Ordovician graptolites from the northern Sierra de Cobachi, Sonora, Mexico

Two Ordovician graptolite assemblages are recognised and described for the first time in the allochthonous siliceous assemblage or succession forming the Guayacan Group of central Sonora in northwestern Mexico. A Middle Ordovician assemblage referable to the Nemagraptus gracilis-Climacograptus wilsoni zonal interval occurs in the upper siliceous shale of unit 1 of the Guayacan Group and an Upper Ordovician assemblage representative of the Dicellograptus ornatus Zone of the Pacific faunal province has been collected from shaly partings in the bedded radiolarian chert constituting unit 2. The stratigraphical make-up of the siliceous assemblage bears a striking similarity to the better known—and correlative—Lower Palaeozoic siliceous succession of central and northern Nevada, giving credence to the hypothesis that the Palaeozoic siliceous assemblage in northwestern Mexico was originally the extension of the Nevadan assemblage into California, which has been transposed to its present position along a left-lateral megashear of the magnitude of at least 1000 km. A review of the proximal development of the spinose species remaining in the form genus Climacograptus Hall has led to the proposal to restrict the genus to Climacograptus bicornis (Hall), its type species, and to two other septate species of early Middle Ordovician age (Llandeilo-early Caradoc) with a primitive type of proximal-end development. The genus Euclimacograptus , with Climacograptus hastatus T. S. Hall as the type species, is proposed for spinose Upper Ordovician climacograptids with an advanced proximal-end development. The genus Ensigraptus , with Climacograptus caudatus Lapworth as the type species, is also proposed for species homeomorphic with those included in Normalograptus Legrand but with a less advanced proximal-end development. Five species are described.