Early Cambrian Stem-Group Brachiopods from South China  

The early Cambrian is a key period in brachiopod evolution.  Members of the crown group make their first appearance, and join the company of many enigmatic stem group forms.  Many stem-group brachiopods and brachiopod-like fossils have been recovered from Lower Cambrian rocks in South China, including some cap-like small shelly fossils (eg., Tianzhushanella and Lathamella), some tommotiids (eg., Micrina and Tannuolina), Heliomedusa and possible halkieriids.  These fossils are important for studying the origin and early evolution of brachiopods. 

Many cap-like SSFs (c. 18 genera) from the pre-trilobitic Cambrian were previously described as brachiopods.  Though their shells may originally have been calcareous, the fossils are usually preserved by secondary phosphatisation.   Only two genera (Tianzhushanella and Lathamella) may conceivably be related to brachiopods.  The other genera are molluscs, junior synonyms of other genera, or enigmatic fossils with whose status as a genus is questionable.  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.  Recent 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 from the Chengjiang Biota was referred to the craniopsids or discinids, but the distinctive pustulose ornamentation on the shell of H. orienta may indicate affinities to the early Cambrian stem-group brachiopod Mickwitzia.