Integral revision of the ichnogenus Daedalus Rouault and the first record in South America

Abstract

The ichnogenus Daedalus is one of the most enigmatic ichnogenus in the literature. It is documented in Ordovician to Silurian successions in both Laurasia and Gondwana. Seilacher (2000) amended the ichnogenus diagnosis; proposed new ichnospecies (D. verticalis), and pointed out the arthrophycid fingerprints. He also modified the diagnosis of D. archimedes (Ringueberg), that was assigned for him to another author and later moved into Daedalus. Then, problems with the first mention, type locality and material exist, while the new ichnospecies is not in agreement with ichnotaxonomical rules and demands a future reexamination before it is validated. The complexity of the ichnogenus ichnotaxonomy has multiple reasons as Daedalus is not the result of the simple activity of a vertical J- shaped tube with lateral displacement as traditionally was interpreted, but a result of a more complex tube morphology and vertical to arcuate, with lateral, spiral or vertical displacement. Sometimes, one specimen exhibits morphological changes that represent a combination of more than one type. Daedalus is characterized by ichnospecies that reflects a simple constructional pattern and others more complex that forms spirocones. These two groups coexisted since the oldest records of the ichnogenus in the Lower Ordovician and until the end in the Silurian. The analysis of the material documented in the Silurian of Argentina, in the Ventana System, constitutes the first record of the ichnogenus in South America. It shows partial resemblance with the D. verticalis from the USA. Nevertheless, these specimens are the result of an animal that displaced vertically, in some cases 0.8 metres and sometimes exhibiting a progressive change in the angle of causative tube. In this sense, specimens with a vertical spreiten with constant width and a pseudo U-shaped morphology, occur with other levels with specimens that increase the width upward. The last two of them, show a progressive reduction of the inclination in the angle of the tube resulting in spreiten that vary the curvature and also the symmetry with respect to both sides of the structure, suggesting a new ichnospecies. The epichnial expressions in some ichnospecies are surfaces crowded of specimens that are extended by kilometers. The ichnogenus has been interpreted as a deposit feeder, a specialized one that had the ability to feeding in the meiofauna. The interpretation of a producer with the head pointing down and searching for food, is acceptable for the simplest morphologies of some ichnospecies but clearly do not explain up to three levels of calices or spindles documented in the type ichnospecies D. desglandi (Rouault). The opportunistic behavior suggested for the ichnogenus is questionable for several reasons. Firstly, large structures demands bioirrigation and walls stabilizations for keeping the oxygenation into the structure. Secondly, ichnospecies with several levels or long vertical penetration suggest a permanent domicile structure, rather than ephemeral occupancy. Finally, in the material of Argentina, was documented that Daedalus obliterate a pioneering bioturbation with Skolithos. The lateral extent by kilometers of beds with Daedalus observed in several parts of the world, suggests time condensation in such levels and the potential use in stratigraphy or paleonvironmental schemes. The revision of the ichnogenus allows to conclude that not gap exist form the oldest to the last one record of Daedalus, and its apparent absence in the Upper Ordovician is the result of some incorrect assignments to other ichnogenus, as is the case of the records in southern Africa.