Structure and Preservation of Chiton Valves: Resolution of a Taphonomic Quandary

Abstract

Chitons are marine molluscs common worldwide that form their shells out of aragonite and are rare in the sediment and fossil records. The rarity of chitons in these records is not surprising, as dissolution readily removes aragonite from the fossil record. Yet, recent studies have suggested that chitons have an unusual capacity to resist dissolution, which implies that dissolution does not remove chiton material from the sedimentary and fossil records. The purpose of this thesis was to address this apparent contradiction of chiton preservation. The valve structure and the effects of abrasion and dissolution on eight New Zealand chiton species (Acanthochitona zelandica, Notoplax violacea, Chiton glaucus, Onithochiton neglectus, Sypharochiton pelliserpentis, Sypharochiton sinclairi, Ischnochiton maorianus, and Leptochiton inquinatus) were examined. Scanning electron microscopy (SEM), X-ray diffractometry (XRD), and Raman spectroscopy were used to determine the mineralogy, ultrastructure, and organic components of chiton valves. The ability of these chiton valves to resist abrasion and dissolution was directly tested by tumbling isolated valves in a rock tumbler for 96 hours and subjecting another group of isolated valves to two treatments of different pH: 8.10 (ambient) and pH 7.70 (projected pH for the year 2100) for 12 days. XRD and Raman spectra confirmed that valves from all analysed species were made of aragonite. The tegmenum was primarily granular and contained one or two carotenoid pigments. The articulamentum was formed by alternating crossed lamellar and spherulitic structures and a ventral-most acicular sublayer. The number of sublayers varied among species, and L. inquinatus displayed a unique crossed lamellar structure in its valves. The valves lost 9-44% of their initial weight after tumbling, which is unusually resistant compared to other molluscs. Abrasion damaged the tegmentum more than the articulamentum. Valves lost 1 to 5% of their initial weight after 12 days in a pH of 7.70 and less than 1% in the ambient pH control. SEM images of valves in the pH treatments revealed that dissolution damaged the articulamentum more than the tegmentum. While these results suggest that chiton valves are resilent, taphonomic forces do not act in isolation. The two different main layers of the valves are vulnerable to different forces; abrasion will remove the tegmentum and expose the articulamentum to rapid dissolution. Chiton valves are estimated to last about 5 years before being rendered unrecognizable. It is likely that chiton valves require a rapid burial event to be preserved since they can be exposed to taphonomic forces for hundreds of years in temperate intertidal and shelf environments.