Three fossilized backbones that spent years in storage after being unearthed in Japan have turned out to belong to an entirely new kind of giant salamander, one that lived about 3.5 million years ago and does not fit neatly into any known branch of the family. Researchers who reexamined the bones concluded they represent a new genus and species, named Limnospondylus ajimuensis, and that the animal favored still lakes rather than the fast-flowing rivers where today’s giant salamanders live. The find rewrites part of the evolutionary story of some of the largest amphibians ever to exist.
Fossils that waited decades for a name
The vertebrae were recovered in the late 1990s from the Ajimu area of Oita Prefecture on the island of Kyushu, preserved in ancient lake sediments. For years they carried only a tentative label, waiting for a closer look that would settle where they belonged.
The bones were first described around 2001 and tentatively lumped in with the living giant salamander genus Andrias, but at the time researchers lacked enough skeletal material and detailed anatomical study to classify them with confidence, according to a research summary from ScienceDaily. For roughly two decades the specimens sat with only a provisional identity, until a new analysis took a closer look with fresh comparisons and methods that were not available when the fossils were first studied.
A new genus and species
Dr. Masahiro Noda and colleagues at Kyoto University reexamined the three vertebrae and determined that they did not belong to Andrias after all, but to a lineage all their own. They named the animal Limnospondylus ajimuensis, combining the Greek words for lake and vertebra with a nod to the Ajimu locality where the bones were found.
As Phys.org reported, the fossils come from the Tsubusugawa Formation, a layer of sediments laid down in a lake roughly 3.5 million years ago during the Pliocene. Reassigning the specimens to a new genus means the salamander represents a distinct branch that had gone unrecognized, rather than an early example of a living group. That distinction matters for reconstructing the family’s history, because it adds a lineage that scientists had not known existed.
A lake-dweller, not a river giant
One of the more intriguing conclusions concerns where the animal lived. The sediments that entombed the bones formed in a freshwater lake surrounded by wetlands and forests, suggesting Limnospondylus ajimuensis inhabited calm, standing water.
That sets it apart from modern giant salamanders, which typically live in cool, fast-moving rivers and streams, a contrast highlighted in a species profile by Sci.News. The difference matters because habitat shapes anatomy and behavior, and a lake-adapted giant salamander points to a wider range of lifestyles in the group’s past than its surviving members display today. It suggests the family once occupied ecological niches that its living representatives have since abandoned or lost.
How big it may have grown
Even from just three vertebrae, researchers were able to estimate the animal’s size. Their analysis suggests Limnospondylus ajimuensis may have reached a total length of about 1.1 meters, or roughly 3.6 feet, placing it firmly among the giant salamanders rather than their smaller relatives.
Living giant salamanders are among the largest amphibians on Earth, with some species exceeding five feet, and their fossil relatives help trace how that impressive size evolved and persisted over millions of years. Reconstructing a body length from a handful of bones relies on comparisons with better-known specimens, but the estimate is enough to place the new species in ecological context and to confirm it was a substantial aquatic predator in its lake habitat.
The value of revisiting old collections
The discovery adds to a growing list of finds that emerged not from new excavations but from a fresh look at material already in hand. Fossils collected decades ago often outlast the tools and knowledge available when they were first studied, and reanalysis can reveal features that earlier researchers could not interpret.
For giant salamanders, whose fossil record is patchy, each reexamined specimen can shift the family tree. Amphibian fossils are often fragmentary because their bones are delicate, which makes every well-studied specimen especially valuable. In this case, a set of bones once filed away under a tentative label now stands as evidence of a lost lineage that swam in Japanese lakes millions of years before the region took its present shape. The find also raises the prospect that other overlooked specimens in Japanese collections could hold further surprises, and it may prompt researchers to revisit fragmentary amphibian fossils that were once considered too incomplete to identify. Modern comparative methods, combined with a richer reference library of known species, can extract meaning from material that earlier generations set aside.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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