First Jurassic brittlestar from Neuquén Basin, Argentina

CAMPETELLA M.D.; PALÓPOLO E.E.; RODRÍGUEZ, MAXIMILIANO NICOLÁS; THUY B.; PONCE, J.J.; CARMONA, N.B.; CASADIO, S.

ACTA PALAEONTOLOGICA POLONICA

INST PALEOBIOLOGII PAN

Lugar: Varsovia; Año: 2023

0567-7920

Articulated fossil ophiuroids from South America were reported for the Devonian,Cretaceous, Eocene, and Miocene. Here we report the first Jurassic record of an articulatedophiuroid from the Sierra Chacaicó Formation (early Pliensbachian–Sinemurian) in NeuquénBasin, Argentina, and discuss the taphonomic processes that allowed its preservation. TheSierra Chacaicó Formation represents the onset of the Early Jurassic extensive marinetransgression in the basin. The basal section comprises shoreface and offshore Gilbert-typedelta system, which was affected by hyperpycnal discharges. The middle and upper sectionsare represented by offshore deposits, affected by storms and eroded by hyperpycnal channel-levee systems. The ophiuroid specimen was found in levels of massive, fine, tuffaceoussandstone beds and covered by coarse sandstone containing a large amount of plant debrisand organic matter. It was preserved, articulated, with a complete disc and almost completearms. Based on the microstructure of the spine-bearing lateral arm plates, the ophiuroid isassigned to Sinosura, an extinct genus of the family Ophioleucidae, widespread in the LowerJurassic deposits of Europe but previously unknown from other parts of the world. Theposture of the ophiuroid, with one arm curved distally and extended in one direction and theother four arms symmetrically oriented in the opposite direction, suggests a walking orescape movement frozen in time. This implies that the ophiuroid was buried alive bysediment thick enough to prevent successful escape. The taphonomic and sedimentologicevidence indicates that the fossil material was found in hyperpycnal deposits accumulated inoffshore positions, which carried a high concentration of sediment in suspension.