Palaecology of Spinicaudatans from the Cañadón Asfalto Formation (Jurassic), Argentina.

MONFERRÁN, M. D.; GALLEGO, OSCAR F.; CABALERI, NORA

Mendoza

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS. The history of life: A view from the Southern Hemisphere September; 2014

Spinicaudatans (?conchostracans?) or ?clam shrimps? are small branchiopod crustaceans that inhabit ephemeral, warm freshwater habitats with pH conditions from neutral to alkaline. They are easily recognizable by their short, laterally compressed bodies. However, their paleoecology has been poorly analyzed, and more work must be done to better understand the relationship between spinicaudatans and their environment. Therefore, our aim is to contribute to their paleoecological study and their importance. The Spinicaudatan fauna is present in the Cañadón Asfalto Basin, located in the extra-Andean region of the Chubut province (Patagonia, Argentina). The outcrops represent the most complete volcano-sedimentary continental Jurassic sequences of South America. Spinicaudatans are the most abundant invertebrate group recorded from the Cañadón Asfalto Formation and are present in both members (Las Chacritas and Puesto Almada). The methodology was based in the identification of species, the measurement of morphological parameters and the study of sedimentary characteristics in several localities. The Las Chacritas Member fauna is composed by the families Euestheriidae and Eosestheriidae, characterized by large (4.5?8.5 mm long) spinicaudatans such as Euestheria volkheimeri Tasch and Volkheimer, Lioestheria patagoniensis Tasch and Volkheimer and Euestheria taschi Vallati. These spinicaudatans are associated with lacustrine facies, interbedded with pyroclastic deposits and basalt flows. The lacustrine systems were the principal environment for the development of larger species/populations, although many levels show smaller individuals with narrow growth band and high growth line densities. In some levels the ?clam shrimps? are associated with ostracods and mollusks. On the other hand, the Spinicaudatan fauna from the Puesto Almada Member is composed by small forms (1-5 mm long) such as Congestheriella rauhuti Gallego et al. and Wolfestheria smekali Monferran et al., where they are recorded in shallow, ephemeral, and shoreline water bodies occasionally with high-energy sedimentations. These species show lower density and wide few growth bands and moreover C. rauhuti appears in association with fishes which are their natural predators. In conclusion, the specimens present in each of the two members differ mainly in species and dimensions, which are probably related to environmental or ecological features. The populations of ?clam shrimps? were regulated by favorable (wide few growth bands) or unfavorable (narrow growth band) conditions as well as predators influence (fishes). The spinicaudatans played an important role in Jurassic ecosystems of Argentina and these results should provide important evidence to build upon for further studies on fossil ?conchostracans?.