Spongiophyton

"Spongiophyton" was a thallose fossil of the early to mid Devonian, which is notoriously difficult to classify.

Affinity

"Spongiophyton" has been mistakenly interpreted as tree resin [Penhallow (1889) in Fletcher (2004)] and lycopod cuticle, [Barbosa (1949) in Fletcher (2004)] and was later identified as the cuticle of a thalloid plant. [Kräusel (1954) in Fletcher (2004)]

It has most recently been interpreted on morphological [cite journal|journal=Int. J Plant Sci. 165(5)|pages=875–881. |year=2004.
id=1058-5893/2004/16505-0018$15.00
doi=10.1086/422129
url=http://www.journals.uchicago.edu/doi/full/10.1086/422129?cookieSet=1
title=SEM Analysis of "Spongiophyton" Interpreted as a Fossil Lichen
author=Wilson A. Taylor, Chris Free, Carolyn Boyce, Rick Helgemo, Jaime Ochoada
volume=165] and isotopiccite journal
author = Jahren, A.H.
coauthors = Porter, S.; Kuglitsch, J.J.
year = 2003
title = Lichen metabolism identified in Early Devonian terrestrial organisms
journal = Geology
volume = 31
issue = 2
pages = 99–102
doi = 10.1130/0091-7613(2003)031] grounds as a lichen - which would make it the earliest known representative of this group. [Retallack (1994, "Were the Ediacaran fossils lichens?") suggested that the Ediacaran biota were lichens, but he has since refined this hypothesis (Retallack 2007, "Dickinsonia"...").]

However, the significance of the isotopic data has been called into question. Jahren "et al". argued that mosses and liverworts had a delta|13|C signature of under -26‰, and lichens were exclusively > -26‰. But in deducing this they relied solely on their own data, neglecting to include published datasets or bryophytes from a range of habitats. They also failed to take into account any adjustment necessary to overcome post-burial alteration of the delta|13|C, or to compensate for the different isotopic composition of the early Devonian atmosphere.cite journal
author = Fletcher, B.J.
coauthors = Beerling, D.J.; Chaloner, W.G.
year = 2004
title = Stable carbon isotopes and the metabolism of the terrestrial Devonian organism "Spongiophyton"
journal = Geobiology
volume = 2
issue = 2
pages = 107–119
doi = 10.1111/j.1472-4677.2004.00026.x] Repeating Jahren's experiments with these factors taken into account shows that most major groups' delta|13|C values overlap significantly, and do not provide a statistically significant case for the inclusion of "Spongiophyton" in any group.

Morphology

The organism displayed dichotomous branching, and a flattened/elliptical cross section with a really thick (20-80μm) upper cuticlular surface. [Fletcher (2004)] It is also perforated with pores resembling those of some liverworts. [Fletcher (2004)]

Habitat

"Spongiophyton" probably grew on the banks of rivers. [Gensel "et al". 1991; Griffing "et al." 2000; in Fletcher (2004)]

Notes and references