The Subcommission on Ordovician Stratigraphy (ICS-IUGS) has voted to define the base of the global Middle Ordovician Series on the first appearance of the conodont Tripodus laevis (=T. combsi); however, the real value of this taxon to allow for that purpose was recently discussed, considering some difficulties on the record of its temporal and spatial (North American endemic?) distributions.

The Argentine Precordillera is distinguished by a superb Lower-Middle Ordovician carbonate sequence that yielded significant conodont faunas, as well as fossil invertebrates. Several sections of the San Juan Formation (on-average 320 m thick) have been resampled in detail, throughout the critical interval spanning the Ibexian-Whiterockian boundary; among these, Peña Sombría, Los Sapitos, Portezuelo Yanso, La Silla, Niquivil and Talacasto, are chosen to complement lateral environmental settings. We recommend the Niquivil section, which has all of the attributes required by a global series and stage stratotype, including interbedded K-bentonite levels, that allow absolute dating.

Subtidal lithotopes and open marine biotic associations prevailed during deposition of the San Juan Formation. Communities are dominated by brachiopods and sponges, while trilobites, echinoderms and gastropods are also abundant components. At Niquivil, the wackestones dominated boundary interval is capped by stromatoporoid-lithistid-algal reef-mounds, which represent the onset of a second transgressive sequence.

About 18 m below the base of the reefal structure, the first appearance of the conodont Texania heligma Pohler is registered, and it is herein proposed as alternative index species for the base of the global Middle Ordovician Series. At same level, just above the last occurrence of Oepikodus evae, the rich conodont association of the O. intermedius Zone includes last records of O. communis. The FAD of T. heligma occurs within the Isograptus victoriae lunatus Zone in Newfoundland, where the base of the series has been correlated for North America. In the Precordillera, first graptolite records correspond to the subsequent I. v. maximus Zone.

Paleoecological studies in the Niquivil section revealed changes in community structures through the reference level, as they were documented elsewhere, after the Late Ibexian demise.