The siphonotretide brachiopod Schizambon from the Early Ordovician of South China: ontogeny and affinity

Schizambon is one of the earliest and most distinctive genera in the Order Siphonotretida. However, current knowledge of siphonotretide phylogeny and early evolution requires understanding of their earliest ontogeny. In this study, the new species Schizambon tongziensis from the Tungtzu Formation at Honghuayuan section in Guizhou Province, South China is described; it is also the first record of Schizambon in South China. The new well-preserved specimens shed light on the ontogeny, palaeobiogeographical distribution, phylogeny and early evolution of siphonotretide brachiopods. Characters of metamorphic shells on both valves, outlined by distinct halos, show that Schizambon had a planktotrophic 'paterinide-type' larva, typical of many early Palaeozoic brachiopods. Based on the newly obtained data it is inferred that the free-swimming larva of Schizambon settled directly on the ventral side of the body, and this type of ontogeny is probably true for other siphonotretides. The divergence of Schizambon from the main siphonotretide lineage probably happened prior to the Guzhangian Age, well before the occurrence of major biodiversification within the order.