Earliest ontogeny of early Cambrian acrotretoid brachiopods - first evidence for metamorphosis and its implications

Background: Our understanding of the ontogeny of Palaeozoic brachiopods has changed significantly during the last two decades. However, the micromorphic acrotretoids have received relatively little attention, resulting in a poor knowledge of their ontogeny, origin and earliest evolution. The uniquely well preserved early Cambrian fossil records in South China provide a great new opportunity to investigate the phylogenetically important ontogeny of the earliest acrotretoid brachiopods, and give new details of the dramatic changes in anatomy of acrotretoid brachiopods during the transition from planktotrophic larvae to filter feeding sedentary juveniles. Results: Well preserved specimens of the earliest Cambrian acrotretoid brachiopods Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis (Cambrian Series 2, Shuijingtuo Formation, Three Gorges area, South China) provide new insights into early acrotretoid ontogeny, and have significance for elucidating the poorly understood early phylogeny of the linguliform brachiopods. A more comprehensive understanding of the applied terminology based on new observation, especially in definition of the major growth stages (embryo, planktotrophic larva, post-metamorphically sessile juvenile and adult), is established. The so-called acrotretoid "larval shell" of both valves of Eohadrotreta demonstrates evidence for metamorphosis (shedding of the larval setae and transitions of shell secretion), during the planktotrophic stage. Therefore, it is here termed the metamorphic shell. The inferred early acrotretoid larval body plan included a bivalved protegulum, secreted at the beginning of the pelagic stage, which later developed two pairs of larval dorsal setal sacs and anterior-posterior alignment of the gut during metamorphosis. Conclusion: The primary larval body plan of acrotretoid Eohadrotreta is now known to have been shared with most early linguliforms and their relatives (including paterinates, siphonotretoids, early linguloids, the problematic mickwitziids, as well as many early rhynchonelliforms). It is suggested that this type of earliest ontogeny can be considered as plesiomorphic for the Brachiopoda and probably first evolved in stem group brachiopods with subsequent heterochronic changes.