Prototaxites

Taxobox
fossil_range = fossil range|420|370Late Silurian to Upper Devonian

---

image_caption = An 1888 illustration of "Prototaxites" in section.
regnum = Incertae sedis
genus = "Prototaxites"
genus_authority = Dawson 1859
subdivision_ranks = Species
subdivision =
*"P. loganii":aut|Dawson 1859
*"P. southworthii":aut|Arnold 1952

The genus "Prototaxites" (pronEng|ˌproʊtoʊˈtæksɨtiːz) describes terrestrial organisms known only from fossils dating from the Silu-Devonian, approximately 420 to 370 million years ago. "Prototaxites" formed large trunk-like structures up to 1 m (3 ft) wide, reaching 8 m (26 ft) in height, made up of interwoven tubes just 50 μm in diameter. Whilst traditionally very difficult to assign to an wict|extant group of organisms, current opinion is converging to a fungal placement for the genus.

Morphology

With a diameter of up to a meter, and a height reaching 8 m, "Prototaxites" fossils are by far the largest from its period of existence. Viewed from afar, the fossils take the form of tree-trunks, spreading slightly near their base in a fashion that suggests a connection to unpreserved root-like structures. Infilled casts which may represent the "roots" of Prototaxites are common in ealry Devonian strata.cite journal | title=Sedimentological evidence for rooting structures in the Early Devonian Anglo–Welsh Basin (UK), with speculation on their producers | year = 2008 | doi = 10.1016/j.palaeo.2008.01.038 | author=Hillier, R, Edwards, D;Other, A.N. | journal=Palaeogeography Palaeoclimatology Palaeoecology ] ; Concentric growth rings, sometimes containing embedded plant material, suggest that the organism grew sporadically by the addition of external layers. It is probable that the preserved "trunks" represent the fruiting body, or "sporophore", of a fungus, which would have been fuelled by a net ("mycelium") of dispersed filaments ("hyphae"). On a microscopic scale, the fossils consist of narrow tube-like structures, which weave around one another. These come in two 'flavours': skeletal "tubes", 20–50 μm across, have thick (2–6 μm) walls and are undivided for their length, and wict|generative "filaments", which are thinner (5–10 μm diameter) and branch frequently; these mesh together to form the organism's matrix. These thinner filaments are septate – that is to say, they bear internal walls. These septa are perforate - i.e. they contain a pore, a trait only present in the modern red algae and fungi.cite journal
doi = 10.1126/science.191.4224.287
pmid = 17832148
author = SCHMID, RUDOLF
journal = Science
volume = 191
issue = 4224
pages = 287–288
year = 1976
title = Septal Pores in Prototaxites, an Enigmatic Devonian Plant
accessdate = 2008-06-24]

The similarity of these tubes to structures in early plant "Nematothallus" has led to suggestions that the latter may represent leaves of "Nematothallus". Unfortunately for this hypothesis, the two have never been found in connection, although this may be a consequence of their detachment after the organisms' death.cite journal | author = Jonker, F.P. | year = 1979 | title = Prototaxites in the Lower Devonian | journal = Palaeontographica, B | pages = 39–56]

History of research

First collected in 1843, it was not until 14 years later that John William Dawson, a Canadian scientist, studied "Prototaxites" fossils, which he described as partially rotten giant conifers, containing the remains of the fungi which had been decomposing them. This concept was not disputed until 1872, when a rival scientist named Carruthers poured ridicule on the idea. Such was his fervour that he rebuked the name "Prototaxites" (loosely translated as "first yew" [The "Taxinaea" (Taxaceae) are the grouping of conifers to which Dawson drew analogy] ) and insisted that the name "Nematophycus" ("stringy algaVerify source|date=February 2008 ") be adopted, a move strongly against scientific convention. Dawson fought adamantly to defend his original interpretation until studies of the microstructure made it clear that his position was untenable, whence he promptly attempted to rename the genus himself (to "Nematophyton", "stringy plant"), denying with great clout that he'd ever considered it to be a tree. Despite these political attempts to re-name the genus, the rules of botanical nomenclature mean that the name "Prototaxites", however inappropriate in meaning, remains in use today.

