STROTHER, PAUL K. Weston Observatory of Boston College, Department of Geology & Geophysics, 381 Concord Road, Weston MA 02493. - The cryptospore record indicates a Cambrian origin for land plants.
Cryptospore tetrads and dyads have been recovered from Middle Cambrian
strata in Arizona (Bright Angel Shale) and eastern Tennessee
(Rogersville Shale). Although many tetrads are irregular in their
configuration, small tetrahedral tetrads similar to Tetrahedraletes
grayae strengthen the likelihood that these are meiotic tetrads
from spore-bearing embryophytes. It is now possible to trace a
cryptospore history from the Middle Cambrian to the Devonian. Late
Cambrian tetrads are known from at least three sites in North America.
The Cambro-Ordovician record in Europe and North Africa is
characterized by the monads, Attritasporites and
Virgatasporites. Middle Ordovician deposits in Saudi Arabia and
the Prague Basin contain dyads, tetrads and other cryptospores,
including spore clusters and cuticle-like fragments. The Caradoc is
marked by evolutionary stasis, based on Charles Wellman’s study of the
type region in the UK. Uppermost Ordovician and lower Silurian strata
contain diverse and widespread cryptospore assemblages. Cryptospores
continue to diversify through the Homerian (middle Silurian) origin of
tracheophytes and decline in abundance in the Early Devonian as
trilete spores begin to dominate terrestrial assemblages. Although
some membrane-enclosed cryptospores have been attributed to possible
freshwater chlorophytes that have strayed into estuarine waters, the
bulk of morphological evidence points clearly to an embryophytic
affinity for these fossils. The affinities of even the earliest
cryptospore tetrads are with hepatics and anthocerophytes. Since
rhyniophytoids are the only plants (living or extinct) known to
produce dyads normally in sporangia, dyads also stand as a proxy for
the embryophytes. (To propose otherwise requires the retention of dyad
sporogenesis from a hypothetical charophycean ancestor.) Thus,
cryptospores continue to serve as a proxy for early land plants that
have yet to reveal a macro or mesophyte record. It now seems plausible
that embryophytes occupied terrestrial habitats throughout the
entirety of the Paleozoic.
Key words: Cambrian, cryptospore, dyad, origin of land plants, palynology, plant evolution