knowledge will be filled following further research on in situ spores and spore wall
ultrastructure, which may shed further light on the evolutionary relationships of early land
plants.
Embryophyte spores from the Cambrian?Recently it has been suggested that the cryptospore fossil record may extend as far back as
the Middle Cambrian. Strother and Beck (2000) described an intriguing assemblage of
palynomorphs from the Middle Cambrian Bright Angel Shale from Arizona, USA. The
palynomorphs have a relatively thick wall, are arranged as monads and polyads (containing
two, three, four, and more than four units), and are often enclosed within a thin envelope.
To a certain extent they resemble cryptospores derived from embryophytes, but they
differ in that the walls are thinner and they are loosely arranged, lacking the rigidity,
regular arrangement, and symmetry of embryophyte cryptospores. Strother and Beck
acknowledge that ‘there are no spores in the Bright Angel assemblage that are
convincingly like spores from any known embryophytes’ (Strother and Beck 2000: p.
417) and ‘there is no direct evidence that the fossils from the Bright Angel Shale represent
either embryophytes or their evolutionary ancestors’ (Strother and Beck 2000: p. 419).
This view is supported by Taylor (1999) who presented preliminary results of an analysis
of wall ultrastructure in the palynomorphs from the Bright Angel Shale, noting that the
walls are quite different from those reported from embryophyte cryptospores.
Strother and Beck interpreted the Bright Angel Shale palynomorphs as deriving from
either freshwater aquatic or subaerial ‘cryptogams’ that colonized a very early terrestrial
landscape. This is based on their interpretation that the muddy component of the
depositional system (incorporating the palynomorphs) was derived from fresh-water
sources washed into estuaries (Strother 2000). Evidence for this includes sedimentological
structures indicative of intertidal to supratidal conditions, and the fact that normal marine
palynomorphs and marine megafossils are absent.
Strother (2000) briefly mentioned a further Middle Cambrian palynomorph assemblage
containing similar cryptospore-like fossils from the Rogersville Shale from Tennessee,
USA. These palynomorphs are more similar to embryophyte cryptospores (personal
observation) but still lack their rigidity and symmetry. More evidence (e.g. further TEM
analysis) is required before they can be accepted as indicative of embryophytes, and until
such time I consider it prudent to regard the earliest fossil evidence for embryophytes to
be Llanvirn cryptospores. I see no reason why the embryophyte cryptospore record could
not extend back beyond the Llanvirn, but more proof is required, as it is quite possible
that these palynomorphs derive from organisms (inhabiting terrestrial aquatic, or even
subaerial habitats) unrelated to the embryophytes.
Dispersed phytodebrisPhytodebris includes fragmentary cuticles and tubular structures that are produced during
plant disarticulation and transported by water into sedimentary environments. Their
affinities have long been controversial (see Banks 1975, and Gray and Boucot 1977, for an
early debate on this subject). The relevance of these fragmentary remains to the study of
134 CHARLES H.WELLMAN