Despite the overwhelming evidence that the organism grew on land, Carruthers' interpretation that it was a giant marine alga was challenged just the once, in 1919, when Church suggested that Carruthers had been too quick to rule out the possibility of the fungi. The lack of any characters diagnostic of any extant group made the presentation of a firm hypothesis difficult, and so the fossil remained an enigmatic mystery and subject of debate. It was not until 2001, after 20 years of research, that Francis Hueber, of Washington's National Museum of Natural History, published a long-awaited paper which attempted to put "Prototaxites" in its place. The paper deduced, based on its morphology, that "Prototaxites" was a fungus.cite journal | author = Hueber, F.M. | year = 2001 | title = Rotted wood-alga-fungus: the history and life of Prototaxites Dawson 1859 | journal = Review of Palaeobotany and Palynology | volume = 116 | issue = 1 | pages = 123–158 | doi = 10.1016/S0034-6667(01)00058-6]

This idea was faced with disbelief, denial and strong scepticism, but further evidence is emerging to support it. In 2007, isotopic analyses by a team including Hueber and Kevin Boyce of the University of ChicagoCite journal|author=Boyce, K.C.; Hotton, C.L.; Fogel, M.L.; Cody, G.D.; Hazen, R.M.; Knoll, A.H.; Hueber, F.M. |title =Devonian landscape heterogeneity recorded by a giant fungus | journal =Geology | volume =35 | issue =5 | pages =399–402 | date =May 2007 | year =2007 | url=http://geology.geoscienceworld.org/cgi/reprint/35/5/399.pdf|doi= 10.1130/G23384A.1] concluded that "Prototaxites" was a giant fungus. They detected a highly variable range of values of carbon isotope ratios in a range of "Prototaxites" specimens; autotrophs (organisms such as plants and algae, that make a living via photosynthesis) living at the same time draw on the same (atmospheric) source of carbon; as organisms of the same type share the same chemical machinery, they reflect this atmospheric composition with a constant carbon isotope trace. The inconsistent ratio observed in "Prototaxites" appears to show that the organism did not survive by photosynthesis, and Boyce's team deduce that the organism fed on a range of substrates, such as the remains of whichever other organisms were nearby.

Ecological context

This organism would have been the tallest living thing in its day by far; the plant "Cooksonia" (pictured in navigational box below) only reached 1 m, and itself towered over the "moss forests"; invertebrates were the only other land-dwelling life. "Prototaxites" became extinct as shrubs and wict|vascular trees rose to prominence.cite journal | author = Selosse, M.A. | year = 2002 | title = Prototaxites: A 400 Myr Old Giant Fossil, A Saprophytic Holobasidiomycete, Or A Lichen? | journal = Mycological Research | volume = 106 | issue = 06 | pages = 641–644 | doi = 10.1017/S0953756202226313] The organism could have used its raised platform for spore dispersal, or if "Prototaxites" really did form its leaves, in competition for light. The presence of bio-molecules often associated with the algae may suggest that the organism was covered by symbiotic (or parasitic) algae, or an alga itself.cite journal | author = Niklas, K.J. | year = 1976 | title = Chemical Examinations of Some Non-Vascular Paleozoic Plants | journal = Brittonia | volume = 28 | issue = 1 | pages = 113–137 | url = http://links.jstor.org/sici?sici=0007-196X(197601%2F03)28%3A1%3C113%3ACEOSNP%3E2.0.CO%3B2-5 | accessdate = 2007-09-20 | doi = 10.2307/2805564] cite journal | author = Niklas, K.J. | coauthors = Pratt, L.M. | year = 1980 | title = Evidence for Lignin-Like Constituents in Early Silurian (Llandoverian) Plant Fossils | journal = Science | volume = 209 | issue = 4454 | pages = 396 | doi = 10.1126/science.209.4454.396]

"Prototaxites" mycelia (strands) have been fossilised invading the tissue of vascular plants; in turn, there is evidence of animals inhabiting "Prototaxites": mazes of tubes have been found within some specimens, with the fungus re-growing into the voids, leading to speculation that the organisms' extinction may have been caused by such activity; however, evidence of arthropod borings in "Prototaxites" has been found from the early and late Devonian, suggesting the organism survived the duress of boring for many millions of years.cite journal
author = Labandeira, C.
year = 2007
title = The origin of herbivory on land: Initial patterns of plant tissue consumption by arthropods
journal = Insect Science
volume = 14
issue = 4
pages = 259–275
doi = 10.1111/j.1744-7917.2007.00152.x] Intriguingly, "Prototaxites" is bored long before plants developed a structurally equivalent woody stem, and it is possible that the borers transferred to plants when these evolved.

References

External links

* [http://www.xs4all.nl/~steurh/engprot/eprototx.html Images and discussion of the classification of "Prototaxites"]
*cite news | first= | last= | coauthors= | title=Mystery fossil turns out to be giant fungus | date=2007-04-23 | publisher= | url =http://www.abc.net.au/news/newsitems/200704/s1905015.htm | work =ABC News Online | pages = | accessdate = 2007-04-23 | language =
* [http://environment.newscientist.com/article/dn11701-mystery-prehistoric-fossil-verified-as-giant-fungus.html Mystery prehistoric fossil verified as giant fungus]