A late Oligocene chiton fauna (Polyplacophora) from a rocky shore ecosystem, Cosy Dell, Southland, New Zealand

Abstract

ABSTRACT Chiton (Class Polyplacophora) fossils are rare globally, mostly because they are restricted to hard habitats such as rocky shores that are taphonomically under-represented in the geological record. New Zealand is rich in Cenozoic marine molluscan fossils, but chitons are very uncommon. The earliest New Zealand records of fossil chiton species are all from the Late Oligocene (Duntroonian) Chatton Formation in Southland. They include Callochiton chattonensis Ashby 1929 [New Zealand fossil Polyplacophora (Chitons). Transactions and Proceedings of the Royal Society of New Zealand. 60:366–369], Acanthochitona (Notoplax) ashbyi (Laws 1932 [New Tertiary Mollusca from New Zealand. No. 2. Transactions and Proceedings of the New Zealand Institute. 62:183–199]) and Rhyssoplax allanthomsoni Mestayer 1929 [Notes on New Zealand Mollusca. Transactions and Proceedings of the Royal Society of New Zealand. 60(4):247–150]. At Cosy Dell farm, Waimumu, New Zealand, the Chatton Formation contains a diverse chiton fauna representing seven families and seven genera, including Callochiton cf. chattonensis, Acanthochitona cf. ashbyi, Ischnochiton sp., Leptochiton cf. inquinatus, Lorica sp., Plaxiphora sp., and Rhyssoplax sp., the highest recorded fossil chiton diversity in New Zealand. All genera described in the fossil assemblage are associated with extant taxa around modern New Zealand rocky intertidal and shallow near shore environments. The study extends the New Zealand stratigraphic range of four genera (Ischnochiton, Plaxiphora, Lorica and Leptochiton) back to the Duntroonian (Late Oligocene), provides the first taxonomic descriptions of each chiton taxon from the site and discusses the paleoecological and biostratigraphic significance of these rarely preserved components of a rocky shore ecosystem.