CRYPTOSPORE DYADS: A NEW PROXY FOR EARLY EMBRYOPHYTES

Cryptospore tetrads bound in the tetrahedral configuration have long been recognized as an embryophytic synapomorphy. It is possible to trace such tetrads back in time to the Middle Cambrian, but by far most of the tetrads seen in the Cambro-Ordovician are planar to irregular in their configuration. This weakens their connection to the embryophytes, because planar tetrads are more typical of the algae. In fact, these planar forms have a record extending well into the Precambrian. Isometric dyads, on the other hand, are not typical of any algae. Their only link to parent plants is their occurrence in sporangia of rhyniophytoids such as Fusitheca fanningiae. Thus, given that rhyniophytoids are true plants, there is no reason to reject dispersed cryptospore dyads as a valid proxy for embryophytes.

Unlike planar and irregular tetrads, cryptospore dyads do not appear to extend into the Precambrian – their first occurrence is in the Middle Cambrian. TEM studies of wall ultrastructure of these earliest dyads (performed by Wilson Taylor at the University of Wisconsin) indicate that they were formed in a sporangium compatible with an embryophytic derivation. Modern studies of sporogenesis in living cryptogamous plants indicate that dyad production could be an ancient character state associated with the lycopods. This hypothesis is reinforces the arguments for the Ludlow age for Baragwanathia; a robust lycopod whose stratigraphic occurrence below much simpler rhyniophytoids has not has not been well received by many paleobotanists. The cryptospore record joins molecular data from extant plants in claiming origination dates for the plant kingdom that are far earlier than the late Silurian dates based on mesofossil remains.