Exceptional preservation of organic periostracum in a Valanginian ammonoid from the Neuquén Basin, central Andes of Argentina

AGUIRRE-URRETA, BEATRIZ; MARIN, L.S.; LAZO, D.G.; TUNIK, M.; PARADA, M.; VENNARI, VERÓNICA V.

Londres

Simposio; 11th International Symposium on cephalopods present and past; 2022

Paleontological Association

We report the exceptional preservation of organic periostracum in multiple specimens of Bochianites neocomiensis (d´Orbigny), a heteromorph ammonoid from the Valanginian of the Neuquén Basin, in the central Andes of Argentina. The fossils were recovered from the Vaca Muerta Formation, a monotonous succession of interbedded mudstones and wackestones. The level with the fossils is a micritic mudstone with abundant nannofossils, calcispheres, scarce foraminifera and undifferentiated fragmented bioclasts. There are only impressions of these ammonoids, and macroscopically it seems that the calcareous layers of the shell are not preserved. The periostracum is preserved as an organic film. Normally coiled ammonoids recovered from the same level are always impressions. The periostracum of Bochianites neocomiensis is dark to amber in color and generally peels off as soon as the surface of the slab with the fossil is exposed. Under the Scanning Electron Microscopy (SEM) the external surface looks smooth, and the thickness is around 2 μm. Preliminary energy dispersive X-ray analysis (EDAX) shows a high percentage of C (65.8%) with lesser amount of O (21.5%), Si (5.6), Al (2.9%) and Fe (2.9%). A combination of bottom entombment in fine grained carbonate sediment, early cementation, anoxia, and sulfate reduction may seem to have played a role in the exceptional preservation of the still soft outer layer of the periostracum.