Divaricate patterns in Cambro-Ordovician obolid brachiopods from Gondwana

A new brachiopod species, Westonia mardini, from the Furongian of Turkey and a new occurrence of Westonia urbiona from the Cambrian Series 3 of Iberian Peninsula are reported. These new finds of 'westoniids' collected in Gondwana allow the discussion of the functional morphology of their terrace ridges. This structure enabled an effective and rapid reburrowing and allowed the occupation of the shallow marine unstable sands and silts in the shoreface environments. This burrowing mechanism, with their pedicle directed downwards, was unknown before the Cambrian Series 2 and was abandoned chiefly in the Early Ordovician. Here we also review the distribution of 'westoniids' in space and time to analyse the diversification, decline and replacement of this important group of brachiopods. They became a significant part of the semi-infaunal marine associations beginning with the Cambrian Series 2 and proliferated in shallow arenaceous shelves during the Cambrian Series 3 and during the Furongian in most palaeocontinents. The acme of 'Westoniid' obolids associations was in the Furongian, but locally relict associations were present in high-latitude Gondwana until the Tremadocian. Decline of westoniid associations began during the Tremadocian, being replaced by very diverse smooth-shelled obolid associations with a novel burrowing mechanism and deeper lifestyle.