Evolutionary framework of early Cambrian cnidarians from South China

Cnidarians were an important component of the paleomarine ecosystem in the early Cambrian period in South China. In this paper, the evolution of early Cambrian cnidarians was discussed primarily based on the fossil records from the Kuanchuanpu, Yanjiahe and Chengjiang biotas in the Yangtze Block. Most early Cambrian cnidarians predominantly belong to the Superclass Medusozoa. By contrast, those of the Class Anthozoa are scarce. All fossil cnidarians in the Cambrian Fortunian Stage of South China are of microscopic sedentary forms with a variety of body radiation symmetries and exoskeletons; most of them are direct developers. In Cambrian Stage 2, the sedentary cnidarians exhibited an enlarged body size with only biradial or tetraradial symmetrical pattern. The fossil record of swimming jellyfishes firstly appeared in Cambrian Stage 3, associated with less sedentary forms, thus representing the emergence of the crown-group medusae and first establishment of complex life cycle with alternation of generations like modern jelly fish. A set of key characteristics, such as the indirect development, nerve ring, thickening of mesoglea, coronal muscles, rhopalium, concaved oral disk, deep subumbrella, tetraradial symmetry and loss of exoskeleton, are closely correlated and subsequently contribute to the step-wise rise of naked, pelagic and predatory medusae in Cambrian Stage 3. The rise of the medusoid stage in the life cycle of medusozoans invented many adaptational advantages. In return, however, it may have subsequently triggered a variety of changes in polyp morphology, structure and function, e.g., the shortening of polypoid stage and the loss of gonads and sexual reproduction. Thus the polyps of medusozoans are substantially different from anthozoan polyps. During the early Cambrian, the body size of the cnidarians and most other co-occurring metazoans increased significantly, mediated probably by various environmental factors, especially increase of ocean oxygen concentration. In the background of Cambrian radiation, the diversity and richness of cnidarians peaked at the dawn of the Cambrian, and then declined in Cambrian Stages 2-3, suppressed likely by the stepwise rising of bilateral animals. Most sedentary cnidarians that flourished in the Fortunian Stage did not extend into Cambrian Stage 3. Therefore, the fossil record of cnidarians from South China indicated the Cambrian explosion is not merely an event of animal radiations as it had witnessed a series of biotic replacements and even extinction of higher taxa in cnidarians. © 2020, Editorial Office of Earth Science Frontiers. All right reserved.