Evidence of drilling predation in oysters from the Lower Cretaceous of the Neuquén Basin (Argentina) and its implications for Mesozoic predation trends

The study of trace fossils recorded on organic substrata is vital to the understanding of past biotic interactions. Here, oyster valves with circular holes are reported from the Lower Cretaceous (Berriasian to lower Valanginian) strata of the Neuquén Basin. From a total of 1318 oyster specimens, 17 valves presented 19 of these traces, which were studied to determine their origin, frequency and whether taxon, size and/or site selectivity existed. The studied traces, identified as Oichnus isp. (O. paraboloides in three cases), were recorded on four oyster species belonging to three genera: Aetostreon, Ceratostreon, and Nanogyra. Boring frequency was low (maximum=0.103), and although traces were recorded exclusively on oysters, no taxon selectivity was found. No differences were found between the size of bored versus intact valves. The traces, although mostly recorded on left valves, showed a random distribution within valves, indicating no size or site selectivity. Hole morphology, absence of attachment scars and multiple holes point to a predatory origin. The lack of stereotypy indicates a generalist or facultative producer. Similar traces are usually ascribed to gastropod producers (i.e., naticids or muricids), but the gastropods co-occurring with the studied oysters lack any extant relative with a drilling apparatus. Hence, the producer remains unknown. This record contributes to the filling of the so-called early Mesozoic gap in drilling predation evolutionary history, during which two predation trends seem to have co-occurred: a background type of predation, carried out by generalists or facultative predators, and the rise of highly specialized predators, associated with the emergence of naticids and muricids