Taxonomic reassessment of fossil Sequoia and Protosequoia from the Upper Miocene of Central Honshu, Japan, with implications for leaf morphological variation in extant S. sempervirens

Abstract

Premise

Since its emergence in the Mesozoic, Sequoia (Cupressaceae) has been considered to possess conserved leaf morphology. However, recent studies have shown that the leaves of extant S. sempervirens become smaller, with a scale form, with increasing tree height. The fossil-genus Protosequoia has a similar scale leaf morphology, but its relationship to co-occurring Sequoia fossils, which bear linear leaves, remains unresolved.

Methods

To resolve this relationship, we re-examined specimens of two fossil species of Sequoia and Protosequoia from the Upper Miocene of Japan, focusing on leaf epidermal morphology and vascular bundle structure in the bract–scale complexes of seed cones.

Results

We identified fossil specimens with two types of shoots that were preserved in organic connection, but they had previously been classified as different genera. As the leaf shape changed from linear to scale, stomatal orientation shifted from parallel to the midrib to irregular, and epidermal cells gradually became shorter. Seed cones contained fewer than 30 bract–scale complexes with rhombic distal surfaces marked by horizontal grooves. In cross section, scales had rhombically arranged vascular bundles and several abaxial resin canals. Seeds had broad wings, and male strobili were mostly spherical.

Conclusions

The connected shoots, which shift in leaf stomatal features, and similarities in reproductive structure of the fossils suggest that the two genera represent the same species. Therefore, we propose a new Sequoia species characterized by densely branched twigs bearing smaller scale leaves, possibly reflecting an adaptation to different climatic conditions, compared to other species.