An ordovician vertebrate from western New South Wales, with comments on Cambro-Ordovician vertebrate distribution patterns

Abstract

A fish plate impression from the Ordovician Rowena Formation in Mutawintji National Park is referred to an indeterminate species of the genus Arandaspis Ritchie & Gilbert-Tomlinson, 1977, genotype of the family Arandaspididae, previously known only from the Middle Ordovician Stairway Sandstone in the Amadeus Basin of central Australia. The Rowena Formation has previously been correlated with units underlying the Stairway Sandstone (Pacoota Sandstone, Horn Valley Siltstone) in the Amadeus Basin, from which a second arandaspid Porophoraspis represents the earliest occurrence in the fossil record that demonstrates two defining developmental characters of the vertebrates (neural crest, epidermal placodes). The new occurrence of Arandaspis is consistent with the Middle Ordovician (Darriwilian) age for the upper part of the Rowena Formation indicated by recent studies of trilobites. The order Arandaspidiformes, also known from the Upper Ordovician of South America (Sacabambaspis), defines a 'Gondwana Endemic Assemblage', which may have been established already by the late Cambrian. Although the first phase of vertebrate biomineralization is not yet documented from the Cambro-Ordovician of Asia, the oldest vertebrates in the early Cambrian Chengjiang fauna and diverse agnathans and gnathostomes by Early Silurian time in South China implicate this as a significant area in the evolution of the earliest vertebrates.