''Nematothallus'' is a form
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
comprising cuticle-like
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s. Some of its constituents likely represent
red algae,
whereas others resemble
lichens.
History of research
''Nematothallus'' was first described by Lang in 1937,
who envisioned it being an
early thallose land plant
The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as sist ...
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. Even so, it was picked up by Jonker (1973), who proposed that ''Nematothallus'' represented leaves of ''Prototaxites'', which he interpreted as a red alga.
Further work failed to draw together all aspects of the organism: Edwards (1982)
and Edwards and Rose (1984)
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,
[ 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,] which are instead referred to as nematoclasts (Graham & Gray 2001).
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.[ 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.][
The genus was later formalised by Strother,][ who discovered better preserved and more complete specimens in ]Pennsylvania
Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...
, America – which appear to show tubes connected to the rims of cuticle. ''Nematothallus'' is widespread from the late Silurian, but similar cuticle is reported from the Caradoc
Caradoc Vreichvras (; Modern cy, Caradog Freichfras, ) was a semi-legendary ancestor to the kings of Gwent. He may have lived during the 5th or 6th century. He is remembered in the Matter of Britain as a Knight of the Round Table, under the ...
epoch (late Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
, ). It is, however, difficult to distinguish ''Nematothallus'' cuticle from that of arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s.[
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. One suggestion based on fossil evidence for their purpose is the release of spores, although this may be the result of spores being trapped between two layers of cuticle.][
]
Habitat
''Nematothallus'' was originally recovered from the Red Downtonian near the base of the Old Red Sandstone, between the clearly marine Ludlow deposits beneath it, and the unambiguously terrestrial Lower Old Red Sandstone.[ Further samples came from elsewhere in the Lower Old Red Sandstone (St Maughams Group, lowermost Devonian), where they co-occur with plant spores.][ Additional, older material occurs with '' Hostinella'', '']Cooksonia
''Cooksonia'' is an extinct group of primitive land plants, treated as a genus, although probably not monophyletic. The earliest ''Cooksonia'' date from the middle of the Silurian (the Wenlock epoch); the group continued to be an important comp ...
'' and '' Steganotheca'' in the Ludlow (upper Ludfordian) Whitcliffe formation.[
]
Affinity
Some material assigned to ''Nematothallus'' closely resembles certain lichens, suggesting a lichenous habit – although as the lichens evolved at least six times independently, this does not fully establish the genetic affinity. The biochemistry of the organism is not inconsistent with an algal affinity, but Edwards (1982) considers it unlikely that algae would be preserved as coalified impressions.[ 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.)][ The most likely affinity, for the aperture-bearing cuticles, is with the coralline red algae; the absence of biomineralization suggests a stem-group level affiliation.] Whereas Smith & Butterfield contend that this material is separate from the charcoalified lichen-like material, Edwards insists that all the material comes from the same lichen-like organism.
Synonyms
''N. taenia'' and ''N. lobata'' were both collapsed into a new form-species ''Laevitubulus laxus''.
See also
*'' Cosmochlaina'', a closely related sister taxon.
*'' Spongiophyton'', a similarly enigmatic organism with a similar cuticular patterning.[ Some material described as ''Spongiophyton'' may in fact be ''Nematothallus''.]
References
*
{{Taxonbar, from=Q1491485
Silurian plants
Devonian plants