DIVERSE STEM-GROUP BRACHIOPOD FOSSILS FROM THE LOWER CAMBRIAN OF SOUTH CHINA

The early Cambrian is an important period for the brachiopod diversification. Not only did major crown groups make their first appearance, but also there existed some stem groups. There are many stem-group brachiopod fossils recovered from Lower Cambrian rocks in South China, including some cap-like calcareous microfossils (e.g., Tianzhushanella and Lathamella), some tommotiids of organo-phosphatic sclerites (e.g., Micrina and Tannuolina), Heliomedusa (organo-phosphatic shell) and Yuganotheca (agglutinated valves), etc. These fossil records provide important material for studying the origin and early evolution of brachiopods.

Many cap-like calcareous SSFs (about 18 genera) from the Terreneuvian of South China were previously described as brachiopods. Through taxonomic scrutiny, only two genera (Tianzhushanella and Lathamella) may be referred to as stem-group brachiopods. Tianzhushanella is often taken as a senior synonym of Lathamella since both genera are cap-like and bear a pair of recessed cavities in the umbonal region. New SEM examination of the holotype of T. ovata shows 7 rows of nodes on the surface of the internal mould, opposing the synonymy of the genera. Unlike the bivalved Apistoconcha and Aroonia from the lower Cambrian of South Australia, both Tianzhushanella and Lathamella from South China have been only recovered with one type of valve, without “ventral” and “dorsal” differentiation. It remains unclear whether they were univalved, or bivalved with two nearly equal valves.

Heliomedusa and Yuganotheca are two exceptionally preserved fossils from the Chengjiang Biota. The unique lophophore organization of Heliomedusa, consisting of a pair of lophophoral arms that freely arch posteriorly, differs from any previously known fossil or living crown brachiopods. The distinctive microstructure and ornamentation of Heliomedusa shell indicate its affinity to the stem-group brachiopod Mickwitzia. Yuganotheca represents a bizarre stem-group brachiopod. It has an agglutinated pair of brachiopod-like valves with a phoronid-type lophophore inside, and a lower bipartite tubular attachment structure with a long coelomic pedicle providing anchorage. Yuganotheca may represent an intermediate form between phoronids and brachiopods.