SPINE ASYMMETRY IN CHONETOIDEAN BRACHIOPODS - AN EXAMPLE OF REITERATED HETEROCHRONIES LINKED TO INTRA-DEVONIAN EVENTS

From the Early Devonian, the distribution of spines along the posterio r margin of the ventral valve of chonetoidean brachiopods, previously symmetrical on each side of the beak during the Silurian, exhibits asy mmetrical examples. The asymmetry of spines affects mainly two familie s which are widely distributed in the Old World Realm during the Devon ian: strophochonetids and anopliids. The detailed vertical range of as ymmetrical forms, compared with conodont zones, suggests close relatio nships with Lower and Middle intra-Devonian events. The geographic dis tribution of the asymmetrical taxa indicates that they mainly characte rize the intertropical zones. The development of spine asymmetry is in terpreted herein as an innovation and a progenetic heterochrony linked to the onset of a pioneer strategy with an epiplanktonic mode of life . The spine asymmetry is repetitive and a consequence of the intra-Dev onian events which contributed to the expression of diversity among ch onetoidean brachiopods.