A new pteraspidomorph from the Nibil Formation (Katian, Late Ordovician) of the Canning Basin, Western Australia

ABSTRACT

Based upon fragmentary remains of dermal armor, a new form of arandaspid fish, Ritchieichthys nibili, gen et. sp. nov., is described from subsurface core material from the Katian (Late Ordovician) Nibil Formation of the Canning Basin, Western Australia. Ritchieichthys nibili represents the first documented record of a fish from the Ordovician of the Canning Basin. Allied to the previous descriptions of arandaspsids from the Amadeus and Warburton basins of the Northern Territory and New South Wales, respectively, this record extends the paleogeographic range of arandaspids across the hypothetical Ordovician Larapintine Seaway and increases the stratigraphic range of the Order Arandaspidiformes into the Katian. The hard tissue histology of Ritchieichthys nibili confirms the presence of a cellular dentine forming the bulk of the dermal armor ornament in arandaspids, a tissue that had not been directly observed previously, and confirms the presence of largely unconnected osteocytes within the dermal bone that forms the majority of the armor.

ACKNOWLEDGMENTS

This work was supported in part by NERC grant NE/B503576/1 (I.J.S.) into the diversification of Ordovician fish from Gondwana. P. Janvier (MNHM, Paris) provided additional information about the structure of Sacabambaspis armor and we are grateful to Z. Johanson and M. Richter (both NHMUK, London) for discussion on the histology of Ritchieichthys nibili and referees D. Elliott (Arizona) and A. Blieck (Lille) for their comments on the originally submitted manuscript. P. Cooper and R. Shelton (both School of Dentistry, University of Birmingham) are thanked for access to the SkyScan 1172 micro-CT and JEOL scanning electron microscope, respectively. P. Hands (School of Geography, Earth and Environmental Sciences, University of Birmingham) prepared the illustrated thin sections. P.W.H. publishes with the permission of the Director, Geological Survey of Western Australia.

Handling editor: Martha Richter.