A new species of Karkenia (Karkeniaceae, Ginkgoales) from the Lower Jurassic of East Siberia (Russia): palaeobiogeographical and evolutionary implications

The family Karkeniaceae is the most ancient of the Ginkgoales. It is known from the lower Permian to the Lower Cretaceous. Cladistic analysis has shown that Karkeniaceae represents a distinct lineage among the Mesozoic ginkgoaleans and is much closer to Trichopitys . However, the absence of the complex ovule shoots of ginkgoaleans with remains of ‘intercalated bracts’ in the palaeontological record makes it difficult to reconstruct the transformation series linking Trichopityaceae to Karkeniaceae. Here, we report Karkenia bracteata sp. nov. from the Lower Jurassic of East Siberia (Russia). We examined impressions and compressions using light microscopy and scanning electron microscopy. Karkenia bracteata consists of the main axis bearing helically arranged bract–ovule complexes, each composed of a long laminar bract subtending and sheathing an ovule stalk with a single ovule. The presence of a bract distinguishes this species from all known Karkenia . However, the ovule structure, stomatal guard cells and megaspore membrane indicate that the new species belongs to Karkeniaceae. Karkenia could have evolved from Trichopitys due to reduction and condensation from an ancestral state. The ovulate short shoots must have fused to form a compact structure, while the main axis was strongly reduced. The leaves of such branches became reduced and further disappeared. Therefore, the leaves of the intermediate form between Trichopitys and Karkenia must have been bracts. Thus, K. bracteata corresponds to the hypothetical intermediate form linking Trichopityaceae to Karkeniaceae and confirms the hypothesis of the cone‐like nature of Karkenia megastrobili.