The Trace Fossil Cruziana and Rusophycus: A Study from the Ordovician Succession of Kalmard Block, Central Iran

Trace fossils are abundant in the clastic succession throughout the Shirgesht Formation of Kalmard Block, Central Iran. At certain horizons there are beds full of Cruziana and Rusophycus. This study presents an evaluation of the arthropod ichnotaxa of the Cruziana and Rusophycus from the Ordovician succession Shirgesht Formation. Apart from the broad range inmorphology and size, Cruziana from Shirgesht Formation show higher behavioral diversity using food sources than has ever been documented in a single section. It consists of a straight, or gently curved, ploughed furrow usually a few centimetres across, which is divided lengthwise into two lobes, each of which is braided transversely by a series of scratches. These vary in arrangement and profile according to ichnospecies. Overall characteristics of recognized ichnospecies suggest that with the exception of C. semiplicata, the ichnofaunas contain all the members of the Cruziana rugosa Group throughout the clastic successions in Shirgesht Formation. It consists of the five members of C. rugosa, C. furcifera, C. goldfussi C. rouaulti and C. yini. The Shirgesht Formation material therefore indicates that these arthropod ichnofossil assemblages are suitable for Cruziana ichnostratigraphy in Shirgesht Formation. Accordingly, the present assemblage could represent the Early to middle Ordovician age. The arthropod traces also, occur with other biogenic sedimentary structures in a fine to medium grained sandstone with minor mudstone interbeds of the Shirgesht Formation. The substrate composition and ichnological evidence places the arthropod ichnofossil-rich unit in the Cruziana ichnofacies.