Ichnofacies of the Stairway Sandstone fish-fossil beds (Middle Ordovician, Northern Territory, Australia)

Abstract

The Stairway Sandstone is a 30–560 m thick succession of Middle Ordovician siliciclastic sedimentary rocks within the Amadeus Basin of central Australia, deposited in the epeiric Larapintine Sea of northern peri-Gondwana. The Stairway Sandstone is significant as one of only two known Gondwanan successions to yield articulated arandaspid (pteraspidomorph agnathan) fish. Herein we use the ichnology of the Stairway Sandstone to reveal insights into the shallow marine habitat of these early vertebrates, and compare it with that of other known pteraspidomorph-bearing localities from across Gondwana. The Stairway Sandstone contains a diverse Ordovician ichnofauna including 22 ichnotaxa of Arenicolites, Arthrophycus, Asterosoma, Cruziana, Didymaulichnus, Diplichnites, Diplocraterion, ?Gordia, Lockeia, Monocraterion, Monomorphichnus, Phycodes, Planolites, Rusophycus, Skolithos and Uchirites. These ichnofauna provide a well-preserved example of a typical Ordovician epeiric sea assemblage, recording the diverse ethologies of tracemakers in a very shallow marine environment of flashy sediment accumulation and regularly shifting sandy substrates. New conodont data refine the age of the Stairway Sandstone to the early Darriwilian, with ichnostratigraphic implications for the Cruziana rugosa group and Arthrophycus alleghaniensis.

Key words:

• arandaspid
• Arthrophycus
• Cruziana
• ichnology
• ichnostratigraphy
• Ordovician

Acknowledgements

NSD and IJS were supported by NERC Grant Ref NE/B503576/1 and are extremely grateful to Colin Gatehouse for his help and advice while conducting fieldwork in the Northern Territory. Chris Howard is also thanked for his assistance and persistence during fieldwork in Watarrka National Park. We are also deeply indebted to John Laurie in sharing information on the geology of the Stairway Sandstone and permitting access to the Geoscience Australia collections in Canberra. Adolf Seilacher assisted with the identification of some of the trace fossils described herein, and John Cope assisted with the taxonomic assignment of Alococoncha. We would also like to thank Steven Holland, Sören Jensen and editor Steve McLoughlin for their insightful comments, which much improved this paper.