TAXONOMY AND EVOLUTION OF EARLIEST ORDOVICIAN GRAPTOLOIDS

The sequence, taxonomy and inferred phylogeny of earliest Ordovician g raptoloids have been determined from stratigraphic sections in Newfoun dland, Norway, China and elsewhere. Rhabdinopora is analysed in terms of population systematics and is envisaged as a diverse complex of int ergrading populations that change in time (stratigraphic subspecies) a nd in ecological space (ecological subspecies). The main lineage, deve loped in sequences representing the deep shelf to continental slope en vironment, comprises the successive subspecies of R. flabelliformis-pa rabola, canadensis, flabelliformis, anglica and, probably, bryograptoi des and is developed through the early and early-late Tremadoc. R. f. parabola, a key graptolite for correlation of the base of the Ordovici an System, is described in derail. The shallow water inshore forms, R. f. socialis and R. f. norvegica are interpreted as ecological subspec ies. The genus is characterized by quadriradiate development type, now known to be present in all subspecies in which the development type c an be determined. The genera Staurograptus and Anisograptus are redefi ned and described and their species interpreted as populations with a wide range of morphologic variance. Triramograptus atavus gen. et sp. nov. is described from Newfoundland. The stratigraphic ranges of all s pecies are revised and their zonal distribution presented.