A new echinoderm fauna from early Tremadocian (Lower Ordovician) of Jujuy province, Argentina

LEFEBVRE, B.; MUÑOZ, D.; NOHEJLOVA, M.; VAUCHER, R.; WAISFELD, B.G.; NARDIN, E.; VACCARI, N.E.

Nagoya

Conferencia; 16th International Echinoderm Conference; 2018; 2018

The Furongian?Floian interval represents a key evolutionary transition with the onset of the Great Ordovician Biodiversification Event (GOBE), soon after the Cambrian Explosion. Few Furongian echinoderm faunas are known worldwide, and all of them consist of low diversity assemblages comprising edrioasteroids, eocrinoids, rhombiferans, solutans, and/or stylophorans. The early?middle Tremadocian fossil record of echinoderms is particularly poor. In marked contrast, several abundant and diverse echinoderm faunas are known from the late Tremadocian?Floian interval, which documents the early diversification of many echinoderm clades (e.g. asterozoans, crinoids). In this context, the recent discovery of an early Tremadocian echinoderm fauna in northwest Argentina (Jujuy Province) brings critical new information on this major evolutionary transition.The echinoderm material was collected in siliciclastic deposits (siltstones withintercalations of sandy layers) belonging to the Upper Member of the Guayoc Chico Group (GCG). An early Tremadocian age was suggested for these levels, based on conodonts and graptolites. Associated sedimentary structures suggest lower shoreface to offshore transition environments influenced by storms, between the fair weather-wave base and the storm wave base. Fossil remains occur exclusively in sandstone layers, interpreted as storm deposits. They seem to be included in the waxing part of the backflow induced by storm activities, thussuggesting a fast and in situ burial. The echinoderm assemblage collected in the Upper Member of the GCG is dominated by eocrinoids and stylophorans. Eocrinoids belong to a new genus. They are exquisitely preserved, with fully articulated thecae, stems and brachioles. Their stems show a unique organization, with an irregular alternation of two different types of columnals. Stylophorans are represented by at least two different taxa. The first one probably belongs to a new species of Thoralicystis (a genus of typical Early?Middle Ordovician scotiaecystid cornutes). The morphology of the second one is not so well preserved, but it suggests some possible affinities with Ponticulocarpus (a primitive cornute previously known only from Cambrian Series 3).