Un nouveau placoderme (Acanthothoraci) du Dévonien inférieur de la Formation de Jauf (Arabie Saoudite)

Un nouvel Acanthothoraci (Placodermi) du Dévonien inférieur d’Arabie Saoudite (Qasr Limestone Member, Jauf Formation), exceptionnellement bien conservé, présentant un endocrâne et un toit crânien complets, est décrit en tant qu’Arabosteus variabilis n. gen., n. sp. Ce nouveau taxon présente une variabilité importante au niveau de l’agencement des plaques dermiques (e.g., une plaque surnuméraire : la plaque centrale postérieure) et de la distribution des canaux sensoriels de son toit crânien, non seulement vis-à-vis des autres Acanthothoraci mais également entre les différents individus représentés au sein de ce taxon. Il permet d’élargir la connaissance des caractères anatomiques des Acanthothoraci et des placodermes en général. Classiquement les arthrodires, l’un des plus anciens clades de placodermes décrits, ont servi de référence dans le débat sur l’établissement d’homologies concernant les plaques du toit crânien des autres groupes. L’interprétation de la distribution des plaques crâniennes d’Arabosteus variabilis n. gen., n. sp. à partir du modèle arthrodire, et l’établissement d’homologies, se révèlent impossibles, à moins de se baser sur des hypothèses préalables (e.g., processus de fragmentation ou de fusion pour le complexe des plaques paranuchales). Le système des lignes sensorielles d’A. variabilis n. gen., n. sp. est instable (e.g., présence ou absence du canal postmarginal) et donc ambigu pour décider d’une terminologie et de l’établissement d’homologies des plaques du toit crânien. Ainsi, une terminologie topographique a été retenue pour une telle tâche en minimisant les références aux caractères instables du système des lignes sensorielles.An exceptionally well-preserved new acanthothoracid (placoderm fish) presenting complete endocranium and skull roof, from Saudi Arabia (Qasr Limestone Member, Jauf Formation), Early Devonian in age, is described as Arabosteus variabilis n. gen., n. sp. Compared to other Acanthothoraci, the new taxon exhibits a skull roof variable in its dermal plate distribution (e.g., a supernumerary plate: the posterior central plate) and sensory line pattern. Even the different specimens of this taxon show such variability between them. Arabosteus variabilis n. gen., n. sp. enlarges the scope of our knowledge of Acanthothoraci and Placodermi in general. Traditionally, the Arthrodira, one of the historically earliest described groups, provide the standard reference model for the placoderm skull roof pattern. The interpretation of the acanthothoracid skull roof pattern of the Saudi taxon from an arthrodiran model and the establishment of homologies is impossible to resolve unless based on prior assumptions (e.g., hypothesis of fragmentation or fusion for the paranuchal plate complex). The lateral line system of A. variabilis n. gen., n. sp. is unstable (e.g., presence or absence of the postmarginal sensory line canal) making it ambiguous to define bone homology and terminology. Consequently spatial distribution of skull roof plates has been retained for such a task with a minimal reference to the instable features of the sensory line groove distribution.</p

A new placoderm fish (Acanthothoraci) from the Early Devonian Jauf Formation (Saudi Arabia)

peer reviewedAn exceptionally well-preserved new acanthothoracid (placoderm fish) presenting complete endocranium and skull roof, from Saudi Arabia (Qasr Limestone Member, Jauf Formation), Early Devonian in age, is described as Arabosteus variabilis n. gen. n. sp. Compared to other Acanthothoraci, the new taxon exhibits a skull roof variable in its dermal plate distribution (e.g. a supernumerary plate: the posterior central plate) and sensory line pattern. Even the different specimens of this taxon show such variability between them. Arabosteus variabilis enlarges the scope of our knowledge of Acanthothoraci and Placodermi in general. Traditionally, the Arthrodira, one of the historically earliest described groups, provide the standard reference model for the placoderm skull roof pattern. The interpretation of the acanthothoracid skull roof pattern of the Saudi taxon from an arthrodiran model and the establishment of homologies is impossible to resolve unless based on prior assumptions (e.g. hypothesis of fragmentation or fusion for the paranuchal plate complex). The lateral line system of A. variabilis is unstable (e.g. presence or absence of the postmarginal sensory line canal) making it ambiguous to define bone homology and terminology. Consequently spatial distribution of skull roof plates has been retained for such a task with a minimal reference to the instable features of the sensory line groove distribution

Palaeacanthaspidae Stensio 1944

Family PALAEACANTHASPIDAE Stensiö, 1944 DIAGNOSIS. — As for the order. To date this is the only family included in the order Acanthothoraci.Published as part of Olive, Sébastien, Goujet, Daniel, Lelièvre, Hervé & Janjou, Dominique, 2011, A new Placoderm fish (Acanthothoraci) from the Early Devonian Jauf Formation (Saudi Arabia), pp. 393-409 in Geodiversitas 33 (3) on page 396, DOI: 10.5252/g2011n3a1, http://zenodo.org/record/537979

Acanthothoraci Stensio 1944

Order ACANTHOTHORACI Stensi&ouml;, 1944 &lt;p&gt; EMENDED DIAGNOSIS (modified from Denison 1978). &lt;b&gt;&mdash;&lt;/b&gt; 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.&lt;/p&gt; &lt;p&gt;REMARKS&lt;/p&gt; &lt;p&gt; Like all members of this order, &lt;i&gt;Arabosteus&lt;/i&gt; 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.&lt;/p&gt;Published as part of &lt;i&gt;Olive, Sébastien, Goujet, Daniel, Lelièvre, Hervé &amp; Janjou, Dominique, 2011, A new Placoderm fish (Acanthothoraci) from the Early Devonian Jauf Formation (Saudi Arabia), pp. 393-409 in Geodiversitas 33 (3)&lt;/i&gt; on page 396, DOI: 10.5252/g2011n3a1, &lt;a href="http://zenodo.org/record/5379795"&gt;http://zenodo.org/record/5379795&lt;/a&gt

Le référentiel géologique de la France : harmonisation géologique et nomenclature litho-stratigraphique

Janjou Dominique, Bonnefoy Denis, Hanot Frank, Nehlig Pierre, Vaslet Denis. Le référentiel géologique de la France : harmonisation géologique et nomenclature litho-stratigraphique. In: Documents des Laboratoires de Géologie, Lyon, n°156, 2002. STRATI 2002. 3ème congrès français de stratigraphie. Lyon, 8-10 juillet 2002. pp. 133-134