Early land plant microfossil recordIn contrast to the early land plant megafossil record, the dispersed microfossil record is
excellent. These microfossils represent organs shed during the lifetime of the plants
(spores) or fragments of recalcitrant tissue released following disarticulation of the plant
(cuticles and tubular structures). Spores have an excellent fossil record because: (i) they
have high preservation potential because they have an outer wall composed of sporopollenin,
an extremely recalcitrant macromolecule; (ii) they are produced in vast quantities
(Table 7.1 presents estimated spore production per sporangium for selected early land
plants); and (iii) they have a huge dispersal potential and are easily transported into
environments favourable for preservation (e.g. lacustrine and river floodplain
environments and shallow marine clastic deposits). There are numerous reports of early
land plant spore assemblages with excellent spatial and temporal coverage (reviewed by
Steemans 1999; Wellman and Gray 2000; Edwards and Wellman 2001). Similarly,
fragmented cuticles and tubular structures that are small and produced in large quantities
may be transported large distances (primarily by water) into environments favourable to
preservation, and are composed of recalcitrant materials that are easily preserved.
Dispersed sporesAs early as 1959 trilete spores, that undoubtably derive from land plants, were reported
from deposits of Llandovery (early Silurian) age (Hoffmeister 1959), and it was
demonstrated that there was an increase in trilete spore diversity throughout the Silurian
(Richardson and Lister 1969; Richardson and Ioannides 1973). These spores were
uncontroversially accepted as deriving from embryophytes, as similar spores are known
Table 7.1 Estimated number of spores per sporangium in selected early land plants (calculated by
dividing the volume of the sporangium by the volume of a spore unit of mean diameter and
assuming 74 per cent close packing)
DATING THE ORIGIN OF LAND PLANTS 129