Trilobite trace fossils from upper Tremadocian strata of the Santa Rosita Formation, northwestern Argentina: revisiting Cruziana stratigraphy of Gondwana

MUÑOZ DIEGO F.; MÁNGANO, M GABRIELA; WAISFELD BEATRIZ G.; BUATOIS LUIS; HERRERA SÁNCHEZ, N; LO VALVO GERARDO; TORO BLANCA A.

Salta

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2022

Universidad Nacional de Salta

Cruziana stratigraphy is a useful tool for Paleozoic successions lacking body fossils, mainly in “Grès Armoricain”-type lithologies. However, precise calibration of trilobite trace fossils relies on fossiliferous interbedded strata providing high-resolution biostratigraphic information. In particular, the stratigraphic range of Cruziana rugosa rugosa and the related ichnotaxa C. rugosa furcifera and C. rugosa goldfussi (i.e. the so-called C. rugosa group) has been the focus of discussion during thelast decades. Despite these ichnotaxa having been traditionally considered Lower Ordovician indicators, specimens from Bolivia triggered the debate about their younger range. As a result, theC. rugosa group is now considered as Ordovician in age without further precision. In turn, the distinctive form Cruziana semiplicata tends to be considered an upper Cambrian (Furongian)- Tremadocian indicator. In addition, Cruziana tortworthi and C. breadstoni were proposed as “transitional ichnotaxa” signaling Tremadocian strata in other Gondwana regions. These ichnotaxa display morphologic features from both C. semiplicata (e.g. external fine scratch imprints and marginal ridge) and C. rugosa (e.g. deeper, regular, subparallel, multiple sets of scratch imprints). New material collected in the Rupasca Member of the Santa Rosita Formation in northwest Argentina indicates that C. semiplicata, associated with C. tortworthi and C. breadstoni, characterizes the early late Tremadocian (Bryograptus kjerulfi graptolite Biozone), providing new precision on Cruziana stratigraphy.