New insights and evolutionary significance of the megasporangiate strobilus of Minostrobus chaohuensis (Lycopsida) from the Upper Devonian of South China

Abstract

Heterosporous lycopsids with monosporangiate strobili are highly diverse in the Carboniferous, but their early evolution is poorly understood. The Late Devonian Minostrobus chaohuensis was included in this plant group, but features of the strobili were unclear in detail. Permineralized material of M. chaohuensis was sectioned and ground in series to reveal details of megasporangiate strobili. The megasporophylls are smooth and borne in a 2/9 helical phyllotaxy. The megasporophyll consists of a pedicel which bears a keel and alations, lamina and heel. The alations extend horizontally and then bend upward to surround the sporangial base. A single megasporangium with a subarchesporial pad is inserted onto the sporophyll pedicel through a narrow attachment. Each sporangium contains four Lagenicula-type spiny megaspores that may be of variable size. On the basis of these new fertile traits, M. chaohuensis is assigned to the Isoёtales sensu lato. This plant is proved to be monosporangiate, and thus Devonian lycopsids are shown for the first time to have possessed megasporangiate strobili. It is confirmed that phylogenetically advanced heterosporous lycopsids with monosporangiate strobili had evolved by the Late Devonian. The type of alations and size variation of the four megaspores per sporangium indicate that Minostrobus may represent an evolutionary form in transition toward the Carboniferous isoёtaleans with monosporic megasporangia more fully enclosed by alations. Evidence suggests a free megaspore dispersal mechanism in M. chaohuensis.