GIGANTISM IN PERMIAN TREPOSTOMES FROM GREENLAND: STABLE ISOTOPE EVIDENCE OF THE ALGAL SYMBIOSIS HYPOTHESIS

Gigantism in organisms can be caused in various ways: by disease, under ideal growing conditions, or symbiosis. It is well documented that symbiotic zooxanthellae algae can cause gigantism in Recent hermatypic corals. However, such symbiotic relationships have never been documented for extant or fossil bryozoans. Unusually large colonies of the trepostome bryozoan Tabulipora sp. have been recovered from the Kungurian (Early Permian) Kim Fjelde Fm. in eastern North Greenland. Branches reach 7 cm in diameter in this species, while other bryozoans in general and other Tabulipora species in particular in the fauna are an order of magnitude smaller. Disease can be ruled out as a causal factor for this example of gigantism, as all individuals in the species exhibited gigantism. Ideal growing conditions can not be completely ruled out as not all other species in the fauna exhibited normal growth sizes. Håkansson and Madsen (1991) hypothesized that the gigantism was caused by algal symbiosis. Their conclusion was based on carbon and oxygen isotopic values derived from coarse sampling that required ≥ 1.5 mg carbonate. In this study using a colony from the same formation and location, a more precise test of their hypothesis was conducted by sampling with 10 µm spatial precision using a computer-driven micromilling device that generated discrete ≥ 20 µg carbonate samples. Skeletal mean d¹³C and d¹⁸O isotope values are 3.8 ‰VPDB and -6.5 ‰VPDB, respectively. Diagenetic effects were evaluated by separately sampling the cements contained within zooecial chambers; skeletal values are significantly higher than the surrounding cements. Taking into account the inferred isotope value of the Permian ocean, we suggest that these isotopic results fail to reject the algal symbiosis hypothesis for gigantism in these bryozoans. On the other hand, morphological and paleoenvironmental support for this hypothesis is equivocal.