Abstract
Cretaceous fossils of derived leptosporangiate ferns (Polypodiales) are exceedingly rare yet they are needed to confirm the hypothesis of a Cretaceous diversification of polypod ferns as predicted by DNA-based divergence time estimates. Here we show that diverse polypod ferns existed in the mid-Cretaceous woodlands of Myanmar. We describe isolated sporangia with a vertical, broken annulus ring containing a differentiated stomium as well as distinguished pinnae fragments. Krameropteris resinatus gen. et sp. nov. is described based on a pinnae-fragment with free branched veins, exindusiate sori and polypod sporangia with trilete spores, and assigned to the early diverging polypod fern lineage Dennstaedtiaceae. Integration of the new fossil evidence in a phylogenetic framework provides support to the hypothesis of a Cretaceous diversification of polypod ferns.
Highlights
Incongruences between divergence time estimates obtained from the fossil record and DNA sequence variation have been recognized for many lineages of organisms (Benton and Ayala, 2003)
This pattern is unlikely to be explained by rock bias against the preservation of ferns because Jurassic and Cretaceous fossils are known for many other fern lineages such as Cyatheales, Gleicheniales, Osmundales, Salviniales, and Schizaeales (Hu & Taylor, 2014; Kvacek et al 2006; Mohr et al, 2015; Skog 2001; Smith et al, 2003; Smith et al, 2015; Tidwell and Ash, 1994; Vera and Herbst, 2015)
As pointed out in studies on the spore record of ferns (Nagalingum et al, 2002), the microfossil record suggests a decline of ferns in the late Mesozoic instead of the Cretaceous to Eocene radiation suggested by DNA-based divergence time estimates (Schneider et al, 2004)
Summary
Incongruences between divergence time estimates obtained from the fossil record and DNA sequence variation have been recognized for many lineages of organisms (Benton and Ayala, 2003). The hypothesis of a diversification of polypod ferns coinciding with the rise of the angiosperms has been established on the basis of DNA based divergence time estimates (Lehtonen et al, 2012; Schneider et al, 2004; Schuettpelz and Pryer, 2009) Precursors of this hypothesis were introduced before the advent of DNA based dating (Lovis, 1977; Rothwell, 1996; Smith, 1972), but were challenged by the rarity of Cretaceous fossils of Polypodiales (Collinson, 1996, 2001; Skog, 2001). The presence of these sporangia is considered as convincing evidence for an occurrence of polypod ferns in the mid-Cretacous and is comparable with the tricolpate pollen type widely recognized as significant evidence for the occurrence of eudicots (Magallón et al, 2015)
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