3.5-million-year-old fossil identified as new species of giant salamander discovered in stratum in Usa City, Oita Prefecture: Kyoto University announces findings

A research group consisting of Research Fellow Masahiro Noda of the Kyoto University Museum, Professor Emeritus Masafumi Matsui of the Graduate School of Human and Environmental Studies at the Kyoto University, and Professor Kanto Nishikawa of the Graduate School of Global Environmental Studies at the Kyoto University has conducted a detailed analysis of a giant salamander family fossil discovered in a Pliocene stratum (approximately 3.5 million years ago) in the Ajimu area of Usa City, Oita Prefecture. As a result, they revealed it to be a new genus and species, naming it "Limnospondylus ajimuensis" (Japanese name: Ajimu-o-sanshouo). This discovery fills a massive gap in the fossil record of Cryptobranchidae in Asia and provides crucial insights into elucidating their evolutionary history. The findings were published online in PeerJ.

The giant salamander family (Cryptobranchidae) is a group of extant amphibians boasting the world's largest body sizes, having persisted for approximately 60 million years from the late Paleocene to the present. Currently, seven species across two genera are known: the genus Andrias, found in Japan and China, and the genus Cryptobranchus (hellbenders) in North America. On the other hand, in Asia, which is the primary distribution area for extant species, there is a chronological gap of more than 20 million years between the most recent Oligocene fossil species and the extant species, leaving their evolutionary history largely unclarified.

Between 1995 and 1997, three fossil vertebrae of a giant salamander (the Ajimu specimens) were discovered in the Upper Pliocene Tsubusagawa Formation (approximately 3.5 million years ago) in the Ajimu area of Usa City, Oita Prefecture. However, at that time, comparative specimens and osteological studies involving both extant and fossil species were scarce. Accurate identification of the skeletal sites and determination of the taxonomic position could not be achieved.

The research group compared the Ajimu specimens with the complete skeletons of extant species within Cryptobranchidae, which revealed that the three fossil vertebrae belonged to a front trunk vertebra, a mid-trunk vertebra, and a caudal-sacral vertebra, respectively. Next, they conducted a detailed comparison between known fossil species and extant species. This revealed that the mid-trunk vertebra possessed a unique combination of morphological characteristics not seen in any other giant salamanders, proving it to be a new genus and species.

The genus name Limnospondylus is derived from the Greek words limne (lake) and spondylos (spine). The specific epithet ajimuensis originates from Ajimu, the locality where the fossil was discovered. It was named based on the fact that the fossils used to propose the new genus and species were vertebrae discovered in a geological stratum of lake sediments distributed throughout the Ajimu area.

Observation of the growth rings remaining on the surface of the prezygapophyses of the mid-trunk vertebra, combined with total length estimations based on measurements of extant species, indicated the possibility that the Ajimu giant salamander reached a total length of approximately 1.1 meters around 18 years of age.

The Tsubusagawa Formation, where the Ajimu giant salamander was discovered, is a stratum formed in a lake and marsh environment about 3.5 million years ago. Many fossils of large animals that do not inhabit present-day Japan, such as elephants and crocodiles, have also been discovered there. The northern part of Kyushu at that time is thought to have been a warmer and more humid environment than it is today, suggesting that the Ajimu giant salamander inhabited environments where such lakes and wetlands spread out. It is possible that the species later went extinct due to changes in its habitat accompanied by global cooling. Meanwhile, the extant giant salamander species Andrias japonicus, which lives in river environments, still inhabits the rivers flowing through the Ajimu area today.

This is an important achievement that clarifies the past diversity of amphibians in Japan while demonstrating the possibility that the giant salamander family once inhabited a wider variety of environments than they do today.

Noda stated: "I am delighted to be able to present these results using a fossil discovered in Japan. Ajimu is the only region in the world where both a fossil and an extant genus of the giant salamander family are found in the same location, demonstrating that Japan is a crucial area for understanding their evolutionary history. In recent years, Japan's extant giant salamanders have faced issues such as hybridization with species native to China and the degradation of their habitat. Through this study, I have once again realized the importance of not only clarifying past diversity but also preserving extant species for the future. I hope to remain involved with giant salamanders from the perspectives of both foundational research and conservation."

Journal Information
Publication: PeerJ
Title: A new genus of giant salamander (Urodela, Cryptobranchidae) from the Pliocene of Japan
DOI: 10.7717/peerj.21362