A 410-million-year-old fossil fungus caught in the act of germination

Buried in a sliver of Scottish chert some 410 million years old, a tiny spheroid less than 150 micrometers across has just rewritten what paleontologists thought they knew about one of the most enigmatic fossils of the early land ecosystems. The fossil, a reproductive unit of the microfossil Windipila spinifera from the Windyfield chert of Aberdeenshire, preserves a single spherical structure inside its cavity from which a narrow hypha emerges, pierces the enclosing wall and mantle, and terminates outside in what appears to be a sporangium complete with a discharge papilla. The specimen, described in the journal Zitteliana, is the first of its kind ever documented for this species, and its architecture bears an uncanny resemblance to a germinated zygospore of the germ-sporangial type seen in certain modern zygomycete fungi.

Windipila spinifera belongs to a family of mantled fungal reproductive units known almost exclusively from the Rhynie cherts, a celebrated Lagerstätte that preserves a terrestrial ecosystem in extraordinary detail. The cherts formed when silica-rich waters from hot springs rapidly permeated and hardened around plants, fungi and microorganisms, entombing them in three dimensions. The fossil in question is a walled spheroid enveloped in one to three layers of tightly interlaced hyphae, from which prominent thin-walled spines and otherwise shaped projections radiate outward. These projections are fewer in the new specimen than in previously described examples, but the mantle remains unmistakable and distinctive.

What sets the new specimen apart is its interior. Inside the central cavity sits a single sphere roughly 55 micrometers in diameter, delineated by a smooth wall up to 0.75 micrometers thick and lacking structured contents. Its position is highly eccentric, so much so that it comes into contact with the inner surface of the enclosing wall, where it appears somewhat flattened. From roughly the center of this flattened portion arises a narrow hypha just 1 to 2 micrometers wide, which evidently passes through the non-hyphal wall of the reproductive unit and continues into the mantle. The exact path through the wall and mantle cannot be traced, but the hypha reappears in the periphery of the mantle, where it expands to as much as 6.5 micrometers wide and becomes tubular and stalk-like, extending some 40 micrometers into the surrounding environment.

At its tip, this stalk-like hypha terminates in a second spherical structure about 46 micrometers in diameter, bounded by a smooth wall and characterized by a single distal, thin-walled papilla-like projection roughly 8.5 micrometers high. This papilla most likely functioned as a discharge apparatus, a feature that in living fungi allows spores to be released in a controlled manner. A second exterior structure, about 38 micrometers across and somewhat asymmetrical, occurs in close proximity in a different plane of the specimen, partially embedded in the mantle and subtended by a short fragment of what appears to be a hypha. Whether this hypha also originates from the interior sphere cannot be determined from the available section.

The morphological correspondence between this fossil and certain modern zygomycetes of the order Mucorales is striking. In these fungi, the sexual stage of the life cycle produces zygosporangia containing single zygospores following gametangial fusion, and sometimes azygosporangia containing azygospores that form parthenogenetically without fusion. When the zygosporangium wall eventually ruptures, the zygospore germinates. If germination is of the germ-sporangial type, the zygospore produces a tube-like structure termed a sporangiophore or promycelium, on the tip of which a germ sporangium develops. After maturation, this germ sporangium ruptures and liberates the spores, which, if they land on a suitable substrate, germinate and develop into a new mycelium. It is conceivable that the Windipila spinifera specimen represents a zygosporangium or azygosporangium containing a zygospore or azygospore, which has germinated and produced a sporangiophore with a terminal germ sporangium. The second exterior structure could be another germ sporangium, and germinated zygospores bearing two sporangiophores have indeed been reported in certain present-day zygomycetes.

Yet the comparison is not without complications. No evidence of gametangia or suspensors, the structures that would positively identify a fossil zygomycete, has been found in any Windipila spinifera specimen. It is possible that these structures were small, inconspicuous and entirely embedded within the mantle. The zygosporangia of certain modern Mortierella species, for example, are surrounded by extensive hyphal coverings arising from the suspensors or from hyphae at their base, rendering the gametangia and suspensors very difficult to recognize. It has also been argued that the interior sphere is too small to be a zygospore or azygospore, because zygosporogenesis involves the development of a complex multi-layered wall in which newly deposited layers follow exactly the shape of the previously deposited ones. The interior sphere is instead somewhat reminiscent of the single oil globules visible in the zygospores of certain mucoralean genera such as Radiomyces and Dichotomocladium, although it seems unlikely that oil globules would become preserved in a recognizable form. It cannot be ruled out that the spore proper shrank during germination and again during the silicification process.

