Looking for the usual suspects: trilobites as Cruziana-Rusophycus producers in Lower Paleozoic sandstones of Northwest Argentina

MANGANO, M.G. AND WAISFELD, B.G.

Trelew - Argentina

Congreso; First International Congress on Ichnology; 2004

Museo Paleontológico Edigio Feruglio

Lower Paleozoic sandstone and shale successions in northwest Argentina are well known for their Cruziana and Rusophycus with distinct Gondwanic affinities. These structures are commonly attributed to the activities of benthic trilobites in Lower Paleozoic rocks worldwide. Several stratigraphic intervals in northwest Argentina contain not only the trace fossils but also body fossils of the potential trilobite producers. Three ichnotaxa have been selected for analyses in different stratigraphic units, ranging from the Upper Cambrian to the Middle Ordovician: Cruziana semiplicata, Rusophycus moyensis and Cruziana rugosa (including its ethological variants). Different candidates for makers of these structures are analyzed based on close association of the trace fossils and trilobites, size range, fine morphology match, and coincident stratigraphic ranges. The particular features of some stratigraphic intervals, such as the paucity and low diversity of trilobite remains point to putative tracemakers. This is the case of the Alto del Cóndor Formation (middle-upper? Arenig, Cordillera Oriental) where large sized Ogyginus is the single trilobite associated with Cruziana rugosa.  In other stratigraphic intervals, however, more than one candidate may be present. In the upper part of the Suri Formation (middle Arenig, Famatina), three trilobite genera (Neseuretus, Annammitella and Merlinia) occur. Only Merlinia, which shares a similar exoskeletal design with Ogyginus, matches in size with the co-occurring C. rugosa. In other intervals, however, more than one candidate may be present. In the Santa Rosita Formation (Upper Cambrian – Tremadoc, Cordillera Oriental), either Angelina or Leptoplastides, might be suggested as possible producers of R. moyensis. Cruziana semiplicata is associated with a variety of trilobite taxa. Although evidence is not conclusive, olenids seem to be the potential producers.  The shift from Cruziana semiplicata-Rusophycus moyensis-bearing strata to Cruziana rugosa-bearing strata is thought to reflect a major trilobite turnover in shallow-marine environments; the replacement of Late Cambrian- Tremadoc olenid-dominated faunas by latest Tremadoc – Arenig asaphid-dominated faunas.