Abstract
The study of six species, (one is new), belonging to the genus Protacanthina Gandl, 1972, Trilobite Asteropyginae from the Lochkovian-Pragian (Lower Devonian) of the Armorican Massif, Spain and North Africa allows to precise the evolution within the genus. This evolution is mainly expressed by an increase of the number of segments on the pygidium (number of axial rings, pleural ribs and lateral spines). The rate of evolution seems to be higher in Spain. It is possible to use these different species to locate the Lower/Upper Lochkovian and the Lochkovian/Pragian boudnaries according to the conodonts, brachiopods and chitinizoans. Their distribution on the carbonate platform in the north gondwanan Domain during the Lower Devonian cofirn that the Armorican Massif, Spain and North Africa belong to the same biogeographic domain.
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