Preservation of cuticle of Protaxius isochela Woodward (Axiidea, Axiidae) from the Agrio Formation (Lower Cretaceous), Neuquén Basin, west-central Argentina

LAZO, DARÍO G.; ANDRADA, ALEJANDRA M.; BRESSAN, GRACIELA S.

Washington D.C.

Congreso; 9th International Crustacean Congress; 2018

Smithsonian Institution

The cuticle of four specimens of Protaxius isochela (Woodward) (Axiidea, Axiidae) contained inellipsoidal carbonate concretions was studied with the objective of knowing its composition andultrastructure. They come from a thin interval of fine sandstones of the marine mixed platform ofthe Agrio Formation (early Hauterivian). SEM and EDS analysis were performed on two of thespecimens, showing an outer smooth surface and revealing presence of massive calcium phosphate,while the composition of the inner part of the propodus was calcium carbonate and isolated smallpyrite crystals were observed. Thin sections of the two other concretions exposed different parts oftwo specimens in cross section. Lamination of the epicuticle and exocuticle could not be identified,while lamination of the endocuticle was clearly observed. Phosphate and calcium carbonate areestablished in irregular layers without a defined pattern. Phosphate layers can disappear laterallybeing replaced by calcium carbonate layers. Cuticle preservation probably included immediate postmortemphosphatization of the cuticle, then shallow entombment of the specimens closing thephosphatization window, anaerobic decay and pyrite precipitation, and finally precipitation ofabundant carbonate cement around the specimens. These processes occurred in the earlydiagenesis, in different microenvironments generated inside the specimens during decay and earlyburial. The lamination preserved in the endocuticle and the presence of pyrite crystals indicate thatthe studied specimens were probably carcasses remains and not moults, whose preservation wereenhaced by early diagenetic processes and early burial.