A micromorphological analysis of Bolonia lata Meunier from the Cretaceous of the Neuquén Basin (Patagonia, Argentina): new insights on the tracemaker

GIACHETTI, LUCIANA M.; FERNANDEZ D.E,; COMERIO, MARCOS A.; GUTIERREZ CAROLINA; PABLO J. PAZOS

SPECIAL PAPERS IN PALAEONTOLOGY SERIES

PALAEONTOLOGICAL ASSOC

Lugar: Londres; Año: 2023

0038-6804

Polychaetes, echinoids, andgastropods have been proposed as tracemakers for Bolonia Meunier, a trace fossil characterized by two lobes composedof biserial, subtriangular pads and a mostly heart-shaped cross-section. Here,the internal structure of Bolonia isdescribed for the first time from shallow-marine Lower Cretaceous intervals ofthe Agrio Formation (Neuquén Basin, Argentina). In transversal thin sections, aconical structure of variable width extends centrally from the base to the topof the trace. In horizontal thin sections, grains are reoriented and reworked,forming a central zigzag structure. The organic matter and clays form grainaggregates representing the internal structure of individual pads of the tracefossil. The sediment has mostly been reoriented, with grains formingsubcircular pods. We interpret the central conical structure as the collapse ofthe medial zone of the trace fossil, which could have destroyed drain tubes,especially in absence of early cementation. Studying horizontal thin sections,we reconstruct the diagnostic biserial pads as biserial concave structures.These are interpreted as backfilling structures produced by the aboral andlateral spines of irregular echinoids, which work together compacting thesediment anteroposteriorly and moving alternatively, explaining the zigzagpattern in the menisci. We propose Spatangoidea or another irregular echinoidgroup (e.g., Cassiduloidea) with similar burrowing ability and spine movementas tracemakers of the analyzed Bolonia samples.Additionally, we provide a valid ichnotaxobase to compare Bolonia with Bichordites insteadof size, as presented in the last revision of Bolonia.