Research Article |
Corresponding author: Michael Krings ( krings@snsb.de ) Academic editor: Alexander Nützel
© 2022 Michael Krings.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
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.e86327
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Windipila spinifera from the Rhynie cherts is a spheroidal microfossil enveloped in a hyphal mantle from which extend prominent spines and otherwise shaped projections. It is believed to be a reproductive unit of a fungus in the Glomeromycota or zygomycetes, but features to determine the systematic affinities have not hitherto been documented. This study describes a new specimen of W. spinifera that contains a single spherical structure from which a hypha arises that extends outside and terminates in what appears to be a sporangium. The specimen is reminiscent of germinated zygospores of the germ-sporangial type, and thus may suggest affinities of W. spinifera to the zygomycetes. However, the interior sphere and its outgrowth could also be a part of another organism that had invaded W. spinifera.
fossil fungi, Glomeromycota, intrusive organisms, zygomycetes, zygospore germination
The Lower Devonian Rhynie cherts (including the Rhynie and Windyfield cherts) of Scotland give detailed insights into fungal diversity in a terrestrial ecosystem c. 410 Ma ago (
Several types of fungal reproductive units have been described from the Rhynie cherts that all possess an ancillary covering in the form of a hyphal investment or mantle. Mantle morphology varies (considerably) among the different types, and thus renders them easy to distinguish from one another (
Windipila spinifera from the Lower Devonian Rhynie cherts (Windyfield chert). A1, 2. Well preserved specimen in two different focal planes (A1: median optical section, A2: section off center), with prominent spines and otherwise shaped projections extending from mantle (focal plane A1 previously published by
Most mantled fungal reproductive units from the Rhynie cherts, including Windipila spinifera, are believed to belong to either the Glomeromycota or the zygomycete fungi based on similar features in modern lineages known to produce spores or sporangia with hyphal mantles (e.g.,
This study describes a new specimen of Windipila spinifera from the Windyfield chert that contains a single spherical structure from which an unbranched hypha is given off that evidently passes through the wall and mantle and terminates in what appears to be a sporangium with a distal discharge papilla. While this specimen does also not clarify the affinities of W. spinifera, it resembles a germinated zygospore or azygospore with a tubular sporangiophore and distal germ sporangium, and thus is suggestive of affinities to the zygomycetes.
The Rhynie chert Lagerstätte is located in the northern part of the Rhynie outlier of Lower Old Red Sandstone in Aberdeenshire, Scotland, in an extensive sequence of sedimentary and volcanic rocks. The cherts occur in the Rhynie Block of the Dryden Flags Formation located northwest of the village of Rhynie. The Lagerstätte is made up of fossiliferous beds containing lacustrine shales and chert that have been 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 specimen described below was identified in a thin section prepared by cementing a wafer of the Windyfield chert to a microscope slide, and then grinding the rock slice until it was sufficiently thin (i.e. c. 60–80 µm thick) to transmit light (for details on thin section preparation, refer to
The specimen (Fig.
The reproductive unit contains a single, more or less spherical structure (henceforth called interior sphere) that is c. 55 µm in diameter, lacks structured contents, and is delineated by a smooth wall up to 0.75 µm thick. Its position within the cavity is highly eccentric in such a way that it comes into contact with, and in vivo perhaps adhered to, the inner surface of the non-hyphal wall of the reproductive unit. The interior sphere is somewhat flattened where it is in physical contact with this wall. No evidence of a subtending hypha or any other kind of parental structure has been found. However, the cavity of the reproductive unit contains shrivelled fragments of thin-walled hyphae or filaments (Fig.
The hypha (rh in Fig.
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 certainly is one of the more peculiar ones (
Windipila has been suggested to have affinities to either the Glomeromycota or the zygomycete fungi (
The most interesting structural element of the only known specimen of Windipila pumila is a walled spherical structure that occurs in the cavity of this fossil (
The fossil presented in this study provides the first evidence of the occurrence of interior spheres also in Windipila spinifera. Moreover, it exhibits several structural features pertaining to the interior sphere that have not been documented in any other Rhynie chert mantled reproductive unit. Said interior sphere gives off a hypha that exits the reproductive unit and terminates in another spherical structure, which is characterized by a papilla-like protrusion that most likely functioned as a discharge apparatus. There is a certain level of morphological correspondence between the fossil and certain zygomycetes of the order Mucorales in which zygospore germination entails the formation of a germ sporangium (e.g.,
One might counter this comparison by pointing out that no evidence of gametangia and suspensors has been found in any of the Windipila spinifera specimens. It is possible, however, that these structures were small, inconspicuous, and entirely embedded in the mantle. For example, the zygosporangia of certain present-day Mortierella (Mortierellaceae, Mucorales) species are surrounded by extensive hyphal coverings arising from the suspensors or from hyphae at the base of the suspensors (
An alternative hypothesis views the interior sphere in Windipila spinifera as a part or life cycle stage of some other fungus or fungus-like organism that had invaded and colonized the reproductive unit, and reproduced by producing a sporangium (or sporangia?) outside of the host. Two lines of evidence seem to support this hypothesis. First, co-occurring with the interior sphere in the cavity of the W. spinifera specimen are fragments of hyphae or filaments, which indicate that, at some point, another fungus has been present in this structure, unless the mantle hyphae had invaded the cavity from the circumference (see below). Second, several different types of small spherules occurring singly or in groups, and probably representing the reproductive units of intrusive fungi, have been reported in the cavity of Zwergimyces vestitus (
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 (
Funding was received from the National Science Foundation (DEB-1441604 subcontract S1696A-A). I thank H. Martin and S. Sónyi (both Munich, Germany) for technical assistance, and Nora Dotzler (Munich, Germany) for insightful comments and suggestions on the manuscript.