Periodicity of reproductive growth in lycopsids: An example from the Upper Devonian of Zhejiang Province, China

Monilistrobus Wang et Berry is a fossil lycopsid reported from the Upper Devonian Wutong Formation of Jiangsu Province, China, and includes only the type species Monilistrobus yixingensis Wang et Berry. Along a single fertile axis, Monilistrobus has cone-like structures separated by and alternated with a length of vegetative region, which is an important character here termed as periodicity of reproductive growth. The alternation of fertile and vegetative growth occurs regularly and produces repeated intercalary (non-terminal) fertile zones. New specimens, identified as Monilistrobus cf. yixingensis, from the same formation but of Zhejiang Province extend the geographic distribution of this genus. These new specimens demonstrate morphological variations within this genus, such as the lack of spiny appendages on the leaves, different measurements of leaf bases, and different shapes and positions of cone-like structures. The distribution of intercalary fertile zones, as summarized for the first time in this paper, occurs in the Drepanophycales, the Protolepidodendrales, the Isoëtales, the Lycopodiales, the Selaginellales, and Wuxia Berry, Wang et Cai and Monilistrobus of uncertain affinities. Thus, this peculiar trait might have evolved in the very early evolutionary stage of the lycopsids, as indicated in the basal group Drepanophycales. However, the presence of repeated, two or more intercalary fertile zones along a single fertile axis is demonstrated only in Monilistrobus and the extant Huperzia Bernhardi.