Abstract
Rocky shores are critical ecosystems that support diverse marine communities and provide essential ecological services. However, their geological history is poorly known because preservation in the fossil record is rare due to the high-energy, erosional nature of rocky intertidal zones. The Late Oligocene Cosy Dell fossil locality in Southland, New Zealand, offers a unique window into an ancient rocky shore ecosystem, preserving an exceptionally diverse assemblage of molluscs and other marine invertebrates. This thesis investigates the paleoecology of three major herbivore groups (chitons, pāua (abalone), turbinids that are characteristic of rocky shores, and the predation profile amongst molluscs, echinoids, barnacles, foraminifera, and ostracods.
Chiton (Class Polyplacophora) fossils are restricted to hard habitats such as rocky shores and are uncommon in the fossil record. The Chatton Formation at Cosy Dell contains the highest recorded fossil chiton diversity in New Zealand with seven families and seven genera, including Callochiton cf. chattonensis, Acanthochitona cf. ashbyi, Ischnochiton sp., Leptochiton cf. inquinatus, Lorica sp., Plaxiphora sp., and Rhyssoplax sp. All genera described in the fossil assemblage are associated with extant taxa around modern New Zealand near shore environments. The recorded New Zealand stratigraphic range of four genera (Ischnochiton, Plaxiphora, Lorica and Leptochiton) is extended back to the Duntroonian (Late Oligocene).
The fossil record of abalone (Haliotis) (Family: Haliotidae) in New Zealand is also sparse. This thesis presents a taxonomic and paleoecological analysis of Haliotis fossils from four localities in southern New Zealand: Cosy Dell, Mt Luxmore, Bluff Hill, and the Castle Hill Basin. Among these, the Cosy Dell assemblage stands out for its exceptionally well-preserved specimens, including a rare juvenile with intact nacreous layers, the smallest fossil Haliotis specimen discovered in New Zealand to date. The morphology of these fossils supports their identification as Haliotis, despite preservation challenges that limit species-level taxonomy. The geological settings of these localities suggest that Haliotis thrived in high-energy, shallow-marine environments, primarily inhabiting rocky shores with boulders and crevices.
The fossil turbinid (Turbinidae) assemblage from Cosy Dell contains opercula and/or body fossils of five genera: Astralium, Bolma, Cookia, Lunella, and Sarmaturbo. The assemblage includes 83 opercula, with Lunella and Sarmaturbo particularly well-represented. Morphometric analysis of opercula reveals size ranges from 5.2 mm to 38.7 mm in diameter, with thickness variations suggesting potential ecological differentiation. The presence of large, robust opercula supports adaptation to high-energy intertidal environments. Additionally, Sarmaturbo at Cosy Dell represents one of the earliest occurrences of the genus in New Zealand.
Shell damage, including crushing and peeling scars, is widespread across macro- and micro-molluscan taxa at Cosy Dell, with many specimens exhibiting healed traces indicative of failed predation attempts. The damage patterns suggest that decapod crustaceans, potentially species of Cancer or Portunus, were key predators, though fish cannot be ruled out. The presence of a fossilized crab claw further supports the role of crabs in shaping this rocky shore ecosystem. A study of predation patterns on fossil invertebrates analysed over 300 drilled specimens across multiple taxa, including molluscs, echinoids, barnacles, foraminifera, and ostracods. Among bivalves, Glycymerita thomsoni exhibits a 14% drilling frequency, with hole diameters ranging from 0.32 mm to 6.87 mm. Predation on gastropods and bivalves is primarily attributed to naticid and muricid gastropods, with some evidence of octopod attacks. The echinoid Fibularia shows a 12.6% drilling frequency, with hole diameters between 0.23 mm and 1.88 mm, suggesting predation by both cassid gastropods and parasitic eulimid gastropods. Barnacles display a low drilling frequency (~10%), with drill hole diameters between 0.18 mm and 0.51 mm, likely caused by muricids or octopods. Foraminifera and ostracods exhibit rare drill holes, attributed to juvenile gastropods. These findings indicate intense and diverse predation pressure in this Oligocene rocky shore ecosystems.
The Cosy Dell fossil locality provides a rare glimpse into a subtropical Late Oligocene rocky shore ecosystem, revealing exceptional molluscan herbivore diversity, intense predator activity and the presence of ancestors of many modern taxa living around New Zealand rocky coastlines today. These findings enhance our understanding of ancient intertidal environments and their preservation in the fossil record.