Abstract

Although onshore to offshore retreat of brachiopods, in terms of their community-level abundance, took place through the Mesozoic and Cenozoic, this study shows that comparable trends also occurred repeatedly on a short time scale and mainly were driven by variations in sediment and nutrient supply. In the Kössen Formation (Upper Triassic), brachiopods retreated to offshore habitats during nutrient-rich, siliciclastic regimes and expand to onshore habitats during nutrient-poor, carbonate regimes. Epifaunal bivalves occupied onshore and offshore habitats during both siliciclastic and carbonate regimes. Infaunal suspension-feeding bivalves expanded to offshore habitats during nutrient-rich, siliciclastic regimes and retreated from offshore habitats during nutrient-poor, carbonate regimes. Thus, the onshore to offshore retreat of brachiopods and the offshore expansion of infaunal bivalves repeatedly coincided with the switch from a nutrient-poor, carbonate regime to a nutrient-rich, siliciclastic regime. Because brachiopods and epifaunal bivalves were abundant in micrite-rich, soft-bottom habitats, the replacements between infaunal and epifaunal communities cannot be explained by variations in substrate consistency alone.

Differences in guild structure between siliciclastic and carbonate regimes and onshore to offshore replacements indicate that distribution of bivalves and brachiopods is related to their differential response to low nutrient supply, turbidity, and, possibly, oxygen levels. Based on actualistic evidence, brachiopods are able to thrive in nutrient-poor conditions due to low metabolic demands and are less tolerant of high-turbidity conditions than bivalves. Epifaunal bivalves that co-occur with brachiopods in nutrient-poor habitats may have been characterized by higher clearance rates in contrast to infaunal bivalves with similar metabolic requirements. Although higher biogenic sediment disturbance or other biotic interactions could play a significant role in the retreat of brachiopods to offshore habitats, this study highlights the importance of varying nutrient supply and turbidity in governing onshore to offshore replacements on short time scales.

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