First record of Podichnus in orthide brachiopods from the Lower Ordovician (Tremadocian) of NW Argentina and its relation to the early use of an ethological strategy

Abstract

Well preserved etchings from the pedicle attachment of brachiopods occur on orthide brachiopod shells from Lower Ordovician (Tremadocian) of NW Argentina. Under the name Podichnus conicus isp. n. this trace fossil is interpreted as a fixichnia produced by the attachment of brachiopod pedicles upon hard substrates. The trace is characterized by an irregular and asymmetrical cluster of circular pits. Each individual pit has a typical conical shape and presents the external mean diameter/length ratio close to 1. The occurrence of P. conicus isp. n. in the Tremadocian is the oldest record of this ichnogenus, and points to orthide brachiopods as Podichnus producers for the first time. The presence of this structure provides new information regarding colonization of biogenic hard substrates during the Early Ordovician and an adaptive strategy of its use by orthide brachiopods.