Wall ultrastructure of the oldest embryophytic spores: Implications for early land plant evolution

摘要

AbstractCryptospores from the Hanadir Member, Qasim Formation (Darriwilian) examined with transmission electron microscopy (TEM), show two distinct kinds of walls: homogeneous and lamellated. A recently described tetrad from the Hanadir Member,Cryptotetras?erugataStrother, Traverse & Vecoli, and the dyad,Pseudodyadosporasp. cf. P.?laevigataJohnson, possess the homogeneous wall type, matching that seen in younger, Silurian species ofTetrahedraletesfrom Laurentia. The lamellated wall structure of a single Hanadir Member monad is also described. Homogeneous walls are characteristic of extant embryophyte sporoderm, which is actively secreted by a tapetum. This implies that these Ordovician cryptospores developed within a sporangium, although fossilized sporangia of this antiquity are not known. Lamellated walls occur in cryptospore dyads found in Siluro-Devonian cryptophytes (as well as fragments of presumed cryptophytes as old as the Ordovician), but this type of wall is also common in Cambrian cryptospores. Therefore, lamellate spore walls appear to be have evolved in algal (charophytic) lineages well in advance of the first plants that possessed an upright axial sporophyte. Isometric, tetrahedral tetrads, likeCryptotetrasandTetrahedraletes, which dominate the Hanadir Member assemblage, express a geometry that reflects simultaneous cytokinesis during sporogenesis. Thus, the Hanadir Member specimens present what we interpret as the earliest evidence of simultaneous meiosis occurring inside a sporangium that possessed an active tapetum.]]>