The status of the Cruziana (trilobite trace fossil) stratigraphy in Western Gondwana: The mixing of Lower and Upper Ordovician Elements in the Central Andean Basin of South America

ACEÑOLAZA, G.F. AND HEREDIA, S.,

Cuadernos del Museo Geominero

Instituto Geológico y Minero de España

Lugar: Madrid, España; Año: 2008 vol. 9 p. 13 - 17

A limited understanding of trace fossil distribution in the stratigraphic record and the partial interpretation of the meaning of this relation, precludes the correct interpretation of the potential values of ichnostratigraphy on “non fossiliferous” strata. Palaeozoic trilobite traces assigned to Cruziana and arthrophycids worm burrows (Arthrophycus, Daedalus and “Phycodes”) are referred as important elements that provide clues evolution and behaviour of organisms during the Paleozoic (Seilacher, 2004). These have been used as important elements on the dating of the nearly barren fossiliferous Early Palaeozoic strata of Northern Africa (ej. Seilacher, 1969; El-Khayal and Romano, 1988), and have been proposed as useful tools in biostratigraphy and palaeogeographic reconstructions (Seilacher, 2005; MacNaughton, 2007). Crimes (1970) and particularly Seilacher (1970; 1992; 2007 with references) provided the conceptual framework for the Icnostratigraphical use of Cruziana, where several forms replace each other along the chronostratigraphy of Gondwana over 200 Million years (Cambrian – Lower Carboniferous). Even though Cruziana rugosa has been historically considered an index fossil for the Lower Ordovician of Gondwana (Crimes, 1970; Baldwin, 1977; Fillion and Pickerill, 1990; Seilacher, 2007 with references), the recent discovery of an Upper Ordovician association including C. rugosa in Bolivia and the current material from northern Argentina confirms that the group reaches on full health the Upper Ordovician in the Western Margin of Gondwana.  C. rugosa, C. furcifera, C. goldfussi C. rouaulti and C. yini occur in successions where conodonts, graptolites, trilobites, brachipods are common elements providing the needed solid biostratigraphic base to define the age of the bearer strata. In addition, the discovery of a new form, Cruziana gutii n. isp., adds a rare new ichnospecie of Cruziana in the Central Andean Basin of South America.