Chapter 7
Dating the origin of land plants
Charles H.Wellman
ABSTRACT
Ascertaining the time of origin of land plants has been a long standing scientific
concern. Initial estimates were based on the early land plant megafossil record, and
suggested a late Silurian origin. However, the megafossil record is notoriously
poor, and subsequent work on the early land plant dispersed microfossil record
(spores and phytodebris) indicates an earlier origin, in the Llanvirn (Ordovician). The
disparity between the plant megafossil/microfossil record is believed to be largely a
consequence of the low preservation potential of the earliest land plants: these are
considered to have been bryophyte-like, and most likely lacked recalcitrant tissues,
such as those containing lignin, that facilitate preservation as megafossils. More
recently, other techniques have been employed to shed light on the time of origin of
land plants. The geochemical record has been examined for signals reflecting global
change considered to have been concomitant with the developmental of a
significant terrestrial vegetation. Such techniques are, however, fraught with
difficulties. Assessing the extent and pattern of global change associated with the
development of terrestrial vegetation is largely conjectural. Furthermore, the
relationships between global change and geo-chemical signals are often poorly
understood. Another approach has been the utilization of molecular clock
techniques. Recent molecular clock analyses suggest that land plants diverged
significantly earlier than expected based on fossil evidence. However, concern has
been expressed that many of the currently utilized molecular clock techniques are
flawed, and indeed that the molecular clock does not exist. None the less, research
into molecular clocks is still in its infancy, and there is hope that, ultimately, more
reliable techniques will appear that will engender greater confidence in results. It is
argued here that currently the most reliable estimate for the time of origin of land
plants remains the minimum estimate of Llanvirn (Ordovician) provided by the
early land plant dispersed microfossil record. In future this benchmark may well be
breached as new fossil finds and new techniques (possibly refined molecular clock
techniques) come to light.