Boys, hold that "large stone" — Researcher who has been a fossil enthusiast since childhood discovers 18-million-year-old seagrass fossils in local Aichi

While casually looking at a fossil catalog during a meal, an error in the description stood out. Professor Toshihiro Yamada, who studies paleobotany at the Hokkaido University Faculty of Science reported that fossils found in Minamichita Town, Aichi Prefecture are 18-million-year-old seagrass fossils. Seagrass fossils are rarely found in complete form because their soft tissues decompose easily and do not remain intact. Although these specimens were described as a type of animal in the catalog, he discovered features unique to plants and realized, "This is seagrass."

Treasures of the strata found at age 5

Yamada, who is from Aichi Prefecture, asked his parents when he was five years old if he could "try digging for fossils." Young Yamada was taken to strata formed in shallow seas in Mizunami City, Gifu Prefecture. When he dug, he found fossils of Genrokusodegai, a type of bivalve. This was his first treasure in life. Young Yamada became captivated by fossil hunting from this experience. When the TV station NHK aired educational programs featuring strata and plate tectonics, one of the theories in earth science, he would watch with intense concentration.

After becoming a junior high school student, he traveled as far as Kumamoto Prefecture with a man he met through his "fossil enthusiasm activities" and headed to Mifune Town, which now promotes tourism as the "Land of Dinosaurs." In that town, there was a rock about the size of two fists, which he received and searched it for fossils at home. First, after drying the rock, he put it in water in a bucket to break it apart. He would sift through the fragments and search for fossils among what remained.

Then, something tooth-like appeared. When he observed it carefully, he could see the irregularities on the chewing surface called ridges and surrounding protrusions, and young Yamada realized that "this is not a tooth from fish or reptiles." When he brought it to the Kyoto University Primate Research Institute (at that time), he was told it was "a tooth from a Mesozoic mammal." As a result of detailed research by graduate students there, it was found to be the tooth of a mole-like creature called an insectivore. This was the first time a Mesozoic mammal fossil was found in Japan. Young Yamada became even more fascinated with fossils and devoted his weekends to fossil hunting.

During his junior high and high school years, he completed Japanese Jurassic shell fossils and created what he called the "Dresser Collection," storing fossils in dresser drawers in phylogenetic branching order. However, because he was too absorbed in his passion activities, he had to repeat a year for his university entrance exams. During his gap year, he forgot about stones and studied "like he was gnawing on rocks," and was able to get into Kyoto University. Even after entering university, he was immersed in fossil hunting. After graduation, he pursued his passion and started on the research path, traveling not only within Japan but also to places like Dong Van County, Ha Giang Province, Vietnam.

Silently visiting strata during the pandemic

The land from Gifu Prefecture to Aichi Prefecture is said to have had deeper seas spreading toward the south. The farmland in Minamichita Town, Aichi Prefecture, consists of uplifted seabed, and in particular, the Morozaki district in the same town has strata from the Early Miocene period, approximately 18 million years ago. There is a book called "Fossils from the Miocene Morozaki Group" (edited by the Tokai Fossil Society, published in 1993) that catalogs fossils from this area. When Yamada was looking at it while eating in 2020, his eyes stopped on a page titled "Unclassifiable Fossils."

He had seen the specimens published in this book when he was in high school. However, when he looked closely again at the fossil photographs labeled "Echiurans?" and "Sea pens?", he noticed features such as the presence or absence of ligules (projections at the leaf base) and structures called short shoots where several leaves are bundled together, leading him to think "these might not be echiurans and sea pens." Echiurans are still used as fishing bait today and have a thick earthworm-like shape.

He contacted the owner of the familiar specimen and had it sent to him, then observed it carefully under a stereomicroscope. The specimen later described as the new species Morozakimukashizangusa showed fibrous leaf sheaths wrapping around the stem in the short shoot sections. The specimen described as the new species Aichiisohaguki had bryozoan and oyster fossils attached to it, and the leaf margins had no serrated pattern. What he had thought were echiurans in high school both turned out to be new species of seagrass.

At this time, COVID-19 was spreading. Yamada, who was at Osaka Metropolitan University, used his private car to travel to Aichi Prefecture as a precaution, even though fossil hunting is not inherently an activity involving meeting people. When he dug in the Morozaki Group, he found fossils of Morozakimukashizangusa and Aichiisohaguki and reported them as new species. The "zangusa" in Morozakimukashizangusa was named after the Okinawan dialect word for seaweed in seagrass beds. The series of results was published in the electronic edition of the Dutch academic journal Aquatic Botany on June 1.

Seagrass fossils connecting to current species: Missing link

Seagrasses inhabit shallow seas, with tropical regions at the center of their distribution. They not only serve as food and a habitat for animals but also play a role in carbon dioxide fixation. It is known that seagrasses had appeared by about 81 million years ago, around the end of the Cretaceous period, but discoveries are rare. This is because seagrass fossils can only form when several conditions overlap: the seagrass must be buried alive in mud, oxygen must be cut off, and decomposition must not proceed.

Furthermore, although genetic analysis suggests that ancestors of current species appeared 30 to 10 million years ago, fossils from this time period had been a "missing link" that had not been found. Seagrass fossils can clarify the status of carbon dioxide fixation in the ocean—known as blue carbon—and allow inference of terrestrial environments. By advancing research on the Morozaki Group strata, it is expected that the conditions of ancient seabeds can be elucidated.

Yamada reflected, "When echiurans are crushed, they become banana-shaped with constrictions, so it's not strange to mistake them for something other than seagrass. However, I had learned botany that I didn't know in the past, and having knowledge of where to distinguish plant characteristics led to this discovery." In the future, he wants to investigate the actual distribution of seagrasses and the ecological conditions of deep seas.

Finally, he gave this advice to fossil enthusiasts: "I think there are fossils in your neighborhood. It's important to collect them yourself, and when doing so, it's good to consider the condition of the surrounding strata as well. How were the strata pushed or pulled? Don't just look at what you want to dig up but observe the surroundings before working. Small stones can be taken home, but large stones should be split along their grain while observing the stone's structure before taking them home."

Throughout our conversation, seeing him speak of his love for his passion (stones), I felt that the saying "love makes one skillful" might have been made for Yamada.