Abstract:
In the most recently revised classification for ferns, Eupolypods, composed of two clades: Eupolypods I and Eupolypods II, include nearly 6000 species—more than half of extant fern diversity. However, the evolutionary history of this lineages re-mains incompletely understood, especially on its diversification scenarios, for example, most DNA-based divergence-time studies indicate that Eupolypods diversification occurred in the Late Cretaceous, but few studies demonstrated that Eupoly-pods originated as early as in the Early Jurassic. On the other hand, although a range of molecular dating methods is now available, they all share a vital dependence on fossils as one of important age calibrations. It is therefore of prime importance to thoroughly document and critically evaluate new and informative fern fossils, especially those fossils from those periods of geologic time that are believed to represent important phases in the establishment and/or radiation of new fern lineages. However, so far, no single study based on fossils from stratigraphic depositions has provided unequivocal evidence for a Cretaceous or pre-Cretaceous occurrence for Eupolypods.
In the recent years, it has been found that approximately 100 million-year-old Myanmar amber provides a unique source of polypod fossils. Eight fossils of different families and numerous sporangia, spores, and scales have been found in Myan-mar amber, including the first compelling eupolypod fossil Holttumopteris burmensis.
Here fern scale fossils have been described in Myanmar amber in mid-Cretaceous for the first time. The scale fossils are basal attached clathrate scales, and assigned to eupolypods based on its cell walls having thick darkened partitions between adjacent cells and thin surficial walls. Based on the latest phylogenetic relationships for extant ferns, the evolutionary trend of scales and their morphological characteristics are discussed. This discovery is nevertheless important because it provided the first unequivocal scale fossil evidence that a diversity of eupolypod ferns was present already in the mid-Cretaceous Myan-mar amber forests, clearly showing that Eupolypods originated before the mid-Cretaceous, probably as early as the Early Ju-rassic, consistent with the few divergence time estimates based on molecular dating.
