Rhizophydites shutei, Krings, Michael, 2024

Rhizophydites shutei nov. sp.

Text-figs 1a, b View Text-fig , 2 View Text-fig , 3 View Text-fig

H o l o t y p e. Globose zoosporangium with a small subsporangial swelling shown in Text-fig. 2i View Text-fig ; slide BAX-1 (the exact position of the specimen is indicated by an arrow on the slide).

M y c o b a n k. MB 855708.

R e p o s i t o r y. SNSB-Bayerische Staatssammlung für Paläontologie und Geologie, Munich, Germany (Robert W. Baxter Rhynie chert slide collection).

E t y m o l o g y. The epithet honors the late Cedric H. Shute (1937–2019) and his important contributions to our understanding of Paleozoic plants and floras.

T y p e l o c a l i t y. Rhynie chert site, Aberdeenshire, Scotland, UK National Grid Reference NJ 494276 (57° 20′ 09.97″ N, 02° 50′ 31.83″ W) GoogleMaps .

T y p e s t r a t u m a n d a g e. Rhynie Block of the Dryden Flags Formation; Early Devonian; early (but not earliest) Pragian to earliest Emsian (Wellman 2006, 2017), 411.5 ± 1.3 Ma ( Parry et al. 2011), 407.1 ± 2.2 Ma ( Mark et al. 2011).

D i a g n o s i s. Rhizoidal system composed of branched rhizoids arising from rhizoidal axis or (subsporangial) apophysis; apophysis, if present, rather small; zoosporangia very different in size and shape, due in part to availability of space in place of growth, ovoid, narrowly to broadly citriform, bulb-shaped, or globose, epibiotic, interbiotic (stalked), or located between host wall components, less than 40 μm high (without stalk) and up to 35 µm at widest point, wall smooth; stalk, if present,erect, short to nearly as long as zoosporangium; mature zoosporangia with a prominent, usually thin-walled apical papilla; post-discharge zoosporangia with a circular apical orifice with slightly raised borders; occurring singly (rarely) or in tuft-like clusters (in most cases) or planar assemblages comprised of multiple individuals at different stages of development. Host: Glomeromycotan acaulospores morphologically similar to Archaeosporites rhyniensis .

D i s t i n g u i s h i n g f e a t u r e s. Rhizophydites shutei differs from all chytrid-like fossils previously described from the Rhynie chert. However, there are some illustrations in the literature of hitherto undescribed and unnamed chytrid-like thalli on Rhynie chert glomeromycotan spores, which could belong to R. shutei on the basis of their overall appearance ( Krings and Harper 2019: fig. 4b, c, Krings 2022: figs 24, 25). Rhizophydites shutei is similar morphologically to Stillula hyphicola M. KRINGS , a monocentric chytrid from the Rhynie chert that colonizes glomeromycotan hyphae ( Krings 2024a). Both forms have epibiotic sessile or stalked zoosporangia with a single apical papilla. However, the papilla is less prominent and usually offset to the side in S. hyphicola . Moreover, S. hyphicola zoosporangia are on average much rounder and do not occur in tuft-like clusters. Two Rhynie chert fossils have previously been described in the genus Rhizophydites , namely R. matryoshkae thriving on land plant spores ( Krings et al. 2021), and R. bicornis M. KRINGS on gomeromycotan spores (Krings in press). Both differ from R. shutei above all in regard to the number of discharge openings. There are 1–4 papillae or short tubes in R. matryoshkae and typically two (rarely one) in R. bicornis . Moreover, secondary sporangia have been reported in both R. matryoshkae and R. bicornis , but there is no evidence of them as yet in R. shutei . Rhizophydites shutei is also somewhat similar to Illmanomyces corniger M. KRINGS et T.N. TAYLOR , which occurs on certain thin-walled glomoid spores from the Rhynie chert ( Krings and Taylor 2014). However, zoosporangia of the latter form are distinctly larger (greater than 60 µm in diameter) than R. shutei and have 4–5 prominent discharge tubes. Yet another Rhynie chert chytrid that is vaguely reminiscent morphologically, but is also on average bigger, is Cultoraquaticus trewinii STRULLU-DERR. , which usually has two discharge papillae ( Strullu-Derrien et al. 2016).

D e s c r i p t i o n. Rhizophydites shutei mostly occurs in tuft-like clusters arising from the wall of the host spore. Tufts consist of 2 to more than 10(–15) individual thalli in different stages of development ( Text-figs 2 View Text-fig , 3a, b, d–f View Text-fig ). Thalli occurring in planar assemblages have also been observed, but are relatively rare ( Text-fig. 3c View Text-fig ). Tuft-like clusters usually occur singly on their substrate (= glomeromycotan spore), but there are also a few hosts with two tufts ( Text-fig. 2a View Text-fig ).

Thalli are bipolar and comprised of an epibiotic or interbiotic (but see below), smooth-walled, sac-like zoosporangium, and an endobiotic rhizoidal system reaching into the host lumen. The morphology of the extramatrical portion of the thallus appears, at least in part, as a function of the surrounding in which it developed. Zoosporangia differ significantly from each other in regard to size and shape; they vary between ovoid, narrowly to broadly citriform, bulb-shaped, and spheroidal, and 10–35(–38) μm high and 7–33 µm wide. Zoosporangia are mostly sessile, but can also be interbiotic and arise from an erect, tubular stalk, which is either short (black arrow in Text-fig. 2c View Text-fig 3 View Text-fig ) or elongate and up to as long as the sporangium itself (e.g., black arrow in Text-fig. 2c View Text-fig 4). A few very small spheroidal or drop-shaped vesicles are also present in many tuft-like clusters (arrows in Text-figs 2a View Text-fig 3, f 3 View Text-fig , h 2 View Text-fig , 3d View Text-fig ); as to whether these structures are mature but underdeveloped, or aborted, or still developing zoosporangia cannot be determined.

