SPATIAL COMPETITION BETWEEN ORGANISMS OF DIFFERENT GROWTH FORMS

Spatial competition between dendritic tetradiid (Rhabdotetradium jiangshanense Lin and Zou, 1980) and massive encrusting siliceous sponge were analyzed in order to understand life-history strategy of branching form organism in competition with that of massive encrusting type. Tetradiid is a problematic Ordovician fossil characterized by mm-sized tubes. Rhabdotetradium is composed of slender single tube dividing into four tubes at long interval. R. jiangshanense analyzed in this study is composed of poorly preserved tube with average diameter of 1.6 mm. Tetradiid-sponge and tetradiid-dominated boundstones from the Upper Ordovician (late Katian) Xiazhen Formation of south China were compared based on transverse serial thin sections. Both boundstones are from the same horizon with close proximity and considered to be formed under comparable environmental conditions. Tetradiid tubes from tetradiid-dominated boundstone show predominance of typical quadripartite fission (69% of axial increase; n=11) with vertical distance between 3 to 7 mm. On the other hand, in tetradiid-sponge boundstone, 56 % of tetradiid tubes occur at the outer margin of sponge showing predominance of atypical bipartite division (66% of axial increase; n=4) with 2 to 4.5 mm of vertical distance. Other 31% of tetradiids are completely enveloped by sponge and dominantly display bipartite fission (63% of axial increase; n=5) with 1 to 2.5 mm vertical distance. There are predominance of bipartite fission and strikingly shorter vertical distance in those from tetradiid-sponge boundstone compared to dominant quadripartite fission with longer vertical distance in tetradiid-dominated boundstone without sponge. These contradistinctive growth patterns of tetradiid appear to be an adaptive survival strategy by ecologic stress derived from competing siliceous sponges. This result provides a new insight for understanding competition between organisms of different growth forms in ancient reefs, and reaffirms the potential of serial thin sections for study of biologic interactions in fossil benthic communities.