Exceptional preservation of decapod Crustacea from the Lower Cretaceous of the Neuquén Basin, Argentina

AGUIRRE-URRETA, M.B., LAZO, D.G. & RAWSON, P.F.

Ankara

Simposio; 9Th International Symposium on the Cretaceous System; 2013

Middle East Technical University

The Neuquén Basin (34° - 39°30?S), located in the eastern foothills of the southern Andes in west-central Argentina, is an important Meso-Cenozoic depocentre forming a large marine embayment. This embayment was connected to the Pacific Ocean to the west through an active volcanic arc. From the Late Jurassic to the Early Cretaceous more than 2,000 m of mostly marine sedimentary rocks accumulated in the basin. In particular, the Agrio Formation represents mixed clastic-carbonate sedimentation in shoreface to offshore palaeoenvironments on a storm-dominated, shallow-marine ramp, spanning from the late Early Valanginian to the latest Hauterivian. The Agrio Formation has an abundant fossil record composed of ammonoids, nautilids, gastropods, bivalves, serpulids, and corals; bryozoans and echinoids are locally abundant while decapod crustaceans occur sporadically throughout the formation. They are represented by body fossils such as carcasses and isolated claws but also by burrowing traces. We report here exceptionally preserved specimens of the palinurid Astacodes falcifer Bell recovered from dark grey shales in the uppermost part of the Agrio Formation. Most specimens consist of still-articulated hard parts, including articulated cephalotorax, abdomen, base of pereiopods and tail fan and remarkably some of them show compound eyes with mineralized cuticle. The eyes are large, nearly circular in shape and are attached to a thickly sclerotized peduncle. The eye surface preserves numerous square facets of the same size, and the vision can be described as superposition optics. Most carcasses are enclosed in calcareous nodules while fragmentary parts of carapaces, usually poorly preserved, may represent remains of exuviae. The excellent preservation of the lobsters, with articulated carapaces and their enclosure within cylindrical nodules reminiscent of decapod burrow systems indicates that they probably died within their burrows, and remained there during the earliest stages of fossilization. Energy Dispersive Analysis X-Ray shows that the stalked eyes are mineralized by calcium phosphate; the phosphatization of such delicate compound eyes requires very rapid, almost in vivo, mineralization, previous to burial and indicates that diagenesis occurred prior to significant decay.