Nematophytes †

Nematophytes, created by Strother (1993), were a group of strange organisms in Silurian and Devonian sediments. They tend to have algal-fungal characteristics that make it difficult to place them in any one category. The "body" of these organisms is a meshwork of various-sized filamentous tubes (nemato means "thread"; phyte means "plant"). Some of these organisms appear to be lichens or fungal-algae symbionts.

Prototaxites †

Found in Ordovician, Silurian, and Devonian sediments, these fossil can be immensely large (8m long, 1.3m wide). They exhibit a pseudoparenchymatous construction, with horizontally arranged tubes or hyphae.

This taxon has been placed in many wide-ranging taxonomic groups
- Phaeophyceae : Penhallow 1889; Pia 1927; Arnold 1947; Gothan and Weyland 1954; Taylor 1981; Meyen 1987; Schweitzer 1990
- Rhodophyceae: Jonker 1979; Schweitzer 1983
- Chlorophyta: Carruthers 1872
- Streptophyta: Dawson 1859, 1873, 1875; Lang 1937; Graham et al. 2010
- Fungi: Carruthers 1872; Church 1919; Selosse 2002;a Selosse and Strullu-Derrien 2015; Berbee et al. 2020
  - Ascomycota: Edwards et al. 2013; Honegger et al. 2018; Retallack 2019
  - Basidiomycota: Hueber 2001; Retallack 2019
  - Mucoromycota: Retallack and Landing 2014; Retallack 2019
An analysis of the evidence argues against an agaricomycete sporocarp or ascomycete lichen association and that the taxonomic identity remains unclear (Nelson & Boyce 2022)
- If Prototaxites was a fungus, it may represent part of an extinct lineage lacking extant descendants (Nelson & Boyce 2022)
Vajda et al. propose that Prototaxites represents a complex hyphal aggregation (rhizomorph) that may have grown horizontally similar to modern complex aggregated mycelial growth forms, such as cords and rhizomorphs. (Vajda et al. 2022)
- Anatomical similarities with modern fungal rhizomorphs Armillaria mellea
- Their main function was possibly to redistribute water and nutrition from nutrient-rich to nutrient-poor areas facilitating the expansion for early land plant communities
Loron et al. (2025) propose that Prototaxites cannot be assigned to any known group of eukaryotes, including fungi, lichens, algae, plants, animals, etc.
- While the tube-like nature of its cells makes the fungal connection seem apparent, there is no evidence of chitin, and "...its anatomy was fundamentally distinct from all known extant or extinct fungi"

Above: microscopic structure of Prototaxites

Below: Reconstruction of Prototaxites thriving in a Devonian landscape (Vajda et al. 2022)

Above: Reconstruction of Prototaxites during the Silurian

Above: F. Hueber, in front of fossil of Prototaxites

Protosalvinia †

Found in Late Devonian sediments, these fossils exhibit thalloid appearance with short dichotomous branching. The branches in the largest species were as much as one centimeter across.

Interpretations seem to narrow this organism to a photosynthetic plant, either a fossil liverwort or brown alga
- No definitive brown algae have been identified from before the Cenozoic era
- Lignin and cutin have been found in the thallus, similar to plants and charophytes
- Embedded in the tissues of the thallus are chambers in which spores (200 micrometre diameter) were produced by meiosis similar to conceptacles
- Spores show no features in common with living groups of brown algae, and have sporopollenin in the spore walls, similar to streptophytes

Parka decipiens †

Found in the Late Silurian and the Early Devonian, it is a compression fossil that is circular or elliptic in shape

The form is 0.5 - 2.0 cm in diameter, showing a reticulated, mesh-like structure on the surface
Small discs present in the meshes of the reticulum, which appear to be sporangia with 30K+ spores
Some specimens show a "holdfast" in the center of disk, and a radial pattern of striations. These structure were probably involved with the attachment to the soil
The systematic place of Parka is is uncertain
- Fossils resemble both extant liverworts and green algae, although chemical composition indicates a relationship with green algae
- This was probably a group of early plant-algae organisms, which colonized near-shore, barren land

Above: Parka decipiens

Nematothallus †

Found in the Middle Silurian and the Early Devonian, it is fossils that resemble sheets of cells, such as cuticle of a plant

Fossils can appear as irregularly-shaped black patches measuring 0.5 to 6 cm in diameter
It is not developed in the way of cell division in higher plants, therefore it is called pseudo-parenchymatous
- Under the cuticle is a mat of thin threads or tubes, which vary in diameter (3-40 µm). Spores have also been found in the mats.
Some researchers have suggested an affinity with lichens or red algae

Above: Nematothallus

Spongiophyton †

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.
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

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