Truncatoflabellum has been considered a free-living genus that exhibits both sexual and asexual phases; divided lower coralla (anthocauli) are specialized for asexual reproduction by transverse division through a decalcification process, whereas the upper coralla (anthocyathi) only undertake sexual reproduction, in a life-cycle strategy that includes a distinct alternation of generations. However, little evidence has been presented to support this idea of its life cycle. We elucidate the life mode of Truncatoflabellum by identifying key fossil characters (e.g., multiple rejuvenations and decalcification records just beneath lateral spines) and statistically analyzing the size distributions of over 500 individual coralla. Results of those morphological and biometric analyses clearly indicate alternation of generations in the life cycle of Truncatoflabellum.