INSIGHT FROM EARLY CORAL-STROMATOPOROID INTERGROWTHS, LATE ORDOVICIAN OF CHINA

One of the earliest endosymbiotic associations within stromatoporoids occurs in the Late Ordovician Xiazhen Formation of southeastern China. Bajgolia, an auloporid tabulate coral characterized by dichotomous branching due to longitudinal fission, is represented by free-living as well as endobiotic forms in various lithofacies representing a wide range of environments. Only two of eight stromatoporoid genera (Clathrodictyon and Ecclimadictyon) hosted Bajgolia, mainly in reef and related facies. The Bajgolia–stromatoporoid association developed occasionally in the lower part of the formation and eventually became persistent in the upper part of the formation. Such Bajgolia–stromatoporoid associations were initiated by larval settlement of the coral onto the growth surface of the stromatoporoid. Growth of Bajgolia generally kept pace with its host, but the faster growth capability of some Bajgolia prevented their envelopment and termination by the stromatoporoids, allowing the establishment and recurrence of an ongoing endosymbiotic relationship between the two organisms. The endobiotic Bajgolia were able to survive while protruding from the host, which is unlike previously studied coral–stromatoporoid associations. It is interpreted that endobiotic Bajgolia may have taken advantage of the host for physical support and elevated feeding positions. The stromatoporoids, however, apparently were neither significantly benefited nor harmed by the presence of these endobionts. The relationships between Bajgolia and stromatoporoids probably represent commensalism, but the possibilities of mutualism and parasitism cannot be entirely ruled out. The Bajgolia–stromatoporoid association in the Late Ordovician of southeastern China represents an important stage in the development of complex ecological relationships and community structure, prior to the common and widespread tabulate–stromatoporoid (‘caunopore’) associations in the Silurian and Devonian.