New Data on the Enigmatic Early Cambrian Brachiopod Heliomedusa orienta from South China
Zhifei Zhang1, Guoxiang Li2, Christian C. Emig3, Jian Han1, Lars E. Holmer4 and Degan Shu1
1 Early Life Institute, State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xian, China
2 Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China
3 20 Rue Chaix, Marseille, France
4 Palaeobiology, Department of Earth Sciences, Uppsala University, Sweden
The organization of the lophophore is a key diagnostic character for the definition of higher brachiopod taxa, and has a pivotal role in phylogenetic analysis of this group. In all extant Lingulata, there is a double row of lophophore filaments at the (early) trocholophe stage, whereas in extant calcareous-shelled brachiopods (Craniiformea and Rhynchonelliformea), the trocholophe stage has only a single row of filaments. However, the lophophore is very rarely fossilized, so organizational evolution can often only be inferred.
Heliomedusa orienta, one of the numerous puzzling brachiopods from the Lower Cambrian Chengjiang lagerstätten of Yunnan, southwestern China, has a well-preserved lophophore set in a striking disposition. Based on a comparative study of lophophore disposition in H. orienta and the extant discinid Pelagodiscus atlanticus, the in- and excurrent pattern and shell orientation of H. orienta are described and discussed. Reconstructions of lophophore shape and function are based on numerous specimens and comparison with P. atlanticus. The lophophore is composed of a pair of lophophoral arms that freely arch posteriorly, rather than coiling anteriorly as commonly seen in fossil and Recent lingulids. Attached to the dorsal lobe of the mantle, it has neither calcareous nor chitinous supporting structures, and is disposed symmetrically on either side of the valve midline. The mouth can be inferred to be located at the base of the two brachial tubes, slightly posterior to the anterodorsal projection of the body wall. The lophophoral arms bear laterofrontal tentacles with a double row of cilia along their lateral edge, as in extant linguiliform brachiopods. In addition to tentacles, the main brachial axes are ciliated, which presumably facilitated transport of mucous-bound nutrient particles to the mouth. In terms of overall shell shape, setal arrangement and external micro-ornament, H. orienta exhibits links to the purported stem-group brachiopod Mickwitzia, which challenges recent interpretations of an affinity to discinids. The combination of morpho-anatomical characters, such as lophophore palisades, dorsal and ventral visceral regions and muscular markings, suggests that Heliomedusa, probably together with the cosmopolitan small shelly fossil Mickwitzia, represents a stem group brachiopod closely associated with lingulates.