Most tuft-like clusters emerge from the outer surface of the host spore, but there are also several specimens in which zoosporangia have developed between the host wall components c1 and c2 ( Text-figs 2f, g View Text-fig , 3e View Text-fig ). Wall component c1 is greatly expanded in these specimens (e.g., black arrows in Text-figs 2g View Text-fig , 3e View Text-fig 2 View Text-fig ), and the enclosed zoosporangia are densely packed and therefore in some cases asymmetrical or somewhat deformed ( Text-fig. 2g View Text-fig [right-hand side of picture]). It remains uncertain what ultimately happened to the envelope when the enclosed zoosporangia matured and released their zoospores. It was presumably destroyed at some point, as there is no evidence in these specimens that the host wall component c1 covers post-discharge zoosporangia (asterisk in Text-fig. 3e View Text-fig 1 View Text-fig ).

A prominent apical (in rare cases somewhat offset or slightly tilted) papilla, up to 4.5 µm high and ca. 4.6 µm wide, is visible in approximately 35 % of the zoosporangia (black arrows in Text-fig. 2a View Text-fig 4 [+inset], d 1, d 2 [magnified in Text-fig. 2e View Text-fig ]; Text-figs 2h View Text-fig 1 View Text-fig , i, j, 3c View Text-fig ), while another ca. 5 % exhibit a distal opening with slightly raised borders that in most cases is more than 10 µm wide (asterisk in Text-fig. 3b View Text-fig 1, d, e 1 View Text-fig ); no evidence of the presence of an operculum was found. The remaining ca. 60 % of zoosporangia lack any evidence of a papilla or other type of preformed discharge apparatus or opening, either due to the plane of the section through the sporangium, or because the sporangium was immature and the discharge apparatus not yet developed at the time of fossilization. The wall of the papilla appears in many cases to be thinner than the wall of the zoosporangium (e.g., Text-figs 2i View Text-fig , 3c View Text-fig ). No evidence of the contents of the zoosporangia was found, with the exception of two specimens that contain a central, albeit only vaguely defined inclusion, which appears to consist of closely spaced individual bodies up to ca. 2(–2.5) µm in diameter (arrows in Text-fig. 3c View Text-fig ).

Remains of the rhizoidal system (= intramatrical portion of the thallus) are recognizable in many specimens, but it is not really well preserved in any of them. The rhizoidal system consists of a tubular or unevenly slightly swollen rhizoidal axis (white arrows in Text-fig. 2c 2 View Text-fig –4 [one focal plane magnified in Text-fig. 3a View Text-fig ]) that arises proximally on the zoosporangium (or from the proximal end of the stalk), passes through the outer two or all three host wall components, and gives off rhizoids (e.g., Text-fig. 2h View Text-fig 1 View Text-fig ). The rhizoidal axis can also form a small (subsporangial) swelling or apophysis (white arrows in Text-figs 2a 2 View Text-fig [+ inset], b 2 [+ inset], b 3 [+ inset], f 1 [+ inset], j, 3d, e 1), from which one or two main rhizoids are then given off that advance further into the host and, in some cases, branch profusely (e.g., Text-fig. 3f View Text-fig ). The shape of the apophysis, and also its position within the host and the course of the main rhizoids, can vary. It may be oval or irregularly elongate in sectional view and located in the translucent region (tr in Text-fig. 1b View Text-fig ) on the inner surface of the host wall component c2 ( Text-figs 2a 2 View Text-fig , 3d, e 1–3 View Text-fig View Text-fig View Text-fig ), with the main rhizoids then initially extending respectively. d: Tuft of zoosporangia seen from above in two different focal planes; black arrows indicate apical papillae. e: Close-up of (d 2), showing one of the papillae. f: Tuft of thalli with several zoosporangia located between wall components c1 and c2 of host in four different focal planes; note apophysis in f 1 (white arrow + inset) and small vesicle in f 3 (black arrow). g: Close-up of (f 2), showing host wall component c1 surrounding zoosporangia (arrow). h: Tuft-like cluster in two focal planes; arrow in h 2 shows small vesicles; note delicate rhizoidal system in spore lumen in h 1. i: Holotype specimen; globose zoosporangium with small subsporangial apophysis (arrow). j: Four zoosporangia seen from above; note two prominent papillae, the one in the lower part of the picture slightly tilted. Scale bars = 20 µm (a, b, c, d, f, h), 10 µm (e, g, i, j).

through the translucent region (e.g., white arrows in Text-fig. 3e View Text-fig 2 View Text-fig ) and in some, but not all, cases later advancing into the host lumen ( Text-fig. 3e 3 View Text-fig [magnified in Text-fig. 3f View Text-fig ]). Conversely, the apophysis may also be more or less circular in sectional view and located in the spore lumen (spl in Text-fig. 1b View Text-fig ) on the inner surface of the host wall component c3 ( Text-fig. 2b 2 View Text-fig , 3 View Text-fig , f 1 View Text-fig ), in which case the rhizoids spread directly in the lumen. The rhizoidal systems in one tuft-like cluster have apparently developed largely between the host wall components c1 and c2, whereby the wall component c 2 in this area has been pushed into the translucent region and the wall component c3 into the spore lumen ( Text-fig. 1b View Text-fig ). No further evidence of host wall deformation by the colonizer has been found, with the possible exception of a slightly more uneven course of the wall component c3 below the tuft of colonizers in the specimen shown in Text-fig. 3e View Text-fig . There is also no indication of a specific host response in any of the host spores.