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The project concerns the study of the algal-plant transition from the perspective of the fossil record. In recent decades biologists have made great strides in the determination of which groups of the traditional "green algae" were the ancestors of the land plants; however, confirmation of the details and the timing of this major evolutionary transition have not been documented in the fossil record. We have been studying cryptospores from Cambrian and Ordovician strata as a proxy for the existence of land plants before the first occurrence of upright axial sporophytes. Small axes bearing terminal sporangia are the minimal set of characteristics traditionally used by paleobotanists as proof of the presence of land plants in the fossil record. Cryptospores retain two fundamentally diagnostic features that enable their use in unraveling phylogenetic relations between the algae and the land plants (embryophytes): 1) the nature of the internal structure of the spore wall, sporoderm ultrastructure, and, 2) the topology, that is the nested relationship, among multiple-walled spore bodies. Both of these essentially morphological features are being used to study the timing and nature of evolution of the algal-plant transition.

On the basis of detailed ultrastructural studies, we have determined that the cryptospores from Cambrian and Ordovician strata tend to be formed as dyad pairs which are surrounded (enclosed) in walls, forming spore packets. These packets represent the results of reduction division (meiotic sporogenesis), but they do not form geometrically uniform tetrads (isometric or tetrahedral) as seen in younger strata and in most spore generation in living plants. We have hypothesized that these cryptospore producers generated spores through successive cytokinesis during meiotic sporogenesis, rather than simultaneous cell division. This hypothesis has become a working model for the interpretation of a wide range of problematic cryptospore forms from Cambrian and lower Ordovician strata. We have used the 100-year-old idea of F. O. Bower, that the gametophyte generation evolved first from the algae as an amphibious generation, and that the spore-bearing generation was interpolated between successive gametophytic generations. Bower's hypothesis required that spore-generating tissues would have evolved before the vegetative porting of the plant sporophyte body. This is in fact exactly what we are now seeing in the fossil record; planar sheets of sporogenous tissues composed of cryptospore dyad packets.

We now believe that the study of the origin of embryophytes must be viewed more as the evolution of embryophyty. The acquisition of the suite of characteristics that define embryophytes needs to be viewed dynamically as a process of serial (evolutionary) acquisition of adaptations to the subaerial habitat, and not as a singularity in evolutionary time. This leads to the support of Bower's ideas and the recognition that the acquisition of the upright sporangiate axis represents the end of the evolutionary transition to embryophyty, not the beginning. One of the beginnings of this process can be found in sporoderm ultrastructure, the second major morphological breakthrough that has come during the tenure of this grant. The sporoderm of many of the Cambrian cryptospore (dyads) examined to date are clearly laminated. Other researchers have used the laminated nature of cryptospore sporoderm as evidence of liverwort (bryophyte) affinity in Late Ordovician fossils. But the laminated sporoderm in Cambrian dyads has now been shown to be homologous with the walls of Riccia and Sphaerocarpos, both liverworts. Given the phylogeny established by the most recent consensus of molecular phylogenomics, this establishes that the charophyte-liverwort split must have occurred before the Middle Cambrian. Thus, our findings now establish the minimum age for the origin of land plants as 510 Ma (Drumian).

This represents a significant departure form the traditionally accepted date for the origin of land plants which is middle Ordovician (Darriwilian), about 465 Ma (± 4 m.y.), based on the recovery of isometric tetrads from Saudi Arabia. But our studies in cryptospore topology, described above, now give us a reasonable explanation for the significance of the origin of tetrahedral (isometric) tetrads – this is the first occurrence of spores produced through simultaneous cytokinesis of the developing sporocyte. Prior to the Darriwilian, the spore record is comprised of spore dyads and dyad packets, representing successive meiosis. Post Darriwilian record begins to pick up tetradedral tetrads and trilete spores generated through simultaneous meiosis as seen in most spore-bearing plants today. Our research is now proposing that the fundamental (basal autapomorphy) character state that should be used to characterize the embryophytes is laminated sporoderm. Having a middle Cambrian origin to the land plants is consistent with present models of carbon dioxide levels of the Early Phanerozoic, and it should promote new thinking about the origin and evolution of the terrestrial biosphere.