A microscale analysis of Bolonia from the Cretaceous of Patagonia sheds light on an enigmatic tracemaker

GIACHETTI, LUCIANA M.; FERNÁNDEZ, DIANA ELIZABETH; COMERIO, MARCOS; GUTIÉRREZ, CAROLINA; HEREDIA, ARTURO MIGUEL; PAZOS, PABLO JOSÉ

Congreso; International Ichnofabric Workshop 16A (virtual); 2021

Bolonia Meunier has been recorded from positive epireliefs in marine deposits from the Ordovician to Eocene of both hemispheres. This ichnotaxon is characterized by two lobes with biserial pads and a heart-shaped cross-section. For many years, this name did not prevail and trace fossils that should have been assigned to Bolonia were assigned to other ichnogenera (e.g., Scolicia, Protovirgularia). The last revision of this ichnogenus was made by Schlirf (2002), who retained the ichnotaxon as valid and emended the diagnosis allowing a clear differentiation from the previously mentioned ichnogenera. Gastropods, polychaetes and irregular echinoids have been suggested as tracemakers. Recently, Bolonia was found in Lower Cretaceous deposits of the Agrio Formation in the Neuquén Basin, Argentina. The specimens were documented in sandstones interbedded with siltstones/claystones and associated with wave and current ripples (including interference ripples) indicating shallow-marine deposition above wave base. Samples of Bolonia were analyzed microscopically through serial thin sections transverse and parallel to the bedding plane (Fig. 1.A). For the first time, the internal structure of Bolonia from one selected sample with high lithological contrast and exceptional preservation is described herein.In transverse section, the outline of the trace fossil is heart-shaped; the sediment has mostly been reoriented, with grains forming subcircular pods (Fig. 1.B). A conical structure of variable width extends centrally from the base to the top of the trace. This central structure is dominated by fine-grained particles associated with organic matter and with no preferential orientation. The area outside the trace fossil is characterized by parallel lamination, with laminae dominated by silt to fine sand interbedded with laminae dominated by organic matter and clay minerals; no reorientation of grains is observed. In contact with the trace fossil, the lamination is slightly deformed and deviates towards its base. In horizontal thin sections, grains are reoriented and reworked forming a central zigzag structure (Fig. 1.C). Next to the central structure, reoriented grains mostly compose linear alignments, though subcircular alignments are common at the trace fossil?s margin. In some cases, the organic matter and clay concentrates form grain aggregates that represent the internal structure of individual pads of the trace fossil.Many authors have associated Bolonia with Bichordites and Scolicia, which are produced by echinoids. The taphonomic processes of trace fossils produced by echinoids have been analyzed by Gibert and Goldring (2008), and they proposed irregular echinoids as the producers of Bolonia, an idea also suggested by Seilacher (2007). Unlike Scolicia and Bichordites, Bolonia does not present drain tubes (structures associated with sanitary tunnels produced by echinoids during burial), and these authors suggested this could be explained by the the lack of early cementation and the consequent collapse of such drain tube. Furthermore, neoichnological experiments have proved that in poorly lithified sediments the drain tubes collapse and therefore, they cannot be recognized (Kanazawa, 1995). The central conical structure of our samples is interpreted as the collapse of the medial zone of the trace fossil, which could have destroyed structures such as medial drain tubes, especially in absence of early cementation. The excellent preservation of the biserial menisci is explained by the granulometric contrast. The seriated horizontal sections allowed reconstruction of the diagnostic biserial pads as biserial concave structures. These biserial menisci are interpreted as backfilling structures. Both Scolicia and Bichordites also present biserial menisci, inferred as backfilling lamination (preservational variant ?Laminites?). The direction of movement of echinoids during burrowing is reflected by the concavity of these menisci. Therefore, the direction of movement of the tracemaker of Bolonia can be inferred both in the sections and the external view of the pads. The biserial pads show different angles between their maximum axes along the same specimen, as the tracemaker changed in depth within the sediment. Taking into account the complete study of serial horizontal sections, it is possible to observe that the change in angle of the pads/menisci is gradual, and they should be produced by particular structures moving simultaneously. When compared with the information gathered from neoichnological experiments with irregular echinoids, we infer such structures would be the aboral and lateral spines, which work together compacting the sediment antero-posteriorly; they move in an alternate way, which explains the zigzag pattern in the menisci. Also, the restriction of movement that each spine or groups of spines possess would explain the sub-circular alignments observed in thin section. Considering these results, we propose Spatangoidea or another irregular echinoid group with similar burrowing ability and spine movement as tracemakers of the analyzed Bolonia samples, supporting the hypothesis of previous authors.ReferencesGibert, J.M. de, and Goldring, R. 2008. Spatangoid-produced ichnofabrics (Bateig Limestone, Miocene, Spain) and the preservation of spatangoid trace fossils. Palaeogeography, Palaeoclimatology, Palaeoecology, 270: 299-310.Kanazawa, K. 1995. How spatangoids produce their traces: relationship between burrowing mechanism and trace structure. Lethaia, 28: 211?219.Schlirf, M. 2002. Taxonomic reassessment of Bolonia Meunier, 1886 (trace fossil) based on new material from the type area in Boulonnais, northern France. Paläontologische Zeitschrift, 76 (2): 331?338.Seilacher, A. 2007. Trace Fossil Analysis, Springer-Verlag, Berlin Heidelberg, 226 pp.