NEW EVIDENCE FOR THE EXISTENCE OF CAMBRIAN EMBRYOPHYTES

Clusters of cryptospores associated with organic coverings occurring in the uppermost Rogersville Shale can be interpreted as fragments of plant sporangia. This material from the well-documented ORNL JOY-2 core (Tennessee, USA) is Middle Cambrian (Ehmaniella biozone) in age. Sporangial fragments consist of clustered sporomorphs covered by simple homogeneous organic layers or distinctly ribbed structures. When extracted from sediments that predate the first appearance of tracheophytes (middle Silurian), highly regular, tetrahedral cryptospore tetrads have become an unquestioned proxy for land plants. In Ordovician through Devonian assemblages, these “perfect” tetrads are always accompanied by a range of morphotypes of alete spores, cryptospores, which vary from relatively simple monads (e.g. Sphaerasaccus, Laevolancis) to various dyads (e.g. Artemopyra, Abditusdyadus, Dyadospora) to more loosely attached tetrads (Steqambiquadrella, Rimosotetras) and tetrads with planar and quasi-planar configurations (unnamed forms). Assemblages throughout the Middle Cambrian of Laurentia contain fewer “perfect” tetrads, but overall, these populations are more similar in character to younger cryptospore assemblages than they are to populations of known (fossil or extant) algae. In combination with these new finds, these fossils seem to indicate that characters associated with adaptation to subaerial habitats may be decoupled from the specific origin of embryos in plants. Clearly, meiotically-produced spores from unilocular sporangia preceded the evolution of upright axial sporophytes. The question remains as to whether such Cambrian “plants” were true embryophytes, but evidence seems to be moving toward a late Precambrian to basal Paleozoic origin to the plant kingdom (sensu Margulis & Schwartz).