New turtle species from the late Cretaceous discovered by second grader in Kuji City, Iwate Prefecture — Waseda University and Kuji Amber Museum conduct joint excavation

A research group led by Professor Ren Hirayama of the Faculty of International Research and Education at Waseda University, and Director Hisao Shinden of Kuji Amber Museum in Kuji City, Iwate Prefecture, verified a new fossil turtle species (Lindholmemys) in the Tamagawa Formation of the Kuji Formation that existed during the Late Cretaceous Period (approximately 90 million years ago). A press conference was held at Waseda University (Shinjuku City, Tokyo) on July 13, 2023.

The fossil of a lower jaw measuring approximately 1 cm, which has never been reported before, was discovered by Tasuku Kubo (a second-grade student at the time of the discovery), who was accompanying his parents who participated in the excavation. The new fossil turtle species was discovered in the Tamagawa Formation of the Kuji Formation (Kokuji Town, Kuji City, Iwate Prefecture) and has been dated to the Late Cretaceous Period through radiometric dating of volcanic ash.

Approximately 2,800 fossils of roughly 30 species of vertebrates have been found, including the teeth of large plant-eating dinosaurs and carnivorous dinosaurs, turtles, crocodilians, choristodera, and sharks. These fossils were discovered along with amber from the same formation. Fossils from various marine and terrestrial organism groups have been excavated from this formation, making it one of the most important and rare formations in the world for researching the biodiversity of the time.

Fossils of turtles, small plant-eating dinosaurs, pterosaurs, and carnivorous dinosaurs have also been discovered at the nearby amber mining experience site of Kuji Amber Museum. According to the degree of degeneration of the amber excavated in the same formation, the Kuji Formation is very soft and has not been exposed to temperatures above 50 degrees Celsius. This is also the reason why heat-sensitive amber can be mined without turning into charcoal, making it highly likely that information from that time has been preserved.

Turtles are an old animal, having existed at least since the Triassic Period of the Mesozoic era (220 million years ago). Today, there are more than 300 species of turtles, 160 of which belong to the Lindholmidae family.

Lindholmemys is considered to be the ancestral group of the Lindholmidae family. The dorsal and ventral carapaces are strongly connected, and the relative depth and width of the scale groove (the boundary of the scales covering the carapace) are several times greater than those in other aquatic turtles. The carapace is small for a Cretaceous terrestrial turtle, with a maximum length of 30 cm. It is distinguished from post-Eocene Lindholmidae turtles by the continuous development of four to three pairs of inferior marginal scales at the border between the ventral and dorsal carapaces.

The fossil of the Lindholmemys turtle is different from those of any previously reported family, in that the anterior-most scale on the dorsal carapace is narrower while the posterior-most scale is wider, and the anterior-most scale on the ventral carapace is smaller.

The researchers confirmed that the occlusal surface is narrow in front and expands in the back, which is a feature not seen in modern turtles. However, this jaw fossil cannot be compared with fossils from other families or from the same family in different localities owing to the scarcity of reports on jaw fossils. This means that the jaw movement is different from that of modern turtles.

The fossil discovered in this study is believed to be the oldest among the Lindholmemys family. Currently, 11 different genera of this family have been confirmed in Asia outside of India, and they lived during a period spanning from the Late Cretaceous to the Paleogene (approximately 90-60 million years ago). This is the second discovery of a new species of vertebrate from the Mesozoic era in Iwate Prefecture, following the discovery of Adkus kohaku, which was reported by the research group in 2021.

Hirayama and his colleagues have continued intensive biannual surveys since 2012. Although several fossils of Lindholmemys turtles have been found since the first survey, last year's survey has revealed the possibility of a new species. The research group will continue to conduct more detailed surveys in the future.