A LOWER ORDOVICIAN CRYPTOSPORE ASSEMBLAGE FROM AUSTRALIA HELPS TO RECONCILE MOLECULAR PHYLOGENETICS AND FOSSIL ORIGINS OF PLANT DEVELOPMENT

The earliest fossil plants are small upright axes of middle Silurian (Wenlock) age, but molecular clock data has been converging on an Ediacaran – Ordovician interval for the origin of the embryophyte crown group. This discrepancy has been attributed to a “missing” fossil record, but a Cambrian origin to the land plants becomes more interesting when the lower Paleozoic record of spore-like microfossils (cryptospores) is considered more broadly. Cambrian cryptospores generally occur as irregular packets of varying numbers of attached spore pairs, which is unlike the geometrically regular tetrahedral spore tetrads that first appear in the Middle Ordovician (Darriwilian). A recently described Lower Ordovician (Tremadocian) assemblage of cryptospores from Samphire Marsh 1 drill core (Canning Basin, West Australia) contains elements of both earlier Cambrian and later, Middle Ordovician forms. This discovery, which includes cryptospore dyads (Dyadospora murusattenuata) and the tetrad Rimosotetras, helps to fill in a temporal gap in the global cryptospore record, linking Laurentian and Gondwanan assemblages. This linkage adds support to the hypothesis that charophycean-style meiosis preceded bryophytic sporogenesis during the evolutionary assembly of the embryophytic genome. More importantly, it opens up the possibility that the evolutionary assembly of the land plant genome (from an evolving charophycean algal complex) might be reflected in the temporal sequence of remains of spores and plant-like debris recovered in the lowermost Paleozoic rock record.