THE TRILOBITE SUBFAMILY MONORAKINAE (PTERYGOMETOPIDAE)

Abstract

The Monorakinae is a subfamily of the Pterygometopidae characterised by the fusion of L2 and L3 in the glabella. The resulting bicomposite lobe is expanded backwards to reach the occipital furrow, displacing L1 from contact with the axial furrow and causing the realignment of S1 to a markedly oblique orientation. The bicomposite lobe is commonly bounded adaxially by a longitudinal furrow containing three pairs of apodemal pits. The Monorakinae was probably derived from the Pterygometopinae, and includes the genera and subgenera Monorakos, Carinopyge, Ceratevenkaspis, Elasmaspis, Evenkaspis (Evenkaspis) and E. (Parevenkaspis), of which Carinopyge, Elasmaspis and Evenkaspis (Parevenkaspis) are known only from limited parts of the exoskeleton. Monorakines have a stratigraphical range of Caradoc–Ashgill. Their known geographical distribution in the Siberian Platform, Taimyr, the Russian Far East, and the Seward Peninsula of Alaska is restricted to areas that in the Ordovician were part of the palaeocontinents of Siberia and Arctida, which must have been connected or situated close together at that time. The occurrence of monorakines in the Taimyr Peninsula but their absence from Baltica does not support the suggestion of some workers that Taimyr was part of Baltica in the Ordovician.