Did an ancient island facilitate migration between the Ryukyu Islands? Compiling the latest geological and biological data, a research team from Tohoku University provides convincing evidence that this is the case.
The Ryukyu Islands stretch from the southwest coast of Kyushu to the eastern side of Taiwan. This island chain is home to many endemic species such as the poisonous snake Habu, or wild black rabbits known as Amami rabbits.
Even within the various islands, rare species can be found. Sitting 300 kilometers southwest of Okinawa, the Miyako Islands are home to the Miyako keelback snake (Hebius concelarus) and the Miyako grass lizard (Takydromus toyamai).
Although separated by the Kerema Gap, these species are closely related to taxa/lineages/populations found on Okinawa and more northern islands and land areas, more so than the Yaeyama Islands, which lie southwest of the Miyako Islands and are closer in distance. Additionally, these species have a limited ability to travel on water, leading to questions about when and how they got there.
Mostly flat, with the highest point standing at 110 meters, the Miyako Islands are completely covered by a type of limestone known as the Ryukyu Group. Based on their distribution location and age, scientists know that the Ryukyu Group was deposited between 1.25 and 0.4 million years ago, during which the sea level fluctuated, repeatedly submerging the Miyako Islands.
Molecular phylogenetic analysis estimates that the Miyako keelback snake became an independent species about 3.7–1.8 million years ago. But this contradicts the evidence that Miyako Island was submerged less than 2 million years ago, and it was about 400,000 years ago that it became the land it is today, after which organisms began to arrive.
Adding to the turbidity of the waters is that the limestone deposits in the caves and fissures contain fossils of Habu, which are not native to the Miyako Islands. These fossils date from 26,800–8,700 years ago.
A new explanation
Yasufumi Iryu, a professor at Tohoku University’s Department of Earth Sciences within the Graduate School of Science who has studied the Ryukyu Group for more than 40 years, proposed a new hypothesis to account for those inconsistencies and in light of the latest geological and biological data. The details of the research are published in the journal Advances in Earth and Planetary Science on July 20, 2023.
“We believe that a piece of land between Okinawa and Miyako existed from 5.5 million years ago to 270,000 years ago. This island served as a transit site for biological migration from Okinawa to the Miyako Islands.”
Iryu and his team call the hypothesis the Okinawa-Miyako Submarine Plateau (OMSP) hypothesis. This includes plate tectonics in the mix of evidence, something that previous explanations surrounding the migration patterns of the Ryukyu Islands have failed to do.
This indicates that the vertical component (up to 1000 m displacement) of the right-lateral strike-slip faulting that formed the Kerama Gap and the corresponding Chinen Disturbance Event drove the uplift. Migration from the OMSP landmass to the Miyako Islands occurred after it was uplifted 400,000 years ago and before the OMSP landmass sank about 270,000 years ago.
Iryu stated that their results rely on the integration of different data sources. “By combining relevant geological and phytogeographic data, we were able to explain the mysterious composition of the modern and Late Pleistocene terrestrial fauna of the Miyako Islands. The study also highlights the high scientific value of the Miyako Island biota and is expected to lead to greater protection and preservation of endemic species.”
More information:
Nana Watanabe et al, Geological history of the land area between Okinawa Jima and Miyako Jima in the Ryukyu Islands, Japan, and its phylogeographical significance for terrestrial organisms in it and adjacent islands, Advances in Earth and Planetary Science (2023). DOI: 10.1186/s40645-023-00567-x
Provided by Tohoku University
Citation: Lost island explains emergence of endemic species in Miyako islands (2023, July 24) retrieved on July 25, 2023 from https://phys.org/news/2023-07-island-emergence-endemic-species-miyako.html
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