The functional biology of Devonian spores with bifurcate processes-a hypothesis

Abstract

Aquatic heterosporous ferns may have grapnel like glochidia, e.g., Azolla, specialized for anchoring a microspore mass to a megaspore. Thus, in an aquatic system, the free floating microspore mass (glochidia) and megaspore are held in close proximity when the sperm cells are released. Similar structures are known from the Cretaceous and Tertiary megaspores and microspores such as Azollopsis and Ariadnaesporites and are considered to have functioned in fertilization. This demonstrate that, as part of the evolution of the aquatic heterosporous habit in the ferns during the Cretaceous, functional and structural elements of the megaspores and microspores evolved early. A parallel evolution event can also be observed in the initial radiation of heterosporous plants during the Late Devonian. Megaspores and microspores, with probable lycopods affinities, demonstrate grapnel-like processes which we suggest were similar to the functional/structural elements known from the Cretaceous aquatic ferns. From this we conclude that many of the Middle and Late Devonian heterosporous plants were aquatic and that there were two parallel evolutionary events, one in the evolution of Devonian aquatic lycopods and a second in the evolution of Cretaceous aquatic ferns. Both of these evolutionary events are characterized by similar functional/structural elements in the megaspores and microspores.