An alternative hypothesis views the interior sphere as part or life cycle stage of some other fungus or fungus-like organism that had invaded and colonized the reproductive unit, reproducing by producing a sporangium outside of the host. Two lines of evidence support this reading. First, co-occurring with the interior sphere in the cavity are fragments of shrivelled hyphae or filaments, indicating that another fungus was present in the structure at some point, unless the mantle hyphae had invaded the cavity from the circumference. Second, several different types of small spherules, probably representing the reproductive units of intrusive fungi, have been reported in the cavity of another mantled Rhynie chert fossil, Zwergimyces vestitus, indicating that mantled reproductive units in the Rhynie paleoecosystem were invaded by other microorganisms and used as habitat. On the other hand, there is no evidence of a physical connection between the interior sphere and any system on which it could have developed, which argues against the hypothesis that the sphere belongs to an intrusive organism. It is also possible that the specimen had already fulfilled its function, producing a spore that subsequently germinated, and was in the process of natural decay at the time of fossilization, in which case the hyphae inside the cavity could belong to saprotrophic fungi or even be outgrowths from the mantle hyphae that invaded the senescing reproductive unit.

The broader context of the discovery is the Rhynie chert Lagerstätte itself, located in the northern part of the Rhynie outlier of Lower Old Red Sandstone in Aberdeenshire, Scotland. The cherts occur within an extensive sequence of sedimentary and volcanic rocks and are interpreted as a series of ephemeral pools within a geothermal wetland, with alkali-chloride hot springs that were part of a complex hydrothermal system in a region affected by volcanic activity. The Windyfield chert site, roughly 700 meters northeast of the original Rhynie chert site, was deposited in an area of hot-spring feeder activity, with paleoenvironments ranging from terrestrial, vegetated aprons of laminated and brecciated sinter to low-temperature pools and marginal aquatic settings. The Rhynie and Windyfield cherts are regarded as broadly coeval, dated to between approximately 411.5 and 407.6 million years old, corresponding to the Pragian to earliest Emsian stages of the Early Devonian.

Eight morphologically different types of mantled fungal reproductive units have so far been described from the Rhynie cherts, of which Windipila spinifera, with its prominent mantle spines, is certainly one of the more peculiar. A similar, albeit less impressive mantle occurs in the little-known Windipila pumila, while a third attributed species, Windipila wimmervoecksii, is characterized by mantle hyphae with vesicle-like inflations from which extend needle-like processes connecting an inner mantle tier with an outer tier of irregularly inflated and interwoven hyphae. Windipila has been suggested to have affinities to either the Glomeromycota or the zygomycete fungi, two groups distinguished in part by the sexual stage of the life cycle, which occurs in zygomycetes as a result of zygosporogenesis following gametangial fusion but has not been observed in Glomeromycota. Several specimens of Windipila spinifera and Windipila wimmervoecksii appear to have developed on simple tubular subtending hyphae, suggesting asexual development by blastic inflation and thickening of a hyphal tip, and hyphal anastomoses and H-branching, common in glomeromycotan fungi but believed to be rare in zygomycetes, have been documented in Windipila wimmervoecksii.

The fossil record rarely gives comprehensive insights into the life history of fungi, which is unfortunate because life history stages can provide valuable information to assess the systematic affinities of fossil fungi in the absence of molecular data. The affinities of the organism that produced Windipila spinifera mantled reproductive units remain conjectural, in spite of the fact that the fossil detailed above shares certain features with germinated zygospores of the germ-sporangial type. Researchers hope that specimens showing these features in greater detail become available as exploration of the Rhynie cherts continues, and that these fossils will clarify the nature of the interior sphere and reveal hitherto unknown aspects that can then be used to determine the systematic position of this enigmatic early fungus.

Subject of Research: The possible germinated zygospore of the Devonian fungal reproductive unit Windipila spinifera from the Rhynie cherts

Article Title: An unusual specimen of the enigmatic fungal reproductive unit Windipila spinifera from the Lower Devonian Rhynie cherts of Scotland

Article References: Krings, M. (2022). An unusual specimen of the enigmatic fungal reproductive unit Windipila spinifera from the Lower Devonian Rhynie cherts of Scotland. Zitteliana, 96, 145-152. https://doi.org/10.3897/zitteliana.96.86327

Image Credits: AI Generated

DOI: 10.3897/zitteliana.96.86327

Keywords: Windipila spinifera, Rhynie chert, Devonian fungi, zygomycetes, zygospore germination, germ sporangium, paleomycology, Windyfield chert, Glomeromycota, fossil reproductive units, Mucorales, Scotland paleontology