First Ordovician vertebrates from the Southern Hemisphere

Fossil remains of primitive vertebrates, preserved in sandstone as natural moulds of the dermal armour, are described from the shallow-water marine Stairway Sandstone of the Amadeus Basin, Northern Territory, Australia. This is the first record of Ordovician vertebrates in the southern hemisphere. Two new genera and species, Arandaspis prionotolepis gen. et sp. nov. and Porophoraspis crenulata gen. et sp. nov., are described. Arandaspis is the most completely preserved of any known Ordovician vertebrate. It is the type genus of the new family Arandaspididae, and is referred to the new order Arandaspidiformes. The new Australian genera are provisionally interpreted as heterostracans, a group of agnathans not previously recorded from Australia. Deposition of the ostracoderm-bearing levels of the Stairway Sandstone is dated as earliest Middle Ordovician on the northern Australian shelly scale, or late Arenigian to early Llanvirnian on the British scale. The Australian forms thus approximate in age the Spitsbergen Anatolepis Bockelie & Fortey 1976, which is at present the oldest confirmed vertebrate fossil. They antedate the North American Harding Sandstone heterostracans Astraspis Walcott 1892 and Eriptychius Walcott 1892 by some 20 million years. Unlike the North American genera, the older forms from Australia and Spitsbergen are protected by a somewhat delicate non-tesserate shield, which invites a critical reassessment of current ideas on the course of evolution of early vertebrate armour and the date of its first appearance. The scale-like sculpture of the newly discovered forms also suggest a possible source of disjunct thelodont-like denticles recovered from Ordovician sediments. The wide geographical separation of the morphologically and temporally similar Australian and Spitsbergen genera, and the consistent association of Ordovician agnathans with marine invertebrates (both body fossils and trace fossils), uphold Denison's (1967, p. 141) conclusions concerning the habitat of early vertebrates: 'They lived in the sea. The regular association of their remains with sandstones and rarely with other sediments suggests that they lived on or above a sandy bottom. They preferred well-lighted, quiet waters at moderate depths in the sublittoral zone.'