Pristiograptus (Graptoloidea) from the perneri - lundgreni biozones (Silurian) of Lithuania

Abstract

Two new forms of Pristiograptus dubius, here designated as varieties A and B, are described and figured. Both were found in core samples representing the perneri through lundgreni biozones of the Wenlock epoch. The cores are from boreholes Šiupyliai-69, Parovėja-9, Likėnai-396, Paežeriai-222 and Sutkai-87 in central and northern Lithuania. The associated graptolites Cyrtograptus perneri Boucek, C. radians Tornquist, C. lundgreni Tullberg, Monograptus flemingii flemingii (Salter), M. testis testis (Barrande), Monoclimacis flumendosae (Gortani) and Pristiograptus pseudodubius (Boucek) (= P. parvus) are all indicative of the perneri, radians and lundgreni biozones of the Wenlock. Two species of Pristiograptus occur in this time-stratigraphic interval: P. dubius and P. lodenicensis. In P. dubius the degree and manner of extension of the thecal apertural lip (thecal hood) onto the succeeding theca distinguish it from otherwise very similar taxa. The thecal morphology of P. lodenicensis differs from that of a typical Pristiograptus in that the apertural lip has a central depression and is rounded into small lobes laterally.