Temporal range: Late Silurian (Wenlock) - Early Devonian (Lochkovian)
|An Ornatifilum-like fossil, identical to those described by Sherwood-Pike and Gray (1985). 250 μm long.|
|O. lornensis from the Silurian of Kerrera, Scotland. 200 μm long.|
Burgess & Edwards 1991
Ornatifilum (Latin ornatis + filum, Ornamented filament) is an artificial form genus, which is used to categorise any small, branched filaments with external ornamentation. It has been applied to microfossils of Devonian age with possible fungal affinities; two "species" have been described, and further Silurian fossils closely resemble it. These Silurian specimens hint that the organisms may have been fungal, placing them among the oldest representatives of this kingdom.
The form genus Ornatifilum was erected by Burgess and Edwards in 1991 to describe tubular fossils retrieved by acid maceration from the late Silurian. It was originally intended as a form genus, to facilitate stratigraphy and environmental reconstruction; the fossils do not display enough features to classify them confidently, even at a kingdom level.
The organisms comprise tubes of around 10 μm diameter, with an ornamented, granular surface texture. These fossils were compared to late Silurian (Ludlow epoch) fossils retrieved from the Burgsvik beds by Sherwood-Pike and Gray, and the genus was used when similar fossils were recovered from the Scottish island of Kerrera by Charles Wellman ten years later. Similar, unornamented filaments are known from the USA.
The type species of the genus consists of flattened filaments - perhaps an artefact resulting from post-burial pressure. Their branching is typically at obtuse angles; the irregularly sized grana, which ornament their surfaces, are concentrated at branching points. They are often found as individuals, but sometimes group together into "wefts", as Wellman has termed them. The filaments are septate, with the septa looking like "pinch points" where the tube is slightly constricted - like a twisted balloon. No sign of perforation was visible in the septa; perforate spores are only found in red algae and fungi, but their absence does not preclude their presence in one of these groups: indeed the perforations are difficult to see or image. There are no other diagnostic features of this species that allow classification in any group. Surface ornamentation is a common convergent feature, found for example in liverwort rhizoids and some fungi, so does not help in classification. The specimens recovered are most common in near-shore environments; however, they are never abundant.
This species has a more complex appearance than O. granatum. For a start, its surface ornament - which covers most of the surface uniformly - takes an array of forms, with "grana, coni, spinae verrucae and occasionally plia"[note 1] present. Further, side-branches and the flask-shaped protuberances occasionally protrude from the tubes, on which the ornament is larger (2.5 μm rather than ~1 μm). Such branching typically occurs in pairs across the main thread.
Sherwood-Pike and Gray's "fungus"
Ornatifilum is compared extensively to microfossil remains recovered from the Ludlow of Gotland by Sherwood-Pike and Gray. These fossils, which have never been formally described, had less prominent grana, but the vase-shaped protrubences are very similar to those exhibited by O. lornensis and they are proposed to belong to the same species.
These fossils allow a classification to be suggested. Firstly, they possess punctate spores, which as mentioned earlier restricts their affinities to the red algae and fungi. Further circumstantial evidence made a fungal affinity look more likely: firstly, they were found in association with fungal spores; further, a "1:1 correlation" was observed with trilete spores diagnostic of land plants. Whilst such spores could easily have been blown or washed into the sea, Sherwood-Pike and Gray consider this correlation to imply a terrestrial habit of the fossils; as the red algæ are solely marine, this would only leave the fungi - dominantly terrestrial today, but with high diversity in marine settings too - as a possible home. Further suggesting a fungal connection, the fossils were found in association with spores that could be assigned to the ascomycetous fungi.
Other early fungi
A rich diversity of fungi is known from the lower Devonian Rhynie chert, but the previous record is absent. Since fungi don't biomineralise, they do not readily enter the fossil record; aside from Ornatifilum, there are only two other claims of early fungi. One from the Ordovician has been dismissed on the grounds that it lacks any distinctly fungal features, and is held by many to be contamination; the position of a "probable" Proterozoic fungus is still not established, and it may represent a stem group fungus. If the case for Ornatifilum's fungal affinity were affirmed, that would make it the oldest known fossil fungus - although, since the fungi form a sister group to the animals, the two lineages must have diverged before the first animal lineages, which are known from fossils as early as the Ediacaran.
- grana: small grains
- coni: small cones
- spinae verrucae: Spiny warts
- plia: small streaky knobs.
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