Spongiophyton

Spongiophyton Temporal range: Early Devonian – Middle Devonian[1] PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom:	Plantae
Phylum:	incertae sedis
Family:	†Spongiophytaceae
Genus:	†Spongiophyton Kräusel, 1954
Species
S. hirsutum S. lenticulare Kräusel, 1954(type) S. minutissimum Kräusel, 1954 S. nanum emend Chaloner et al, 1974

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

Spongiophyton displayed dichotomous branching, and a flattened/elliptical cross section with a thick (20–80 μm) upper cuticular surface.^[2] It is also perforated with pores resembling those of some liverworts.^[2] It probably grew on the banks of rivers.^[3] Spongiophyton has been mistakenly interpreted as tree resin^[4] and lycopod cuticle,^[5] and was later identified as the cuticle of a thalloid plant.^[6] It has most recently been interpreted on morphological^[7] and isotopic^[8] grounds as a lichen - which would make it the earliest known representative of this group.^[9]

The significance of the isotopic data has, however, been called into question. Jahren et al. argued that mosses and liverworts had a δ¹³C 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 wide range of habitats. They also failed to take into account any adjustment necessary to overcome post-burial alteration of the δ¹³C, or to compensate for the different isotopic composition of the early Devonian atmosphere.^[2] Repeating Jahren's experiments with these factors taken into account shows that most major groups' δ¹³C values overlap significantly, and do not provide a statistically significant case for the inclusion of Spongiophyton in any group.^[2]

Notes

1. Taylor, W. A.; Free, C.; Boyce, C.; Helgemo, R.; Ochoada, J. (2004). "SEM Analysis of Spongiophyton Interpreted as a Fossil Lichen". International Journal of Plant Sciences 165: 875. doi:10.1086/422129.
2. Fletcher (2004)
3. Gensel et al.. 1991; Griffing et al. 2000; in Fletcher (2004)
4. Penhallow (1889) in Fletcher (2004)
5. Barbosa (1949) in Fletcher (2004)
6. Kräusel (1954) in Fletcher (2004)
7. Taylor et al. (2004)
8. Jahren et al. (2003)
9. Retallack (1994) suggested that the Ediacaran biota were lichens, but has since refined this hypothesis (Retallack, 2007).

References

• Fletcher, B. J.; Beerling, D. J.; Chaloner, W. G. (2004). "Stable carbon isotopes and the metabolism of the terrestrial Devonian organism Spongiophyton". Geobiology 2 (2): 107–119. doi:10.1111/j.1472-4677.2004.00026.x.
• Jahren, A. H.; Porter, S.; Kuglitsch, J. J. (2003). "Lichen metabolism identified in Early Devonian terrestrial organisms". Geology 31 (2): 99. Bibcode:2003Geo....31...99J. doi:10.1130/0091-7613(2003)031<0099:LMIIED>2.0.CO;2. ISSN 0091-7613.
• Retallack, G. J. (1994). "Were the Ediacaran fossils lichens?" (PDF). Paleobiology 20 (4): 523–544.
• Retallack, G. J. (2007). "Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil" (PDF). Alcheringa: an Australasian Journal of Palaeontology 31 (3): 215–240. doi:10.1080/03115510701484705.
• Taylor, Wilson A., Chris Free, Carolyn Boyce, Rick Helgemo, Jaime Ochoada (2004). "SEM analysis of Spongiophyton interpreted as a fossil lichen". International Journal of Plant Sciences 165 (5): 875–881. doi:10.1086/422129.