Palaeoecology and taphonomy of an extraordinary whale barnacle accumulation from the Plio-Pleistocene of Ecuador

Abstract

The exceptional assemblage of cirripedes from the Late Pliocene–Pleistocene sediments of Canoa and Tablazo Formations includes more than 80 specimens of the whale barnacle Coronula diadema, a species that today lives mainly in the skin of humpback whales ( Megaptera novaeangliae). In light of the exclusive ectoparasitism of Coronula on whales and mainly on humpback whales, we infer that during the Late Pliocene–Pleistocene the Canoa Basin was on the seasonal migration route of these cetaceans and that whales remained in the Canoa Basin for sufficient time (breeding?) for large accumulations of Coronula to form. Support for this hypothesis comes from current use of this part of the Ecuadorian coast as a breeding ground for Megaptera and from a rib fragment of ? Megaptera from the Coronula-rich Tablazo Formation. Indirect palaeoclimatic implications may be derived from this as humpback whales require at least 25 °C for breeding. Thus, in the Late Pliocene and the Pleistocene, the Humboldt Current apparently had a northern extension similar to the present, with a 25 °C summer isotherm in the same latitudinal position along the Ecuador coast as today. On the basis of preservation, it is concluded that the Coronula-bearing molluscan shell beds in the Canoa and Tablazo Formations are of two basic types: biogenic and sedimentologic. Coronula shells deposited in the biogenic shell beds are little affected by physical processes (fragmentation, peeling, edge preservation), while modified by biological processes like bioerosion and encrustation. The exterior and interior surfaces of the shells display a variety of bioerosional features and encrusting taxa including barnacles, serpulid polychaetes, small oysters and bryozoans indicating that they were deposited during periods of sedimentary quiescence. In comparison, Coronula that accumulated within sedimentologic concentrations are typically heavily abraded and highly fragmented with little or no encrustation.