MULTIPLE LINES OF EVIDENCE SUPPORT THE PRESENCE OF CAMBRIAN LAND PLANTS

Cryptospores are spore-like microfossils that pre-date trilete spores characteristic of primitive tracheophytes. Perfectly tetrahedral cryptospore tetrads occur in the middle Ordovician (Llanvirn), setting the presently accepted date for the origin of land plants (embryophytes). The cryptospore record in Laurentia begins in the late early Cambrian Rome Fm (Tennessee). The Cambrian cryptospores are generally dismissed as "algal" in origin, but several independent lines of evidence strongly support bryophyte over algal affinity for these microfossils, including: 1) the recovery possible sporangia - spore clusters attached to several varieties of recalcitrant tissues, 2) the recognition of a wide variety of non-tetrahedral tetrad topologies that persist throughout the entire range of the cryptospore record, 3) Comparative morphology of Cambrian and younger cryptospore dyads, 4) Topologies of multi-layered walls that are consistent with spore development in liverworts in which the free archesporal sporocytes undergo one or two mitoses prior to meiosis, 5) Cytological studies in cryptogamic embryophytes that retain ancestral dyad formation, or bipolar spindle pairs, during sporogenesis, 6) Cloverleaf-style microtubule arrangements in some developing bryophyte sporocytes that mimic thickenings seen in dispersed Cambrian and younger cryptospore tetrads, 7) Studies in the sporogenesis of extant bryophytes suggestting that cytological control of sporogenesis was an important component of Natural Selection during the origin and early diversification of the bryophyte groups. An early origin of land plants is also supported by recent observations on the nature of shifts in the mineralogy of weathered clays in the Proterozoic. The transition from aquatic to subaerial habitats has been accomplished by a number of phylogenetically disparate chlorophytes, but these algae today are limited in their distribution and none of them gave rise to the Embryophyta. It is the evolution of embryonic development, beginning with ancestral bryophytes, which enabled the full exploitation of subaerial habitats on Earth. Evidence from cryptospores now indicates that ancestral embryophytes, some of which were at a bryophyte grade of evolution, were abundant colonizers of the land surface by Middle Cambrian time.