Japan's oldest plant fossil discovered in a 400-million-year-old geological stratum in Ofunato City, Iwate Prefecture

A research group led by Assistant Professor Julien Legrand and Dr. Ahmed Maher of Faculty of Science at Shizuoka University has discovered Japan's oldest spore fossil assemblage from a stratum in Ofunato, Iwate, dating from about 390−410 million years ago (Early Devonian Paleozoic). The fossils were also the oldest record of plant fossils in Japan and more than 10 million years older than the previous record. This work was published in the e-edition of Paleontological Research of the Palaeontological Society of Japan.

Plants expanded their habitat and emerged onto land around 480 million years ago. The ancestors of vascular plants also diversified rapidly during the Early Devonian, and the ancestors of the plants we see today emerged. The diversification of vascular plants during this period was a major event comparable to the Cambrian explosion; however, the oldest plant fossils known in Japan were those of Prunus spinulosa (tree Lycopodiales) from the Late Devonian (approximately 360−380 million years ago), reported in Iwate, Fukushima, Gifu, Kumamoto, and other prefectures. There were no plant fossil data at all during this important period in Japan.

As a stepping stone in the clarification of the vegetation in Japan in the Early Devonian, the research group searched for spore fossils in the Nakazato Formation distributed in Ofunato City, Iwate Prefecture. The Nakazato Formation was deposited in the sea environment, but it has long been known to contain charcoal-like substances abundantly. Rocks from the formation were ground and spore fossils were extracted. Typically, spore fossils are observed under an optical microscope, but the extracted spore fossils were opaque because they were charred due to heat and pressure. Therefore, scanning electron microscopy was used to observe the spore fossils.

As a result, they found cryptospores (a member of the Marchantiaceae family), which are aggregates of multiple spores, and spores with Y-shaped marks on their surfaces. Subclassification of the spores with Y-shaped markings revealed that they include Rhynia (a genus of primitive vascular plants), Lycopodiales, Zosterophyllum (a sister genus to Lycopodiales), and Trimerophytes (ancestors of modern ferns and seed plants). All were herbaceous, with no plants growing into trees. This means that there were extended grasslands consisting of primitive vascular plants in the hinterland of the sea where the Nakazato Formation was deposited.

This spore fossil assemblage is the oldest plant fossil record in Japan and more than 10 million years older than the previous record. A number of species found were in common with species of spore fossils in the same period reported from South China. This finding suggests that the Nakazato Formation was deposited in close proximity to South China and is important to estimate the paleogeography of the time.

Studies on early vascular plants have been conducted mainly in Europe and North America. Because of the relative paucity of data from Asia, the plant diversification that occurred during the Early Devonian is not understood on a global scale. Further studies in Japan on not only the Early Devonian but also older or slightly newer strata are expected to deepen our understanding of the early evolution of plants.

Journal Information
Publication: Paleontological Research
Title: Early Land Plant Spore Assemblage from the Devonian Nakazato Formation of the South Kitakami Belt, Northeast Japan
DOI: 10.2517/PR230018