Spongiophyton

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

Spongiophyton Temporal range: Early Devonian – Middle Devonian[1] PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Kingdom:	Plantae
Division:	incertae sedis
Family:	†Spongiophytaceae
Genus:	†Spongiophyton Kräusel (1954)
Type species
†Spongiophyton lenticulare[2][3] (Barbosa) Kräusel (1954)
Species
†S. articulatum Kräusel (1954) †S. hirsutum Kräusel (1954) †S. lenticulare (Barbosa) Kräusel (1954) †S. minutissimum Kräusel (1954) †S. nanum Kräusel (1954)

Spongiophyton displayed dichotomous branching, and a flattened/elliptical cross section with a thick (20–80 μm) upper cuticular surface.[4] It is also perforated with pores resembling those of some liverworts.[4] It probably grew on the banks of rivers.[5] Spongiophyton has been mistakenly interpreted as tree resin[6] and lycopod cuticle,[7] and was later identified as the cuticle of a thalloid plant.[8] It has most recently been interpreted on morphological[9] and isotopic[10] grounds as a lichen—which would place it with Winfrenatia among the earliest known representatives of this group.[11]

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.[4] 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.[4]