The effaced Ordovician trilobite Svobodapeltis and the earliest history of illaenimorph trilobites in the Ordovician of the Prague Basin (Bohemia)

摘要

Revision of the Middle Ordovician (Darriwilian) illaenimorph trilobite Svobodapeltis avus (Holub, 1908) results in a substantially emended diagnosis of the genus Svobodapeltis ?najdr, 1957 and its assignment to the Family Hemibarrandiidae Courtessole & Pillet, 1975. The general illaeniform morphology of Svobodapeltis and its relative Hemibarrandia Prantl & P?ibyl, 1949, with effaced cephalon and pygidium, the connective suture on the cephalic doublure indicating the presence of a rostral plate, no articulating furrows in the axis, and the fulcrate thorax are in distinct contrast with the presence of pleural furrows on all thoracic segments. The glabella that is defined anteriorly, the presence of border furrows on the cephalon, lack of the lunette, and furrowed segments are regarded as convincing evidence for separation of the Hemibarrandiidae from other illaenoids. The illaenid Caudillaenus advena (Barrande, 1872), a specimen of which was previously assigned to S. avus that influenced the concept of the genus, is redescribed together with its ancestor C. aff. advena .