Newswise — A research group from Tohoku University has presented compelling evidence suggesting that an ancient island played a significant role in facilitating migration among the Ryukyu Islands. By analyzing the most recent geological and biological data, they have shed light on this historical connection.
The Galapagos of Asia
The Ryukyu Islands extend from the southwestern coast of Kyushu to the eastern part of Taiwan. This island chain boasts a rich variety of endemic species, such as the venomous Habu snake and the wild black Amami rabbits.
Even within the distinct islands of the chain, one can discover unique species. The Miyako Islands, situated about 300 kilometers southwest of Okinawa, harbor the Miyako keelback snake (Hebius concelarus) and the Miyako grass lizard (Takydromus toyamai).
Surprisingly, despite being separated by the Kerema Gap, these species show close genetic relationships to taxa or lineages found on Okinawa and other more northern islands and land areas. They are more closely related to these regions than to the Yaeyama Islands, which are located southwest of the Miyako Islands and much closer in proximity. The limited ability of these species to travel over water raises questions about the timing and mechanism of their migration to the Miyako Islands.
Inconsistencies
The Miyako Islands, characterized by their predominantly flat terrain, reach a maximum elevation of 110 meters. These islands are entirely covered by a type of limestone called the Ryukyu Group. Geological research has shown that the Ryukyu Group was deposited between 1.25 to 0.4 million years ago, a period marked by fluctuating sea levels that repeatedly submerged the Miyako Islands.
According to molecular phylogenetic analysis, the Miyako keelback snake evolved into an independent species approximately 3.7 to 1.8 million years ago. However, this timeline seems to contradict the evidence indicating that Miyako Island was submerged before 2 million years ago and only began to emerge as the present-day landmass roughly 400,000 years ago. It was after this emergence that organisms started arriving on the island.
Adding to the complexity of the situation, limestone caves and fissures have yielded Habu snake fossils, which are not native to the Miyako Islands. These fossils date back to a period ranging from 26,800 to 8,700 years ago, further puzzling researchers about the historical dynamics of the region.
A New Explanation
Yasufumi Iryu, a distinguished professor at Tohoku University's Department of Earth Sciences in the Graduate School of Science, has devoted over four decades to studying the Ryukyu Group. In response to the discrepancies arising from the latest geological and biological data, Professor Iryu has put forth a novel hypothesis.
According to his proposal, a land area existed between Okinawa and Miyako approximately 5.5 million years ago until around 270,000 years ago. This landmass acted as a crucial transit site, facilitating the migration of biological species from Okinawa to the Miyako Islands. This hypothesis aims to reconcile the inconsistencies and shed new light on the historical dynamics of the region.
The Okinawa-Miyako Submarine Plateau (OMSP) hypothesis, as coined by Iryu and his research team, introduces a novel perspective that takes into account plate tectonics as a significant factor in understanding migratory patterns to the Ryukyu Islands. Unlike previous explanations, this hypothesis incorporates the vertical component of right-lateral strike-slip faulting responsible for shaping the Kerama Gap and the Chinen Disturbance Event. The faulting led to an uplift, with up to 1000 meters of displacement, forming the landmass between Okinawa and Miyako, which they refer to as the OMSP.
Migration from this OMSP landmass to the Miyako Islands took place after the uplift around 400,000 years ago and before the OMSP was submerged approximately 270,000 years ago. This timeline aligns with the observed patterns of the terrestrial fauna found on the Miyako Islands, shedding light on their composition.
Iryu emphasizes that their findings are the result of a comprehensive integration of various data sources, combining geological and phytogeographic data. This approach has enabled them to explain the intriguing mix of modern and Late Pleistocene terrestrial fauna on the Miyako Islands. Furthermore, the study underscores the immense scientific value of the Miyako Island biota and highlights the importance of protecting and conserving the endemic species that inhabit the region.
Details of the research were published in the Journal Progress in Earth and Planetary Science on July 20, 2023.
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