Epibiotic relationships in Mesoprotero zoic fossil record: Caoyuzh uang Formation, Ch i na Lee Seong-Joo UN/R 5561 CNRS Biogeosciences, University of Bourgogne, 6 Boulevard Gabriel, 21000 Dijon, France Stjepko Golubic Department of Biology, Boston University, Boston, lVlassachusetts 02215, USA Eric Verrecchia UMR 5561 CNRS Biogeosciences, University of Bourgogne, 6 Boulevard Gabriel, 21000 Dijon, France ABSTRACT Epibiotic coccoid microfossils ofprobable cyanobacterial affinity were discovered in cherty stratiform stromatolites of the 1400-1500 Ma Gaoyuzhuang Formation, Hebei Province, north- ern China. Sessile coccoids with polarized growth settled on cyanobacterial sheaths and devel- oped dense populations. These microfossils represent the earliest known occurreirce ofepibiosis in the fossil record with characteristics similar to those observed in modern aquatic environ- ments. The reported fossil epibionts are also the first known occurrence of Proterozoic cyano- bacteria of chamaesiphonalean affinity. The presence of specialized epibiotic communities in Gaoyuzhuang stromatolites demonstrates that epibiosis was well established and diversified by Mesoproterozoic time, reflecting early adaptation in response to competitive pressures for mi- crobial attachmeni sites in ancient, densely populated shallow-water benthos or, possibl¡ the onset of specialized mutualistic relationships among cooccurring microorganisms. INTRODUCTION In nature today, organistns of one species often grow physically attached on those of another species. Such epibiont-basibiont rela- tionships between different organisms, called epibiosis, exist on plants (epiphytic) and ani- mals (epizootic), including macroscopic and microscopic forms. They ale ubiquitous in modern terrestrial and aquatic environments, especially in substrate-bound, space-limited communities (Ballantine, 1979; Baker and Orr, 1986; Wilmotte et al., 1988). In aquatic envi- Ionments. epibiosis occurs as an extension or special case of benthic growth habit, in which epibiotic organisms take advantage of the space already occupied. Epibiotic lelationships be- tween microfossils of different taxonomic affinities have not been identified in the Ple- camblian fossil recold. Over the past 40 yr paleontological studies of ancient benthic mi- clobial fossils have greatly improved our un- derstanding of origin, distribution, and diver- sity of ancient life; however, relatively limited attention has been paid to mutual relationships of cooccurring fossil microorganisms, includ- ing selective colonization of different types of substrates. Such lelationships ale revealed only when entile populations of microorganisms are presewed in situ together with the sedimentary strLrctul'es surrounding them (Seong-Joo and Golubic, 1999). We report here on abundant and diverse epibiotic assemblages of coccoid microfossils settled on cyanobâcterial sheaths from the ca. 1400 Ma Mesoproterozoic Gao- yuzhtrang Folmation of northern China. GEOLOGTC SETTTNG AND MATERIAL Little-metamorphosed Paleoploterozoic to Mesoprotelozoic sedinientary sequences are de- veloped along the northwestern margins of the North China platform; the rocks crop out in the alea of the Pangjapu iron mine in Hebei Province ofnoltheln China, about 115 km northwest of Beijing. This seQuence, which in this region in- cludes the Changcheng and Jixian Gloups, un- confolmably oveilies metamorphosed Archean ++ Archean basement Mesozoic granite Paleoproterozoic-Mesoproterozoic sequence Jurassic sedimentary rocks Figure 1 . Locátion map of fossil epibionts preserved in cherty stromatolites near c¡ty of Pang- japu, northern China. Star represents measured section of Gaoyuzhüang Formation (modified from Zhang,1986). 40.40'N 1 15.10'E Pangjapu o Yellow Sea Jb _tc HINA I E WWVVVWWVVV VVWWVVWVW WVWWVWWVVW VVVVVVVVVVV VVVWWVVVWVVWWV VVVWVVVVVWVVVV VVVVWWVWWVWWWV VVVVVVVWWWVVV VVVVVWVVVWVVWW WVVWWVWVWVVVV VVVVVVWWVWVV VWVWVVVWVVV VVVWVVVVVV WWVWVVV VVVWVVV VVVVWVW N t lkm + + + + + ++++ ++ ++++ ++ +++ C e o I o gy : Decetnber 1999 ; v. 21 ; no. 1 2; p. 1059 -1062; 5 fi gules. I 059