Ichnotaxonomical and age-related implications of a recent, detailed microscale analysis of Bolonia from the Cretaceous of Patagonia and its proposed tracemaker

FERNÁNDEZ, DIANA E.; GIACHETTI, L. M.; PAZOS, PABLO J.; GUTIÉRREZ, CAROLINA; ARAMBARRI, GABRIEL A.

Congreso; 6th International Palaeontological Congress; 2022

Bolonia Meunier is a monotypic ichnogenus that includes horizontal to subhorizontal, epichnial, non-branching, occasionally tapering at the ends, bilobed structures. Its outline in cross-section is trapezoidal to heart-shaped (Meunier, 1886; Schlirf, 2002). For many years, this name did not prevail, and trace fossils that should have been named Bolonia were instead assigned to Scolicia Quatrefages, Protovirgularia McCoy, Bichordites Plaziat and Mahmoudi, etc. The last revision of Bolonia retained the ichnotaxon as valid, amended the diagnosis allowing a clear differentiation with the mentioned ichnogenera, and re-located some Paleozoic material to Bolonia (Schlirf, 2002). It has since been considered that this ichnotaxon is recorded from the Ordovician to the Eocene in shallow marine deposits of both hemispheres. Gastropods, polychaetes and irregular echinoids have been suggested as tracemakers. Recently, an in-depth macro and microscale analysis of Bolonia samples from the Lower Cretaceous of Neuquén Basin (Patagonia) supported the hypothesis of previous authors that this ichnogenus is produced by irregular echinoids (Giachetti et al., 2021). In the diagnosis amendment, Schlirf (2002) mentioned that the “top of (the) trace fossil rarely shows an elliptical depression, which is slightly wider than the rest of the burrow, in respect to the long axis, and lacks distinct pads”. In the light of the recent findings on the tracemaker affinities, it is clear that those structures are assignable to Cardioichnus Smith and Crimes, echinoid resting trace fossils echinoids, which are not uncommon to find in association with other ichnotaxa known to have been produced by these animals. The cases described by Schlirf (2002), and also found in our material, are therefore examples of compound trace fossils (Bolonia-Cardioichnus-Bolonia), and the diagnosis of Bolonia should be modified accordingly. The fossil record of irregular echinoids begins in the Jurassic. Therefore, the recent proposal of tracemaker does not apply to pre-Jurassic samples currently assigned to Bolonia. Nevertheless, in the published photographs and descriptions of these Paleozoic specimens, the diagnostic characters of Bolonia are absent, or the observed characteristics differ from those expected in Bolonia. We consider that pre-Jurassic samples assigned Bolonia should be revised.