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 posterior margin of the ventral valve of chonetoidean brachiopods, previously symmetrical on each side of the beak during the Silurian, exhibits asymmetrical examples. The asymmetry of spines affects mainly two families which are widely distributed in the Old World Realm during the Devonian: strophochonetids and anopliids. The detailed vertical range of asymmetrical forms, compared with conodont zones, suggests close relationships with Lower and Middle intra-Devonian events. The geographic distribution of the asymmetrical taxa indicates that they mainly characterize the intertropical zones. The development of spine asymmetry is interpreted 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-Devonian events which contributed to the expression of diversity among chonetoidean brachiopods.