Una propuesta preliminar sobre la distribución estratigráfica de las faunas triásicas de insectos y conchostracos del extremo sur de América del Sur

GALLEGO, O.F.; MARTINS-NETO, R.G.

Resistencia

Otro; Reunión de Comunicaciones Científicas y Tecnológicas; 2006

In this work the authors present a preliminary and tentative approach on the stratigraphical distribution of the non-marine Triassic fossil insects and conchostracans (crustaceans). The knowledge of the South American Triassic paleoentomofauna has increased in the last years (Martins-Neto et al., 2003). Besides, the Triassic insect record from Argentina, the South American record is very poorly known, contrasting with at least two of the richest insect faunas from the Gondwana ones, Ipswich Coal Measures (Australia), and Molteno Formation (South Africa). Many problems exist for dating and correlation of the Triassic continental sedimentary rocks in South America. The paleoentomofauna furnishes another probable and alternative point of view. Typical Triassic insect groups as the Family Dyrmorphoptilidae (Hemiptera) have resulted in notably similar stratigraphic distribution pattern with the Coleoptera. Adding other known Argentinean Triassic insects taxa, the consistency of the distribution is remarkable. This indicates that stratigraphically, at least in partly, the fauna from the upper unit of the Los Rastros Formation (Gualo) is the same of that from the lower section of the Cacheuta Formation (Uspallata). Also, the insect fauna from the middle to upper third of the Ischichuca Formation (Quebrada de Ischichuca Chica) and from the lower section of the Santa Juana Formation (Bio-Bío river, Chile) are similar with the mentioned above. These data are partially coincident with the palynological ones, referred to the chronostratigraphy of those units (Morel et al., 2001). On the other hand, the fauna from the lower part of the Los Rastros Formation (Los Chañares) is more different (and probably older) than the mentioned above. The Potrerillos Formation insect fauna (lower as well as the upper unit) is the most different (and may be the oldest) (as proposed for these units in Morel et al., 2001). The stratigraphic distribution of the conchostracan fauna is not totally coincident with that from insects. Also, the Eosestheriidae and Polygraptidae conchostracan faunas have a similar distribution from the La Ternera Formation, the La Coipa Beds, and the Santa Juana Formation (all from Chile) with the Vera Formation (Los Menucos group) (Gallego et al., 2005). The Ulugkemiidae, Loxomegaglyptidae and Euestheriidae faunas are distributed from the upper section of the Potrerillos Formation to the lower section of the Cacheuta Formation (the first two families from Uspallata and the third from Cerro Cacheuta) and across the Ischichuca to the Los Rastros formations. So, some insect taxa are specially useful for local and regional correlations, as is the case of the genus Ademosyne (having a wide distribution in other Gondwanic localities), and other genera within the Coleoptera as well as Gallegomorphoptila (Hemiptera Gallegomorphoptilinae). Nevertheless, the conchostracans taxa are only useful for local correlation, due to their endemic condition. Otherwise, the species Euestheria forbesi, E. minuta and E. mangaliensis are also recorded from the Cuyo Basin (Potrerillos and Cacheuta formations), the Triassic from Africa, India and the Australian Carnarvon Basin (Blina Shale Formation, Lower Triassic). The Molteno Fomation and Ipswich Coal Measures conchostracan faunas are completely different to the Argentinean and Chilean ones. In correlation terms, the Dysmorphoptilidae (Hemiptera), Scytinomorpha (Hemiptera), Polycitellidae (Jurinina), and Permosynidae (Coleoptera) are indicating that the Potrerillos and Los Rastros formations are closely correlated with the Ipswich Coal Measures (Australia), and positioning at the lower Carnian (after Anderson and Anderson, 1993). On the other hand, both faunas are not directly correlated with the Molteno Formation (Africa), during at least the middle Carnian, (Anderson and Anderson, 1993).   Anderson, J.M., and Anderson, H.M., 1993, Terrestrial flora and fauna of the Gondwana Triassic: Part 1. Occurrences: in Lucas, S.G., and Morales, M., eds., The Nonmarine Triasic: Bulletin, New Mexico Museum of Natural History and Sciences, v. 3, p. 3-12. Gallego, O.F., Martins-Neto, R.G., and Nielsen, S.S., 2005, Conchostracans and Insects from the Upper Triassic of the Bio-Bío River (Santa Juana Formation), from southern Chile: Revista Geológica de Chile, v. 32(2), 293-311. Martins-Neto, R.G., Gallego, O.F., and Melchor, R.N., 2003, The Triassic insect fauna from South America (Brazil, Argentina and Chile): a checklist (except Blattoptera and Coleoptera) and descriptions of new taxa: Acta Zoologica Cracoviensia, v. 46 (suppl.), p. 229-256. Morel, E.M., Artabe, A.E., Zavattieri, A.M., and Bonaparte, J.F., 2001, Cronología del Sistema Triásico: in Artabe, A.E., Morel, E.M., and    Zamuner, A.B., eds., El Sistema Triásico en la Argentina: Fundación Museo de La Plata ``Francisco Pascasio Moreno``, La Plata, p. 227-253.