The recent discovery of giant salamander fossils in Japan has rewritten the family's history, revealing a new genus and species that challenges our understanding of their evolutionary past. This finding highlights the importance of preserving extant species for the future, as Japan's native giant salamanders face challenges such as hybridization with foreign species and habitat destruction.
The fossils, found in the Fukami River in Oita Prefecture, Kyushu, date back to the Late Pliocene, approximately 3.5 million years ago. The sediments were once a warm, wet lakeland, now unrecognizable from the cooler Japan of today. The team of researchers, led by Masahiro Noda, used a micro-CT scanner to examine the internal details of the fossil vertebrae, revealing a bundle of features that did not match any known genus, extinct or alive.
The mid-trunk vertebra, in particular, stood out with its compact profile and unusual joints. Its centrum, the barrel-shaped core of the vertebra, was short from front to back yet tall from top to bottom, giving it the most compact profile of all the salamanders measured. The joints linking it to its neighbors flared out sideways, sitting on unusually broad, heavy bases, and the lower rims of its two side wings lined up into a single straight edge, a detail unmatched by its relatives.
The team named the new genus and species Limnospondylus ajimuensis, a name derived from the Greek words for lake and vertebra, with the species name paying homage to Ajimu, the site where the fossils were found. The largest of the three vertebrae suggests a decent-sized animal, with a body length of approximately 3.6 feet, although the estimate is provisional due to the limited number of modern species for comparison.
The study's findings have broader implications for our understanding of the evolutionary history of giant salamanders. The fossil's discovery in a lake environment suggests that the species may have ranged more widely than its river-bound cousins, and the presence of growth rings in the bone indicates that the giant salamander was roughly 17 or 18 years old at the time of its death.
The good conditions of the Pliocene period did not last, as the world cooled, and the Pleistocene period brought colder weather and shifting land, leading to the decline of lakes and wetlands. This change likely dealt a fatal blow to the lake-dwelling salamander, while its river-loving relative, Andrias japonicus, survived and continues to thrive in the same Fukami River today.
Before this discovery, only five genera of giant salamander had been described, living and extinct combined. The new genus stretches the known shape and range of the group, and the authors suspect that some of the previously dumped fossils into the genus Andrias may be unrecognized species still waiting to be sorted out.
The study's publication in the journal PeerJ highlights the importance of preserving extant species for the future, as Japan's native giant salamanders face challenges such as hybridization with foreign species and habitat destruction. The discovery serves as a reminder of the need to protect and study these fascinating creatures to better understand their evolutionary history and ensure their survival.
