Evolution of Mesozoic-Cenozoic terrestrial ichnofaunas from southern South America

JORGE FERNANDO GENISE; RICARDO NESTOR MELCHOR; EDUARDO SERGIO BELLOSI; MARÍA VICTORIA SÁNCHEZ; JAVIER MARCELO KRAUSE; LAURA SARZETTI; MARIANO VERDE; EMILIO BEDATOU

Cracovia

Congreso; Ichnia 2008; 2008

Jagiellonian University, Polonia

Southem South America, and particularly Patagonia, contains a protracted record of continental deposits bearing diverse invertebrate ichnofossils. The study of these deposits, mostly composed by primary and recycledvolcaniclastic materials, and ranging in age from Jurassic to Miocene, allows to trace the evolution of Mesozoic-Cenozoic terrestrial ichnofaunas. The Late Jurassic to Early Cretaceous Bajo Grande Formation and Baqueró Group contain similar ichnofabrics dominated by large vertical, lined "Y´ burrows (Loloichnus) and meniscate burrows of similar diameter amibuted to crayfish producers. This ichnofabric may he overprinted by a boxwork of unlined, small burrows (sometimes meniscated), and rare chambers, which is assigned to the work of earthworms. A less common ichnofabric from these deposits are the first known crayfish nesting or recruitment trace fossils(Dagnichnus and Cellicalichnus). During the Late Cretaceous, as exemplified from the Bajo Barreal and Laguna Palacios Formations, the crayfish and earthworm ichnofabrics continue to be important and well developed. In addition,the first insect trace fossils appear, including pupal chambers (Rebuffoichnus, Pallichnus) and bee nests(Cellicalichnus). The K-T event produced a change from crayfish - earthworm dominated ichnofaunas to insect-dominated ones. The late Paleocene-middle Eocene Rio Chico Group shows few trace fossils, probably due totaphonomic causes: Eatonichnus (brood structure of coprophagous beetles), Feoichnus (cicada feeding chambers),and rare occurrences of the earthworm ichnofabric. In contrast witb the paucity of trace fossils in paleosols, contemporaneous early to middle Eocene floras from Laguna del Hunco and Rio Pichileufi display a large variety of insect traces in leaves. These include leaf-cutting, mining, galls, bite marks and oviposition traces, suggesting alarge insect diversity that parallels the enormous diversity of plants. The probably coetaneous paleosols of theAsencio Formation from Uruguay (early Eocene) also display a large diversity of insect ichnofossils, essentiallydominated by bee and coleopteran trace fossils (Coprinisphaera, Celliforma, Corimbatichnus, Ellipsoideichnus,Palmiraichnus, Teisseirel, Uruguay, Rebuffoichnus and Monesichnus). The producers of this ichnofauna, ocurringat a lower latitude, probably reached Patagonia later, resulting in the burst of insect trace fossils from the middleEocene-Miocene Sarmiento Formation and the early Miocene Pinturas Formation. Paleosols of these units containa highly diversified ichnofauna witb abundant insect trace fossils, especially brooding balls of dung beetles(Coprinisphaera). Other recorded trace fossils include Teisseirei, Celliforma, Pallichnus, Feoichnus, Lazaichnusand rare Loloichnus and earthworm traces. The dominance of Coprinisphaera in the Sarmiento Formationichnofauna, along with other sedimentologic, paleopedologic and paleontologic evidence, records the appearanceand evolution of one of the oldest (middle Eocene) open grasslands ecosystems worldwide.