New progress on the leaf evolution in early-diverging ferns----Nanjing Institute of Geology and Palaeontology Chinese Academy of Sciences

Devonian witnesses great increases of the land plant diversity and the plant type. All kinds of plants except angiosperms have fossil records in the Devonian. The main euphyllophyte lineages (i.e. ferns sensu lato, progymnosperms and gymnosperms) had evolved laminate leaves by the Late Devonian. The evolution of laminate leaves, however, remains unclear for early-diverging ferns, largely represented by fern-like plants.

Recently, a cooperative study was carried out by Prof. XU Honghe from Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (CAS) and researchers from Peking University and Institute of Botany, CAS. The study presents a new fern-like taxon with pinnules, which provides new insights into the early evolution of laminate leaves in early-diverging ferns. In the study, a new fern-like taxon, Shougangia bella is described from the Upper Devonian (Famennian) Wutong Formation of Anhui and Jiangsu Provinces, South China and represents an early-diverging fern with highly derived features. Shougangia has a partially creeping stem with adventitious roots only on one side, upright primary and secondary branches arranged in helices, tertiary branches borne alternately or (sub)oppositely, laminate and usually lobed leaves with divergent veins, and complex fertile organs terminating tertiary branches and possessing multiple divisions and numerous terminal sporangia.

Shougangia provides unequivocal fossil evidence for laminate leaves in early-diverging ferns. It suggests that fern-like plants, along with other euphyllophyte lineages, had independently evolved megaphylls by the Late Devonian, possibly in response to a significant decline in atmospheric CO2 concentration. Among fern-like plants, planate ultimate appendages are homologous with laminate pinnules, and in the evolution of megaphylls, fertile organs tend to become complex.

The study was published as a cover paper in the recent issue of Annals of Botany.

Part and counterpart specimens of the vegetative Shougangia

The preparation processes and the reconstruction (right) of the fertile portion of Shougangia