Draft:Satsumamukashiumigame

Satsumamukashiumigame sea turtle was discovered in November 2020 by the paleontologist Satoshi Utsunomiya. Fossilized bones were found within a nodular limestone mass in the Cretaceous strata in the southwestern part of Shishi Island, Nagashima Town, Kagoshima Prefecture.

Subsequently, research conducted by Associate Professor Yasuhisa Nakajima of Tokyo City University revealed that it is the oldest sea turtle fossil in Japan (East Asia), dating back approximately 100 million years. The findings were announced at the Japan Paleontological Society meeting held from January 26, 2024.

Sea turtle fossils are mainly found in Cretaceous strata in various locations in Japan, such as Hokkaido, Kuji City in Iwate Prefecture, and Awajishima in Hyogo Prefecture. Fossil records older than 85 million years ago are rare, and the discovery from strata dating back approximately 100 million years establishes the oldest record in Japan (East Asia). This finding is of great significance in exploring the global distribution history of sea turtles. The research team has named the sea turtle fossil from the discovery site in Kagoshima Prefecture, formerly known as Satsuma, as the "Satsumamukashiumigame."

Overview

 * Ｎick Name: Satsumamukashiumigame.
 * Discovery Location: Shishijima, Nagashimachou, Kagoshima Prefecture
 * Stratum: Goshoura Group, Hekizuka Formation
 * Age: Late Cretaceous Period (Approximately 100 million years ago)
 * Discoverer: SatoshiUtsunomiya
 * Discovery Date: November 20, 2020

Discovery location
The fossil was obtained from the "Hekizuka Formation," a Cretaceous marine deposit (approximately 100 million years old) found along the Hekizuka (Hegushi) coast in the south-southwest part of Shishi Island, Nagashima Town. During Mr. Utsunomiya's investigation of the surrounding strata, he discovered nodules on the surface of the formation containing fossilized bones. Associate Professor Nakajima of Tokyo City University conducted CT scans and cleaning processes for analysis, revealing that it is the oldest sea turtle fossil in Japan (East Asia).

Preservation site of the fossil
The preserved parts of the bones include one point of the neural arch portion of the cervical vertebrae (approximately 25 mm in length anterior-posterior), and an incomplete left ventral scute (width of the preserved portion is about 42 mm). Based on the size of these bones, it is estimated that the carapace length reached approximately 70 cm or more. In the neural arch of the cervical vertebrae:

1）There is a developed long anterior articular process and a short posterior articular process, 2）The neural spine is developed but low, 3）The anterior-posterior length is slightly larger than the left-right width.

These features match with the 4th cervical vertebra or the preceding and following cervical vertebrae of the Cheloniidae family, such as the extant sea turtle family and extinct Thalassemydidae like Toxochelys. Additionally, the anterior part of the ventral scute has a thin, plate-like shape, and the loose articulation surfaces between adjacent bones are consistent with those of the extant sea turtle family and the extinct Thalassemydidae such as Desmatochelys.