Extreme habitat adaptation by boring bivalves on volcanically active palaeoshores from North Atlantic Macaronesia

Abstract

Extensive bivalve borings are described in detail for the first time from basalt rockgrounds in the North Atlantic volcanic islands of Macaronesia. They occur on a Middle Miocene rocky shore of a small islet of Porto Santo (Madeira archipelago of Portugal), as well as on Plio-Pleistocene rocky shores on Santiago (Cape Verde archipelago). A basalt substrate is widely penetrated by clavate-shaped borings belonging to the ichnogenus Gastrochaenolites interpreted as dwelling structures of suspension-feeding bivalves. Some of these borings still retain evidence of the alleged trace-makers preserved as body fossils, while others are filled with their casts. The ichnofossil assemblage present on these bioeroded surfaces belongs to the Entobia ichnofacies. Recognition of Gastrochaenolites borings in volcanic rocks provides useful palaeoenvironmental information regarding an expanded strategy for hard-substrate colonization. Preliminary results from fieldwork in the Cape Verde Islands indicate that such borings are more widespread through Macaronesia than previously thought.