Abstract:

Brachiopoda, as a key phylum of lophotrochozoan animals, plays an important role in studying the biodiversification of benthic communities during the Cambrian evolutionary radiation. Eight genera and twelve species of brachiopods, with the exception of three indeterminate taxa, are revealed and recovered from the Cambrian Series 2 carbonate facies near the Ediacaran-Cambrian boundary in the Yangtze Platform. They represent one of the earliest assemblages or components of brachiopods from South China. Based on the brachiopod data, the shell morphology and textural structures of the earliest variety of brachiopod groups and their occurrences and dispersals are discussed here to explore the origin and biodiversification of phosphatic-shelled brachiopods. We then summarised the recent research advances on the development of Cambrian brachiopods known from the Xihaoping Member of the Dengying, Shuijingtuo, and Yu’anshan formations of the Yangtze Platform. Lines of studies demonstrated that the linguliform brachiopods achieved adaptive diversity in the ornamentation of metamorphic and mature shells and shell structures, and in the configuration of pedicle foramen during the Cambrian explosion. In addition, three ontogenetic stages, including the pedicle foramen formation stage, pedicle foramen enclosure stage, and intertrough increasing stage, are observed in Cambrian Epoch 2 acrotretides. Compared with the ontogenetic developmental processes in different genera and species of acrotretides, the post-metamorphic allometry of shell valves was recognized as it played an increasingly significant role in the morphological diversification of different acrotretide clades. Furthermore, our recent studies on the earliest ontogenetic characteristics, preserved in the metamorphic shells in the oldest known lingulides and acrotretides, demonstrated that metamorphosis of planktotrophic larva is plesiomorphic for all brachiopods while the living lingulids are modified to obtain secondary larva with direct development. Interestingly, separate and diachronous occurrences of the oldest known brachiopods in the Cambrian palaeocontinents implied that linguliform brachiopods probably originated in East Gondwana and South China during the early Cambrian Epoch 2 and achieved a cosmopolitan dispersal subsequently. More studies on the diversity, ontogeny and biostratigraphy of the earliest phosphatic-shelled brachiopods will help us improve our understanding of their origination and the evolutionary changes of different lineages to uncover the correlation of Cambrian strata in detail.

Key words: brachiopods, phosphatic-shelled, ontogeny, diversity, Cambrian Epoch 2, South China