Nematothallus

Nematothallus is a form genus comprising cuticle-like fossils. It was first described by Lang in 1937^[2], who envisioned it being an early thallose land plant with tubular features and sporophytes, covered by a cuticle which preserved impressions of the underlying cells. He had found abundant disaggregated remains of all three features, none of which were connected to another, leaving his reconstruction of the phytodebris as parts of a single organism highly conjectural.

Further work failed to draw together all aspects of the organism: Edwards (1982)^[3] and Edwards and Rose (1984)^[4] both provided thorough descriptions of the cuticular aspects of the plants, while Pratt et al. (1978) and Niklas and Smocovitis (1983) focused on the anatomy of the tubes. Indeed, some workers suggested that the name Nematothallus should only apply to the tubes,^[5][6] until Strother (1993) found more complete specimens, with tubes attached to the cuticle. He attempted to unite and formalise the genus, and extended it to include banded tubes,^[1] which are instead referred to as nematoclasts (Graham & Gray 2001).^[7]

It is possible that Nematothallus consisted of two layers of cuticle, although fossils giving this impression may in fact represent two layers which happened to overlap one another and become stuck. It is not readily established what the cuticle represents.^[4] Lang (1945) had it as an epidermal layer, similar to the waxy cuticle of plants today, covering a parenchymatous layer. Alternatively, Edwards (1982) proposed that the inner tissue of Nematothallus comprised stringy tubes, with the cellular patterning produced by their ends.^[3]

The genus was later formalised by Strother,^[1] who discovered better preserved and more complete specimens in Pennsylvania, America - which appear to show tubes connected to the rims of cuticle.^[6] Nematothallus is widespread from the late Silurian, but similar cuticle is reported from the Caradoc epoch (late Ordovician, 450 million years ago).^[8] It is, however, difficult to distinguish Nematothallus cuticle from that of arthropods.^[6]

Further work by Edwards and Rose has identified oval-shaped growths in places on the cuticles of a limited number of Nematothallus fragments, which develop into holes - whose purpose is unknown. Since they are not found in all Nematothallus individuals, it is unlikely that they were involved in gas exchange - perhaps they were involved in the release of spores? One fossil gives the impression, which may well be an illusion, of spores being trapped between two layers of cuticle.^[4]

Affinity

There is a possibility that the genus represents a lichen, rather than a plant - although this is not yet supported by firm evidence.^[9] The biochemistry of the organism is not inconsistent with an algal affinity,^[10] but Edwards (1982) considers it unlikely that algae would be preserved as coalified impressions.^[3] However, Edwards does note that the surface patterning could have been produced in a similar fashion to surface layers in green algae - that is, by the ends of tightly packed filaments causing indentation on the surface layer. (Just because they were formed in the same way doesn't mean they were formed by green algae, though.)^[3]

See also

• Cosmochlaina, a closely related sister taxon.

• Spongiophyton, a similarly enigmatic organism with a similar cuticular patterning^[3]

References

McGregor and Narbonne (1978): "Upper Silurian trilete spores and other microfossils from the Read Bay Formation, Cornwallis Island, Canadian Arctic"

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