A new Placoderm fish (Acanthothoraci) from the Early Devonian Jauf Formation (Saudi Arabia) Author Olive, Sébastien Author Goujet, Daniel Author Lelièvre, Hervé Author Janjou, Dominique text Geodiversitas 2011 2011-09-30 33 3 393 409 http://dx.doi.org/10.5252/g2011n3a1 journal article 10.5252/g2011n3a1 1638-9395 5379795 Order ACANTHOTHORACI Stensiö, 1944 EMENDED DIAGNOSIS (modified from Denison 1978 ). — The skull is moderately long and narrow with subparallel sides and a deep embayment on the posterior margin bounded by strongly projecting paranuchals. Dermal bones may be well developed and cover the cranial roof, or they may be separated or overlain by tesserae; two pairs of paranuchals are present. The orbits are lateral or dorso-lateral, and the nostrils are dorsal, bounded anteriorly by a premedian plate and surrounded postero-mesially by a small rostral plate. The suborbital plates are short and deep, without postorbital laminae. The lateral line sensory system occupies wide, open grooves or closed canals. An exoskeletal cranio-thoracic articulation is absent. The perichondraly ossified endocranium typically is moderately long and narrow with a rostrum projecting below and anterior to the rhinocapsular ossification. Dermal bones and scales are ornamented with tubercles that are commonly stellate. REMARKS Like all members of this order, Arabosteus n. gen. shows a typical sigmoid skull roof occipital border with a medial deep symmetric embayment and the posterior projection of the posterior paranuchal plate; large orbits oriented laterodorsally and a narrow interorbital wall are also special features present in most Acanthothoraci .