CRYPTOSPORES AND THE FOSSIL RECORD OF EARLY LAND PLANT EVOLUTION

The study of the transition to a land flora has a long botanical history, extending back to the work of Bower in 1908. The study of the fossil record of the origin of land plants commenced in the 1970s with the work of Jane Gray and others who began recovering spore tetrads in the Medina Group in New York and the Tuscarora Fm in Pennsylvania. These "permanent" tetrads and "dyads" only later became know as cryptospores, a term which was coined in a 1984 paper by John Richardson and colleagues. Tetrahedraletes Strother & Traverse 1979, described originally from a section of the Tuscarora Fm exposed along route 322 at Mann Narrows (Jacks Mountain), has been recovered literally worldwide and has become the taxon that is now thought to represent the earliest spore record of land plants. Current research places the earliest occurrence of these tetrahedral tetrads in the Darriwilian of Saudi Arabia, and this date is now generally accepted as the time of origin of the embryophytes. There are much older spore-like remains, but they do not form the isometric tetrads that are reflective of sporogenesis in land plants. Ongoing studies of early Ordovician to middle Cambrian spore-like palynomorphs in combination with extant ontogenetic studies, seem to fit a model of land plant origins proposed by Bower over 100 years ago - the antithetic hypothesis. It now appears that the origin of land plants was not a singular moment in time, but, rather, took place in a more gradual fashion as separate sporophytic characters were acquired during the early Cambrian to lowermost Devonian interval.