Spongiophyton

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. It is also perforated with pores resembling those of some liverworts. It probably grew on the banks of rivers. Spongiophyton has been mistakenly interpreted as tree resin and lycopod cuticle, and was later identified as the cuticle of a thalloid plant. It has most recently been interpreted on morphological and isotopic grounds as a lichen - which would make it the earliest known representative of this group.

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 2998740000000000000♠−26‰, and lichens were exclusively > 2998740000000000000♠−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. 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.