Abstract

SORRENTINO, Leonor; BENEDETTO, Juan L  and  CARRERA, Marcelo G. Brachiopod taxonomic diversity and morphotype distribution in the Ahtiella Argentina Zone (Middle Ordovician), San Juan Formation, Argentina. Ameghiniana [online]. 2009, vol.46, n.3, pp.481-493. ISSN 1851-8044.

The upper part of the San Juan Formation in the Cerro Viejo area (NE of Jáchal city) was deposited in a middle to distal carbonate ramp setting during the lower Darriwilian. This interval, c. 24 meters thick, shows a general deepening upwards trend, ranging from bioclastic packstones-wackestones in the base to nodular mudstones in the upper part. Most of the fossiliferous levels have yielded rich rhynchonelliformean brachiopod assemblages. The dominant forms are the plectambonitoids Ahtiella argentina (the index fossil of the biozone), Leptella (Petroria) rugosa, Taffia (Chaloupskia) anomala, and the orthoids Paralenorthis vulgaris and Orthidium geniculatum. Other taxa, such as Productorthis cienagaensis, Martellia mesocosta, Inversella (Reinversella) arancibiai and Sanjuanella plicata are less common and display more restricted stratigraphic ranges. In order to evaluate the correlation between the mode of life of brachiopods and lithofacies (reflecting a set of environmental parameters), species were grouped into four general morphotypes: pedunculate erect, pedunculate recumbent, semi-infaunal, and liberosessile. The umbo-down, permanent-attached pedunculate species are distributed throughout the section, whereas liberosessile, semi-infaunal and recumbent forms are dominant or common in the nodular mudstones and wackestones. This implies that the low-attached permanent pedunculate forms (i.e Paralenorthis) were more ubiquitous ecologically than the liberosessile and semi infaunal forms, which inhabited preferably soft-bottom, low-energy substrates. Overall brachiopod species richness attains higher values in the nodular mudstone levels, which were deposited during a high stand sea level episode. Such an increase of richness may have resulted from a pulse of immigration coupled with newly evolved species during this interval.

Keywords : Diversity; Morphotypes; Brachiopods; Ordovician; Precordillera Argentina.