Encrusted-barnacles on fossil cetacean bones as first evidence of the reef stage of a whale-fall from shallow marine deposits, early Miocene of Patagonia, Argentina

FARRONI, NICOLÁS D.; CUITIÑO, JOSÉ I.; BUONO, MÓNICA R.

Alcalá de Henares (Madrid)

Congreso; 6th TWG-ICAZ and the 9th TAPHOS congress; 2022

Cetacean bone encrustation has been interpreted as the final (reef) stage of the ecological succession in deep marine whale-falls. However, little is known about the communities that develop in association with carcasses deposited in shallow marine environments. Knowledge about fossil and modern whale-fall studies in Argentina is null. Recent prospecting fieldworks on the lower Miocene Gaiman Formation in northeast Patagonia (Chubut province) resulted in the discovery of a large number of fossil cetaceans; strikingly, three of them from the same horizon show attached encrusting acorn barnacles. Based on palaeontologic, taphonomic, and sedimentologic descriptions, we aim to examine the possible factors that controlled the preservation of these encrusted cetacean bones and to discuss their ecological impacts on the shallow seafloor. The specimens analyzed correspond to small odontocetes from Cañadón del Puma (CDP) locality: CDP-10 is a 60 cm-long skull of a juvenile Notocetus sp. in dorsal-up position (Fig. 1A,B), with a low degree of corrosion, no abrasion nor bioerosion, and a moderate degree of encrustation; CDP-05 is an at least 50 cm-long skull in ventral-up position (Fig. 1C,D), with a moderate to high degree of corrosion, no abrasion nor bioerosion, and a low to moderate degree of encrustation; and CDP-04 is composed of almost-articulated postcranial elements (17 vertebrae and 4 ribs) (Fig. 1E,F), with low degree of corrosion and encrustation, and no abrasion nor bioerosion. The three specimens were found within bioturbated shell-beds with muddy sandstone matrix, displaying molds of fossil invertebrates such as turritellid gastropods, carditids, corbulids and venerids bivalves. These sediments are interpreted as deposited in a shallow-water, inner shelf setting, characterized by moderate energy and soft bottom. The associated shark teeth next to CDP-05 and the fossil invertebrates may indicate the presence of mobile-scavenger and opportunistic stages of the whale-fall ecological succession. However, data integration suggests that these accumulations are the result of background environmental conditions in shallow marine environments under low sedimentation rate conditions, producing stratigraphic condensation (time-averaging). Thus, they didn’t necessarily inhabit the seafloor because of the organic matter input from the cetacean carcasses. In contrast, the occurrence of encrusted-barnacles on fossil bones is inferred as direct evidence of a well-developed reef stage, indicating a prolonged exposure of the specimens prior to final burial. Thus, whale-fall stage limits in shallow marine ecological successions are not only controlled by paleoenvironmental distribution of the associated fauna and its paleoecological dynamics but also by sedimentologic, stratigraphic and taphonomic factors.