the earliest land plants has recently been reviewed by Gray (1985), Gensel et al. (1991),
Wellman and Gray (2000), and Edwards and Wellman (2001).
Many post-Early Silurian dispersed cuticles clearly derive from embryophytes, and some
even possess structures such as stomata (Gensel et al. 1991; Edwards and Wellman 1996).
Earlier forms, however, are more controversial. It has been suggested that some
represent a cuticular covering of nematophytes (Lang 1937). These enigmatic ‘plants’
have a unique tubular anatomy (possibly with a cuticular covering). There is evidence to
suggest that they possibly represent pathogens or decomposers (Edwards et al. 1996b,
1998; Edwards and Richardson 2000), and it has been suggested that at least some have
fungal affinities (Hueber 2001). More recently, it has been suggested that some of these
cuticles may derive from early embryophytes (i.e. bryophytes). Experiments were
conducted in which extant bryophytes were exposed to high temperature acid hydrolysis
in order to study the fragments that survived (i.e. recalcitrant tissues that one would
expect to survive in the fossil record) (Kroken et al. 1996; Graham and Gray 2001;
Kodner and Graham 2001). These authors noted similarities between some of the extant
bryophyte remains and fossil cuticles, and suggested that some of the early dispersed
cuticles may derive from primitive embryophytes. This is a distinct possibility and
requires further examination. However, it should be noted that geo-chemical studies by
Edwards et al. (1996b) showed that some of the cuticles were markedly different in
chemical composition from coeval ones derived from axial higher land plants, suggesting a
fundamentally different affinity.
The dispersed tubular structures are even more problematic. They exhibit a variety of
forms and occur in complex associations, and almost certainly derive from diverse
sources. Some probably represent fungal hyphae (Sherwood-Pike and Gray 1985;
Wellman 1995). Others appear to derive from nematophytes (Gray 1985; Burgess and
Edwards 1991; Wellman 1995; Wellman and Gray 2000). Recently, following
experimental disaggregation of extant bryophytes (see above), a similarity between some
of the surviving extant bryophyte remains and some of the fossil tubular structures was
noted, and it has been suggested that some of the fossils may represent fragments of
ancient bryophytes (Kroken et al. 1996; Graham and Gray 2001; Kodner and Graham
2001). I am yet to be convinced by this argument owing to problems regarding the size
and symmetry of the remains. However, few extant bryophytes have been examined to
date, and this interesting avenue of research should be explored further.
Dispersed phytodebris does not extend the embryophyte fossil record back beyond the
Llanvirn. The earliest fragments of cuticle are reported from the Caradoc (Gray et al.
1982), although the age of these deposits has been questioned (Richardson and McGregor
1986). It is not until the Llandovery that such cuticle becomes abundant. Tubular
structures, on the other hand, extend back to the Llanvirn where they co-occur with
cryptospores. As noted by Graham and Gray (2001), the differences in the stratigraphical
ranges of cuticles and tubular structures suggests that they have a different origin.
ConclusionsThe embryophyte fossil record, whilst not always easy to interpret, provides tangible
evidence for the timing of the origin of land plants. However, it must be interpreted with
DATING THE ORIGIN OF LAND PLANTS 135