First Ordovician vertebrates from the Southern Hemisphere

Abstract

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 Spitsb...