A reconstruction of the Ajimu giant salamander, which is believed to have inhabited the lakes and marshes of the Ajimu region approximately 3.5 million years ago when its environment was warmer and more humid. Today, this area is home to Andrias japonicus, the Japanese giant salamander endemic to Japan.
Image Credit: Kanon Tanaka
Scientific Frontline: Extended "At a Glance" Summary: What Is Limnospondylus ajimuensis?
The Core Concept: Limnospondylus ajimuensis is an extinct, newly identified genus and species of giant salamander that inhabited the freshwater lakes and marshes of Japan approximately 3.5 million years ago.
Key Distinction/Mechanism: Initially misclassified under the extant genus Andrias, this salamander is distinguished by unique morphological characteristics found in its mid-trunk vertebra, separating it from all other known species in the Cryptobranchidae family.
Major Frameworks/Components:
- Taxonomic Reclassification: The identification relied on precise comparative skeletal analysis of an anterior trunk vertebra, a mid-trunk vertebra, and a sacro-caudal vertebra against extant Cryptobranchidae species.
- Paleoclimatology: The presence of this species in the Tsubusugawa Formation indicates the Pliocene environment of Kyushu was significantly warmer and more humid than modern Japan.
- Extinction Dynamics: Researchers hypothesize that climactic cooling during the transition from the Pliocene to the early Pleistocene drove the genus to extinction, though its relative, the Japanese giant salamander (Andrias japonicus), survived.
Branch of Science: Paleontology, Evolutionary Biology, and Herpetology.
Future Application: Understanding the historical climate adaptations and evolutionary history of extinct amphibians provides critical baseline data for modern conservation efforts aimed at protecting extant, endangered giant salamander species from habitat loss and anthropogenic climate change.
Why It Matters: The Cryptobranchidae family previously contained only five described genera. This discovery substantially expands the known phylogenetic diversity of giant salamanders and highlights the massive scientific value of re-evaluating unclassified or misidentified historical fossil archives.
In the late 1990s in the Ajimu region of Japan's Oita Prefecture, researchers discovered three fossilized vertebrae belonging to the giant salamander family Cryptobranchidae. These were embedded in the Tsubusugawa Formation, Pliocene-era strata of lake deposits dating back approximately 3.5 million years. The strata have also yielded fossils of animals that no longer roam Japan, such as elephants and crocodiles, revealing a glimpse of an era much warmer and more humid than Japan's current climate.
Researchers originally assigned the three Ajimu specimens to the genus Andrias, which includes the world's largest living amphibians, but at the time, a lack of comparative specimens and research prevented their precise taxonomic identification. Now, more than two decades later, a new research team at Kyoto University has succeeded in shedding more light on these mysterious fossils.
After comparing the Ajimu specimens to the skeletons of extant Cryptobranchidae species, the team found that the three fossils consisted of an anterior trunk vertebra, a mid-trunk vertebra, and a sacro-caudal vertebra. Further comparisons revealed that the mid-trunk vertebra possessed unique morphological characteristics not seen in other Cryptobranchidae species. This led the researchers to conclude that the Ajimu specimens represent a new species and genus.
"Ajimu contains the only site in the world where fossils of the giant salamander family and extant genera have both been found," says first author Masahiro Noda. "I'm delighted that this study has highlighted Japan as a crucial region for understanding their evolutionary history."
The team named the new species Limnospondylus ajimuensis, derived from the Greek words limne for lake and spondylos for vertebra, with ajimuensis referencing Ajimu. This species of giant salamander inhabited the extensive freshwater lake and marsh environments that characterized the island of Kyushu a few million years ago and may have reached a length of approximately 1.1 meters by adulthood.
To date, only five genera of the Cryptobranchidae family have been described, so the discovery of a previously unknown genus represents a significant breakthrough highlighting giant salamander diversity. Fossils belonging to the Cryptobranchidae family have been found across the Northern Hemisphere, but a lack of sufficient knowledge for accurate identification led to many of them being classified under the extant genus Andrias. This study suggests that other fossils may conceal numerous previously unknown species.
Climatic cooling during the transition from the Pliocene to the early Pleistocene and subsequent habitat changes may have contributed to the extinction of Limnospondylus ajimuensis. However, its cousin Andrias japonicus, the Japanese giant salamander, still inhabits the rivers flowing through the Ajimu region and other parts of Japan.
"Recently, Japan's native giant salamanders have faced challenges such as hybridization with foreign species and habitat destruction," says Noda. "This research has given me a renewed appreciation for the importance of preserving extant species for the future."
Published in journal: Paleontology and Evolutionary Science
Title: A new genus of giant salamander (Urodela, Cryptobranchidae) from the Pliocene of Japan
Authors: Masahiro Noda, Masafumi Matsui, and Kanto Nishikawa
Source/Credit: Kyoto University
Edited by: Scientific Frontline
Reference Number: pal071026_01