Ordovician microvertebrate remains from the Amadeus Basin, central Australia

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Abstract
Microvertebrate remains are described from five formations (Pacoota Sandstone, 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 material is referred to indeterminate species of the agnathan genera Arandaspis 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 assigned to the Chondrichthyes. Scales of this species apparently lack a base, and are made of an atubular laminar surface tissue of uncertain histology. Mode of scale growth was similar to that of the Early Devonian form Polymerolepis. If correctly interpreted, this taxon is the oldest chondrichthyan and/or gnathostome known from the fossil record. The form taxon Apedolepis tomlinsonae, gen. et sp. nov. contains scales with 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 pteraspidomorphs may be indicated by a similar system of pores opening to the surface. A pore-canal system, and bone rather than aspidin, may have been primitively present in agnathans. Indeterminate remains from the early Arenig and Caradoc (Horn Valley Siltstone, Stokes Formation) may include endoskeletal ossification. These new microvertebrate faunas are assigned to four assemblages ranging in age from early Arenigian to Caradocian (Bendigonian-Eastonian). They have biostratigraphic potential for age control in Ordovician marginal marine sediments, and are important in demonstrating new combinations of hard tissue types at the beginning of the vertebrate fossil record.