Radulaceae is a family of liverworts, and the only family in the order Radulales. The family comprises three genera: Radula, Cladoradula, and Dactyloradula, recognised as distinct following a 2022 taxonomic revision. Distinguishing features include bilobed leaves arranged in two rows, with the smaller lobe folded under the larger one, and rhizoids (root-like structures) uniquely emerging from leaves rather than stems. The family lacks underleaves, which are common in other liverwort families. Fossil evidence from Burmese amber indicates the family had diversified by the Cretaceous period, approximately 98 million years ago, with molecular studies suggesting its divergence from related groups occurred during the Permian period. While Radula occurs worldwide from sea level to 4,000 metres elevation, Cladoradula shows a disjunct distribution across tropical and temperate regions, typically growing on tree bark and shaded rocks in submontane and lower montane forests between 400 and 2,100 metres. Dactyloradula is endemic to Japan and uniquely features finger-like appendages at the base of each leaf lobe. The family includes both rare and widespread species, with several Macaronesian endemics facing significant conservation challenges. Among these, Radula visianica is assessed as Critically Endangered, with fewer than 50 individuals remaining in Austria, while others are threatened by climate change, habitat modification, and increased fire frequency. Recent molecular phylogenetics analysis has refined understanding of relationships within the group, with the genus Porella identified as its closest relative.
Systematics
Historical taxonomy The family Radulaceae was first described based on the genus Radula, established by the Belgian botanist Barthélemy Charles Joseph Dumortier in 1822. A significant early treatment came from Richard Spruce in 1885, who established the subgenus Cladoradula based on specimens from tropical America. Spruce defined this group based on its short branches bearing reproductive organs (gynoecia without innovations (new shoots growing from the reproductive branches), relatively short protective sheaths around the reproductive organs (perianths), and nearly spherical spore capsules. Spruce later elevated the group to Radulineae, defined by keeled leaves with two unequal lobes, completely lacking underleaves, with perianths dorsiventrally flattened and truncate at the mouth, positioned at the end of the main shoot. The upper leaf lobe is 3-–4 times larger than the lower lobe, resulting in succubous leaf arrangement, and rhizoid bundles arise from the base of the lower lobes. For nearly 200 years, Radula remained the only genus in the family, making Radulaceae a monogeneric family within the order Porellales. Herbert Castle's 1936 worldwide monograph of Radula took a broader view of Cladoradula, expanding it to include all species with reproductive structures on short branches, regardless of other features. This expansion increased the subgenus to 15 species with an almost worldwide distribution. However, later botanists, including Eustace W. Jones (1977), Kohsaku Yamada (1979), and Rudolf Mathias Schuster (1980), returned to a narrower definition more aligned with Spruce's original concept.
Modern molecular studies DNA sequencing techniques developed in the late 20th century revealed new relationships within Radula. A comprehensive molecular phylogenetics study by Devos and colleagues in 2011 analysed DNA sequences from plants collected worldwide. This research consistently identified seven distinct evolutionary lineages (clades) within Radula, which were formally recognised as subgenera:
subg. Amentuloradula subg. Cladoradula subg. Dactyloradula subg. Metaradula subg. Odontoradula subg. Radula subg. Volutoradula Each of these subgenera had unique combinations of morphological features. For example, Amentuloradula was characterised by specialised branches resembling catkins, while Odontoradula typically had pointed leaf tips. The study revealed that subgenera Cladoradula and Dactyloradula were the earliest diverging lineages, representing the oldest evolutionary splits within the genus. Of these seven groups, Radula and Cladoradula showed the widest geographical distributions, being found almost worldwide. Metaradula and Volutoradula were mainly tropical but extended into southern temperate regions. Amentuloradula and Odontoradula were largely restricted to Asia, Australasia and Oceania, while Dactyloradula was found only in East Asia and western North America.
2022 taxonomic revision Matthew Renner, Robbert Gradstein, and colleagues split the family Radulaceae into three genera in 2022, based on molecular and morphological evidence. Several key findings supported this taxonomic revision. Cretaceous fossils in Burmese amber (98 million years old) revealed the ages of different Radula groups. These studies revealed that the divergences between some groups were remarkably ancient, with some lineages being as old as many flowering plant families. The two groups that were elevated to genus rank, Cladoradula and Dactyloradula, were found to have diverged from the main Radula lineage during the Jurassic or early Cretaceous periods. The original genus Radula predates most plant genera, with flowering plants, ferns, and other liverworts typically originating within the past 30–40 million years. Additional factors influenced this taxonomic decision. The researchers identified several distinct morphological features that set these groups apart. Particularly significant was the way the leaf lobes (modified leaves characteristic of liverworts) attached to the stem. Both Cladoradula and Dactyloradula show a transverse (sideways) attachment, while all other species show a longitudinal (lengthwise) attachment. Additionally, both new genera possess a unique stem structure with an extra layer of cells called a subepidermis, which is absent in Radula. The revision divided the family into three genera: Radula (comprising most species), Cladoradula (seven tropical and temperate species), and the monotypic Dactyloradula (D. brunnea from Japan). This reclassification aligned the family's taxonomy with both its morphological diversity and deep evolutionary divisions.
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