Palaeobiogeographical implications

The palaeogeographic and temporal origin of Dipterocarpaceae and their association with ectomycorrhizal fungi have frequently been discussed. It is typically suggested that dipterocarps originated in eastern Africa or Madagascar and drifted northward on the Indian platform, reached Asia during the Eocene and spread (Dutta et al., 2011). Alternatively, an origin in Southeast Asia has been proposed and taken into account (Lakhanpal, 1970; Sasaki, 2006).

However, the monophyly of the three subfamilies of Dipterocarpaceae and of the Sarcolaenaceae, along with their consistent association with ectomycorrhizal fungi, suggest that the potential to form ectomycorrhizas is an ancestral character of the Dipterocarpaceae family (Ducousso et al., 2004;

Moyersoen, 2006). Ectomycorrhizal symbioses may have conferred a selective advantage for some tropical tree species (McGuire, 2007), even in early tropical broadleaf rainforests, and the high diversity and abundance of Dipterocarpaceae in Asia might be based on their potential to associate with ectomycorrhizal fungi.

Ectomycorrhizal associations are considered to be unstable evolutionarily dynamic associations that evolved independently in several major clades of fungi (Hibbett & Matheny, 2009) as well as several times within the angiosperm clade that includes Rosids and Asterids and within the Pinaceae (Fitter &

Moyersoen, 1996; Hibbett & Matheny, 2009). Consequently, E. cenococcoides itself is not necessarily an ancestral mycobiont of its host. The only previously reported fossil record of ectomycorrhizas is actually from the roots of Eocene (c.

50 million yr old) Pinaceae from Vancouver Island (LePage et al., 1997). Our find provides evidence that angiospermous ectomycorrhizal associations in the Paleogene tropics occurred contemporaneously with gymnospermous ectomycorrhizal associations in the Nearctic.

Acknowledgements

The authors would like to thank David A. Grimaldi (New York), Matthias Gube (Jena), Heike Heklau (Halle), Jouko Rikkinen (Helsinki), Kerstin Schmidt (Jena) and Gerhard Wagenitz (Göttingen) for their helpful comments; Jes Rust (Bonn) for advice; and

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Wolfgang Dröse (Göttingen) and Dorothea Hause-Reitner (Göttingen) for assistance with histology and the field emission microscope. We are grateful to the anonymous reviewers for their constructive suggestions. H.S. and R.S.R. thank Ashok Sahni (Lucknow) for his unfailing encouragement and kind advice. H.S. would like to recognize Naresh Chandra Mehrotra, the director of the Birbal Sahni Institute of Palaeobotany (Lucknow), for his support of laboratory and field work. R.S.R. thanks the Department of Science and Technology, Government of India. This is publication number 65 from the Courant Research Centre Geobiology, funded by the German Initiative of Excellence.

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Chapter 4: Conditioning films in rock fractures of the Äspö