Nematothallus

Nematothallus Temporal range: Ludfordian–Downtonian PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom:	Plantae (?)
Phylum:	Nematophyta
Class:	†Nematophytina Strother 1993
Order:	†Nematophytales Lang 1937[1]
Family:	†Nematothalaceae Strother 1993[2]
Genus:	†Nematothallus Lang 1937
Species
N. pseudo-vasculosa Lang 1937 N. radiata Lang 1937

Nematothallus is a form genus comprising cuticle-like fossils.

History of research

Nematothallus was first described by Lang in 1937^[1], 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.^[3] 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.^[4]

Further work failed to draw together all aspects of the organism: Edwards (1982)^[5] and Edwards and Rose (1984)^[6] both provided thorough descriptions of the cuticular aspects of the plants, while Pratt et al. (1978)^[7] 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,^[8][9] 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,^[2] which are instead referred to as nematoclasts (Graham & Gray 2001).^[10]

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.^[6] 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.^[5]

The genus was later formalised by Strother,^[2] who discovered better preserved and more complete specimens in Pennsylvania, America – which appear to show tubes connected to the rims of cuticle.^[9] Nematothallus is widespread from the late Silurian, but similar cuticle is reported from the Caradoc epoch (late Ordovician, 450 million years ago).^[11] It is, however, difficult to distinguish Nematothallus cuticle from that of arthropods.^[9]

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.^[6]

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.^[1] Further samples came from elsewhere in the Lower Old Red Sandstone (St Maughams Group, lowermost Devonian), where the co-occur with plant spores.^[6] Additional, older material occurs with Hostinella, Cooksonia and Steganotheca in the Ludlow (upper Ludfordian) Whitcliffe formation.^[5]

Affinity

There is a possibility that the genus represents a lichen, rather than a plant – although this is not yet supported by firm evidence.^[12] The biochemistry of the organism is not inconsistent with an algal affinity,^[13] but Edwards (1982) considers it unlikely that algae would be preserved as coalified impressions.^[5] 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.)^[5]

Synonyms

N. taenia and N. lobata were both collapsed into a new form-species Laevitubulus laxus.^[14]

See also

• Cosmochlaina, a closely related sister taxon.

• Spongiophyton, a similarly enigmatic organism with a similar cuticular patterning.^[5] Some material described as Spongiophyton may in fact be Nematothallus.^[15][16]

References

1. ^ ^{a b c} Lang, W. H. (1937). "On the Plant-Remains from the Downtonian of England and Wales". Philosophical Transactions of the Royal Society B: Biological Sciences 227 (544): 245–226. Bibcode 1937RSPTB.227..245L. doi:10.1098/rstb.1937.0004. JSTOR 92244.  edit
2. ^ ^{a b c} Strother, P. K. (1993). "Clarification of the genus Nematothallus Lang". Journal of Paleontology 67 (6): 1090–1094. doi:10.2307/1306131. JSTOR 1306131.  edit
3. ^ Lang, W. H. (1945). "Pachytheca and some anomalous early plants (Prototaxites, Nematothallus, Parka, Foerstia, Orvillea n.g.)". Botanical Journal of the Linnean Society 52: 535. doi:10.1111/j.1095-8339.1945.tb01351.x.  edit
4. ^ Jonker, FP (1979). "Prototaxites in the lower Devonian". Palaeontographica Abteilung B (Schweizerbart'sche Verlagsbuchhandlung) 171 (1–3): 39–56. 
5. ^ ^{a b c d e f} Edwards, D. (1982). "Fragmentary non-vascular plant microfossils from the late Silurian of Wales". Botanical Journal of the Linnean Society 84 (3): 223–256. doi:10.1111/j.1095-8339.1982.tb00536.x.  edit
6. ^ ^{a b c d} Edwards, D.; Rose, V. (1984). "Cuticles of Nematothallus: a further enigma". Botanical Journal of the Linnean Society 88: 35. doi:10.1111/j.1095-8339.1984.tb01563.x.  edit
7. ^ Pratt, L. (1978). "Evidence of non-vascular land plants from the early Silurian (Llandoverian) of Virginia, U.S.A". Review of Palaeobotany and Palynology 25 (2): 121–149. doi:10.1016/0034-6667(78)90034-9.  edit
8. ^ Gensel, P. G.; Johnson, N. G.; Strother, P. K. (1990). "Early Land Plant Debris (Hooker's "Waifs and Strays"?)". PALAIOS 5 (6): 520–547. doi:10.2307/3514860. JSTOR 3514860.  edit}}
9. ^ ^{a b c} Strother, P. K. (1988). "New Species of Nematothallus from the Silurian Bloomsburg Formation of Pennsylvania". Journal of Paleontology 62 (6): 967–982. doi:10.2307/1305385. JSTOR 1305385.  edit
10. ^ Graham, L.E.; Gray, J. (2001). "The Origin, Morphology, and Ecophysiology of Early Embryophytes: Neontological and Paleontological Perspectives". Plants Invade the Land: Evolutionary and Environmental Perspectives (Columbia University Press). ISBN 9780231111614. http://books.google.com/?id=xN8fK6kDkeAC&pg=PA140&dq=The+Origin,+Morphology,+and+Ecophysiology. Retrieved 2008-05-06. 
11. ^ Edwards, D.; Kerp, H.; Hass, H. (1998). "Stomata in early land plants: an anatomical and ecophysiological approach". Journal of Experimental Botany 49 (90001): 255. doi:10.1093/jexbot/49.suppl_1.255.  edit
12. ^ Retallack, G. J. (2007). "Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil". Alcheringa: an Australasian Journal of Palaeontology 31 (3): 215–240. doi:10.1080/03115510701484705.  edit
13. ^ Niklas, K. J. (1976). "Chemical Examinations of Some Non-Vascular Paleozoic Plants". Brittonia (New York Botanical Garden Press) 28 (1): 113–137. doi:10.2307/2805564. JSTOR 2805564.  edit
14. ^ Burgess, N. D.; Edwards, D. (1991). "Classification of uppermost Ordovician to Lower Devonian tubular and filamentous macerals from the Anglo-Welsh Basin". Botanical Journal of the Linnean Society 106: 41. doi:10.1111/j.1095-8339.1991.tb02282.x.  edit
15. ^ "Spongiophyton z dolnego dewonu Pol­ski [On Spongiophyton from the Lower Devonian of Poland]" (in Polish). Acta Palaeobotanica 19 (1): 13–20. 1978. http://www.botany.pl/ibwyd/acta_paleo/act-p19.htm. 
16. ^ Filipiak, P.; Zatoń, M. (2010). "Plant and animal cuticle remains from the Lower Devonian of southern Poland and their palaeoenvironmental significance". Lethaia: no–no. doi:10.1111/j.1502-3931.2010.00251.x.  edit

• McGregor, DC; Narbonne, GM (1978). "Upper Silurian trilete spores and other microfossils from the Read Bay Formation, Cornwallis Island, Canadian Arctic". Canadian Journal of Earth Sciences (NRC Research Press) 15 (8): 1292–1303. doi:10.1139/e78-136.