Triassic/Jurassic bivalve extinction and recovery in the Neuquén Basin, Argentina

DAMBORENEA, S.E.; ECHEVARRÍA, J.; ROS-FRANCH, S.

San Luis Potosí

Congreso; 10th International Congress on the Jurassic System; 2018

International Commission on Stratigraphy, IUGS

The endTriassic extinction event is one of the ?big five? global crisis in the historyof life in the marine realm. Nevertheless, that extinction and the subsequentbiotic recovery are not so well known as others, and most of the publishedanalyses were based on data from the Northern Hemisphere. Bivalves are one ofthe best studied groups in relation to the recovery after the end-Triassicextinction event. We analyze the Late Triassic extinction and Early Jurassic recoveryof bivalve faunas within marine environments in the Atuel river area of theNeuquén Basin, Argentina (Riccardi et al., 1988; Lanés, 2005). The nearlycontinuous presence of marine estenohaline major taxa such as cnidarians,rhynchonelliform brachiopods, echinoderms and cephalopods indicate normalsalinity throughout (Damborenea & Mancenido, 2005). Data were collectedfrom a thick and exceptionally well-exposed latest Triassic-earliest Jurassicsection of the Andes, which allows a high-resolution reconstruction of thelocal diversity dynamics. Four phasescan be clearly distinguished on the basis of the analysis of bivalve diversitythrough time from Rhaetian to Early Sinemurian (Fig.), each characterized bythe relative relationships between regression lines of cumulative first andlast appearance data (FADs and LADs respectively) for each recorded speciesagainst section thickness: a) Triassic equilibrium phase (Rhaetian), b) extinctionphase (late Rhaetian?), followed by a long interval with no recorded benthonicfauna (Early Hettangian), c) recovery phase (Middle to early Late Hettangian)and d) Jurassic equilibrium phase (from latest Hettangian). The bivalve faunarecovery was relatively rapid, within the Middle and lowermost Late Hettangian. Thetaxonomic composition analysis through time (at the generic level) suggeststhat the recovery was mainly triggered by immigration into the basin of widelydistributed genera, and the origination of new taxa was restricted. Bivalvepalaeoecologic diversity seems to have been fairly homogeneous along thesection, being dominated most of the time (after extinction) by attachedepifaunal bivalves. Strikingly, since the abrupt mid-Hettangian diversification,relative diversity of the different main life habits shows little variation,though some minor trends could be identified. One main difference betweenTriassic and Jurassic faunas is the abundance (both relative and absolute) ofshallow burrowers, more frequent during the Rhaetian than on subsequent stages.This new set of local data can be compared with information from otherlatitudes and contribute to future global analyses.