D ispersed megaspores with affinities to aquatic heterosporous ferns are relatively common in mesofossil assemblages from the Early Cretaceous to the Recent. Extant heterosporous ferns are free floating or shallow rooted freshwater plants, with a dominantly tropical to warm-temperate distribution (Tryon and Lugardon, 1991). Their heterosporous life cycle (including both megaspores and microspores) is likely to be an adaptation to their aquatic habit (Collinson, 1991; Hemsley et al., 1999; Kar and Dilcher, 2002). Thus the abundance of heterosporous fern megaspores, or the presence of heterosporous fern macrofossils, within a stratigraphic interval may be indicative...

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