Fossil callimothalloid fungi: Revised taxonomy, modern equivalents and palaeoecology
Introduction
Fungi are one of the kingdoms of eukaryotic organisms that probably appeared in the late Proterozoic (Taylor et al., 2015). In the fossil state fungal hyphae, sporocarps, and spores are usually found as coalified compressions and permineralizations, rarely also as impressions and even casts. Well-preserved fungal remains, sometimes almost unaltered are found embedded in amber (Halbwachs, 2019) and are common in palynological samples (Elsik, 1996; Worobiec et al., 2009). An important group of fossil fungi are epiphyllous fungi found both on cuticles of fossilised leaves and as detached specimens during palynological analysis. Most of the fossil epiphyllous fungi usually found as sporocarps are classified as members of the order Microthyriales or in the families Asterinaceae, Microthyriaceae, Micropeltidaceae, and Trichothyriaceae (Cookson, 1947; Elsik, 1978; Tripathi, 2012; Worobiec and Worobiec, 2013). The taxonomy of fossil epiphyllous fungi is generally based on morphological features. However, they are usually preserved as sporocarps alone without any asci, ascospores, and attached mycelium. This makes the taxonomy of fossil epiphyllous fungi especially difficult since only few diagnostic features are available. Molecular phylogenetic approaches are generally not applicable for the taxonomy of fossil species since DNA is not preserved in specimens estimated to be several million years old. Considering these limitations, similarly to many groups of fossil organisms, the taxonomy of fossil epiphyllous fungi is based on an artificial morphological taxonomic system (Elsik, 1978). The determination of the generic position of epiphyllous fungi in many cases has led to serious mistakes (e.g. García-Massini et al., 2004 see section “Ecology and climatic inferences of callimothalloid fungi”). Fossil fungi, however, could be useful as a palaeoecological proxy (Dilcher, 1965; Lange, 1976; Elsik, 1978; Bera and Mandal, 2014; Conran et al., 2016; Worobiec and Worobiec, 2017; Worobiec et al., 2018) and recently fossil fungi have also been applied for the calibration of phylogenetic trees obtained using molecular clock methods (Beimforde et al., 2014; Hongsanan et al., 2016; Liu et al., 2017) assuming that they are exactly dated, well preserved allowing identification, and can be assigned to modern taxonomic groups. We conducted a detailed investigation aimed at the taxonomic revision of the remains of epiphyllous, so far enigmatic callimothalloid (with sporocarps bearing porate cells) fungi, which have usually been considered to be the representatives of the family Microthyriaceae (Dilcher, 1965) or the order Microthyriales (Kalgutkar and Jansonius, 2000). They are frequently reported both during palynological investigations (Kumaran et al., 2001) and on the cuticles of fossil leaves (Du et al., 2012) in Upper Cretaceous up to Holocene deposits. We reinterpreted the taxonomic position of the fossil-genera Callimothallus Dilcher ex Janson. and Hills and Cribrites R.T. Lange and fossil-taxa described under these fossil-genera. This is followed by a discussion on recent counterparts and the palaeoecological significance of these fungi. The study of fossil callimothalloid fungi is based on collected material from the Oligocene of Hungary and the Miocene of Poland (Callimothallus pertusus Dilcher) and on data published (C. pertusus, Callimothallus quilonensis K.P. Jain & R. Gupta and Cribrites).
Section snippets
Geological setting
The investigation of the fossil-species C. pertusus is based mainly on specimens collected from two European localities: the Oligocene deposits of Csolnok, Hungary and the lower and upper Miocene deposits of the Bełchatów Lignite Mine, Poland. Remains of Callimothallus were found both on cuticles isolated from leaf compressions and in palynological samples.
The Hungarian locality, Csolnok (47°41′37″N 18°42′18″E), is situated in the Dorog Basin, N Hungary, important for mining of brown coal of
Taxonomy
Dothideomycetes.
Muyocopronales Mapook, Boonmee & K.D. Hyde (2016).
Muyocopronaceae K.D. Hyde in: Hyde et al. (2013).
Neomycoleptodiscus Hern.-Restr., J.D.P. Bezerra and Crous (2019).
Type species. Neomycoleptodiscus venezuelense Hern.-Restr., J.D.P. Bezerra and Crous (2019).
Fossil-species. Neomycoleptodiscus pertusus (Dilcher) G. Worobiec, comb. nov. Plate 1.
MycoBank number: MB 836230
Basionym. Callimothallus pertusus Dilcher, Palaeontographica B 116 (1–4): 13.1965. Jansonius and Hills, Fossil
Conclusions
The commonly recorded fossil callimothalloid fungus, Callimothallus pertusus, previously thought to be related to the fungal family Microthyriaceae or to some modern algae (Phycopeltis and Ulvella), is found as almost identical to the modern Mycoleptodiscus disciformis and N. venezuelense. To accommodate fungal remains of the C. pertusus type a new combination is proposed, Neomycoleptodiscus pertusus. Contrary to C. pertusus, another fossil callimothalloid species C. quilonensis has pores found
Acknowledgements
This work was supported by the W. Szafer Institute of Botany, Polish Academy of Sciences, Kraków, Poland through the statutory funds to Grzegorz Worobiec and Elżbieta Worobiec, and by the National Research, Development and Innovation Office (NKFIH-OTKA 120123) and German Academic Exchange Service (DAAD) to Boglárka Erdei. Dr. Marie-Stéphanie Samain (Editor, Acta Botanica Mexicana), Dr. Vandana Prasad (Director, Birbal Sahni Institute of Palaeosciences) and Dr. Amit K. Ghosh (Editor, The
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