The first record of ostracods in the lowermost Tremadocian.

SALAS, MARIA JOSE; VACCARI, EMILIO

Brasilia

Simposio; 16 ISO- International Symposium on Ostracoda; 2009

Despite the abundance of ostracods in the fossil record, their origin remains enigmatic. The first appearance of accepted ostracods is in the middle Tremadocian (Paltodus deltifer Zone). However, this fauna is relatively diverse (Palaeocopas, Binodicopas and possibly other groups are present) and geographically widespread, suggesting a Cambrian or at least early Tremadocian origin for the group. A recent finding in the western Puna region (Northwest of Argentina) contributes to the knowledge of the early stage of the group. New material, assignable to ostracods, was found in the Las Vicuñas Formation (Furongian-lower Tremadoc), associated with the world widespread trilobite Jujuyaspis, indicating a lowermost Tremadoc age. The Las Vicuñas Formation, 200 m thick, consists of pyroclastic rocks interbedded with siltstone and calcareous shell beds. The material is still limited (only a few valves), and its morphology is very simple and featureless. However, the morphological similarity with the carapace of the post tremadocian ostracods led us to assign it to the group. The new material has a calcium carbonate carapace. The outline of the valves is elliptic and amplete. The maximum length of the valves varies from 0.87 to 0.97 mm. The dorsal margin is straight and long. The ventral and lateral ones are rounded. The lateral surface is evenly convex and smooth. Though it is too early to discuss about the meaning of the finding with regard to the origin of the ostracods, the record of these forms in the Cambrian-Ordovician transition adds important data that would help to resolve the question about the early development of the group.