Fossil oysters from the southernmost coast of Brazil: Bioerosion and palaeoenvironmental implications

Fossil oysters are commonly found in Rio Grande do Sul Coastal Plain (RSCP), where the main species are Ostrea cf. puelchana d’Orbigny, 1841 and Crassostrea praia Ihering, 1907. These fossils are transported towards the beach by storm events throughout the year, being available in the swash zone. The objective of this article is to reconstruct the Holocene palaeoenvironment by analysing these fossils. A total of 1898 valves, collected along Cassino and Hermenegildo beach, both in southernmost Brazil, were described. It was observed a prevalence of C. praia at Cassino beach (39.20%) and O. cf. puelchana at Hermenegildo beach (89.02%). Hermenegildo beach sample were statistically greater for C. praia height (p-value: 0.1736) and length (p-value: 0.4604) and for O. cf. puelchana height (p-value: 0.0602) and length (p-value: 0.07394). The predominance for northern and southern regions for each species indicates a palaeoenvironment setting that favoured these organisms, probably related to temperatures and salinity variation. The most common ichnogenera found in the oysters were Caulostrepsis and Entobia present in both sites, observed in over 40% of internal and external sides of the samples’ shell. According to radiocarbon modelled age, the studied samples coexisted during the Mid-Holocene (8.290–8.130 cal. yr. BP), with the subsequent sea-level rise and fall the shells became bioavailable. The lack of samples with closed valves and high percentage of bioerosion in the inner side of the valve, indicates that the palaeoenvironment in both Cassino and Hermenegildo beaches gradually changed, not as result of an extreme event.