Paper Number: 944
New Small Shelly Fossils Acanthocassis from the Early Cambrian
Yunhuan Liu1, Tiequan Shao1, Qi Wang2, Li Yuan2, Yanan Zhang2, Yingying Ma2, Hanhua Tang2, Meng Wei2, Tingting Yang2, Bo Hu2, Zhiguo Dong2, Jiangtao Chang2
1College of Earth Science and Resources, Chang’an University, Xi’an 710054, China; Key Laboratory of Western Mineral Resources&Geological Engineering Ministry of Education, Xi’an 710054, China, yunhuanl@chd.edu.cn
1College of Earth Science and Resources, Chang’an University, Xi’an 710054, China; Key Laboratory of Western Mineral Resources and Geological Engineering Ministry of Education, Xi’an 710054, China,stotto@163.com
2 College of Earth Science and Resources, Chang’an University, Xi’an 710054, China
Acanthocassis is a microfossil reported from the early Cambrian small shelly fauna of South China. It is polyp-like and is composed of an apical main branch and an abapical aggregate of several sub-branches arranged in radial symmetry. Isolated sub-branches are common and were assigned to different genera by different authors. The affinity of Acanthocassis remains a matter of debate, though several hypotheses were presented.
We describe Acanthocassis orthacanthus from the early Cambrian of South China. A. orthacanthus has a long, cylindrical main branch that is ornamented with irregularly and sparsely distributed small nodes at the abapical portion. The sub-branches range in number from three, four, five, six, seven, to eight. They were originally soft and flexible, and merged at their bases to form a common plane among them. They are coniform and hollow internally, and their surfaces are ornamented with irregularly distributed small nodes/spines as well as densely-spaced longitudinal lines. These anatomic features permit a detailed revision of the generic diagnosis of Acanthocassis. Previous studies regarded Acanthocassis as disarticulated hard parts for support, or cuticular armourings for defense. Here, we reinterpret Acanthocassis as an independent organism, and tentatively assign it to the stem group of Hydrozoa. Acanthocassis might be a sessile hydropolyp with radial symmetry, hollow tentacles, no mesenteries (gastric setpae) and no perisarc (athecate). The nodes/spines on the surfaces might be nematocyst batteries that accommodate cnidae functioning for defense and food capture. No mouth or hypostome is developed in the present specimens, indicating that they were non-feeding and implying that they formed colonies and got nutrient from other feeding polyps. Acanthocassis might represent the oldest known hydrozoan, and it indicates that representatives of crown-group cnidarians (anthozoans, cubozoans, scyphozoans and hydrozoans) have emerged simultaneously in the early Cambrian.
Acknowledgements:
Supported by the National Natural Science Foundation of China (No. 41572009) and College Students' innovative training program of Chang'an University (No. 201510710060, 201510710056, 201510710179).