ORDOVICIAN MICROVERTEBRATE REMAINS FROM THE AMADEUS BASIN, CENTRAL AUSTRALIA

Microvertebrate remains are described from five formations (Pacoota Sa ndstone, Horn Valley Siltstone, Stairway Sandstone, Stokes Formation, Carmichael Sandstone) in the Ordovician sequence of the Amadeus Basin, central Australia. Two new genera and species are erected for scales and other elements with distinctive morphology and histology. Other ma terial is referred to indeterminate species of the agnathan genera Ara ndaspis and Porophoraspis previously described from central Australia, and Sacabambaspis previously described from the Ordovician of Bolivia . The most common vertebrate from the Stokes Formation is referred to Areyonga oervigi, gen. et. sp. nov., a form taxon provisionally assign ed to the Chondrichthyes. Scales of this species apparently lack a bas e, and are made of an atubular laminar surface tissue of uncertain his tology. Mode of scale growth was similar to that of the Early Devonian form Polymerolepis. If correctly interpreted, this taxon is the oldes t chondrichthyan and/or gnathostome known from the fossil record. The form taxon Apedolepis tomlinsonae, gen. et sp. nov. contains scales wi th possible osteocyte spaces in the basal tissue, and a crown composed of dentine covered with an enamel-like surface layer which also lines chambers of a pore canal system. Affinity to arandaspidid pteraspidom orphs may be indicated by a similar system of pores opening to the sur face. A pore-canal system, and bone rather than aspidin, may have been primitively present in agnathans. Indeterminate remains from the earl y Arenig and Caradoc (Horn Valley Siltstone, Stokes Formation) may inc lude endoskeletal ossification. These new microvertebrate faunas are a ssigned to four assemblages ranging in age from early Arenigian to Car adocian (Bendigonian-Eastonian). They have biostratigraphic potential for age control in Ordovician marginal marine sediments, and are impor tant in demonstrating new combinations of hard tissue types at the beg inning of the vertebrate fossil record.