Two new species of Mesozoic tree ferns (Osmundaceae: Osmundacaulis) in Eurasia as evidence of long-term geographic isolation

Abstract

The fossil trunks and rhizomes of Osmundaceae provide important information about its evolutionary history. Due to limited records of the Mesozoic trunks and rhizomes in the Eurasia of the Northern Hemisphere, our understanding on the fossil diversity of the Osmundaceae is hindered. Two new species of the Osmundaceae trunks, Osmundacaulis asiatica sp. nov. and Osmundacaulis sinica sp. nov. representing the first discovery of the Mesozoic tree fern genus Osmundacaulis in Eurasia, are described from Wudalianchi and Qiqihar, Heilongjiang Province, Northeast China, which enriches the plant diversity of the Osmundaceae in the Eurasia, and provides vital evidence for studying the distribution, radiation and evolution of the genus during the Cretaceous. The fossil records suggest that Osmundacaulis species may have evolved from a common ancestor, which first appeared in the Australian portion of Pangaea, and then spread to ancient northern North America and ancient East Asia. Since then, they developed into different species through their own evolutionary lines. The Chinese species have a special local feature that the outer cortex is thicker than the inner cortex, in contrast with reported Osmundacaulis species having thinner outer cortex and thicker inner cortex. Long-term geographic isolation may have led to the radiation of diverse Osmundacaulis species and the appearance of region-specific features, such as the thick outer cortex and the thin inner cortex of the Chinese species. Among all reported Osmundacaulis species, the two new species found in China, O. nerii from the Jurassic of Australia and the O. lemonii from the Jurassic of the USA, have special groups of mucilage-sacs inside sclerenchyma ring of petiole base. Mucilage sacs probably originated independently among taxonomic groups, representing convergent adaptations to similar habitats, rather than indicating genetic inheritance from a common ancestor.