raveri Dunkleosteus terrelli Dunkleosteus amblyodoratus Heterostiidae Heterosteus ingens Yinosteus major Aspinothoracidi Tapinosteus heintzi Bullerichthys fascidens Kendrickichthys cavernosus Bruntonichthys multidens Dinichthys herzeri Hadrosteus rapax Gorgonichthys clarki Selenosteidae Heintzichthys gouldii Pachyosteus bulla Gymnotrachelus hydei Stenosteus angustopectus Brachyosteus dietrichi Melanosteus occitanus Rhinosteus parvulus