PRESERVATION OF CUTICLE OF PROTAXIUS ISOCHELA (WOODWARD) (AXIIDEA, AXIIDAE) FROM THE AGRIO FORMATION (LOWER CRETACEOUS), NEUQUÉN BASIN, WEST-CENTRAL ARGENTINA

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

Washington, D.C.

Congreso; Ninth International Crustacean Congress; 2018

The cuticle of four specimens of Protaxius isochela (Woodward) (Axiidea, Axiidae) contained in ellipsoidal carbonate concretions was studied with the objective of knowing its composition and ultrastructure. They come from a thin interval of fine sandstones of the marine mixed platform of the Agrio Formation (early Hauterivian). SEM and EDS analysis were performed on two of the specimens, 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 small pyrite crystals were observed. Thin sections of the two other concretions exposed different parts of two 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 are established in irregular layers without a defined pattern. Phosphate layers can disappear laterally being replaced by calcium carbonate layers. Cuticle preservation probably included immediate post-mortem phosphatization of the cuticle, then shallow entombment of the specimens closing the phosphatization window, anaerobic decay and pyrite precipitation, and finally precipitation of abundant carbonate cement around the specimens. These processes occurred in the early diagenesis, in different microenvironments generated inside the specimens during decay and early burial. The lamination preserved in the endocuticle and the presence of pyrite crystals indicate that the studied specimens were probably carcasses remains and not moults, whose preservation were enhaced by early diagenetic processes and early burial. This is contribution R-249 of IDEAN.