Triassurus: a larval stem salamander from Triassic Kyrgyzstan

Triassurus sixtelae is a small amphibian from the Middle or Late Triassic Madygen Formation of Kyrgyzstan. The original fossil was puzzling because it was immature and incomplete. A second specimen described in 2020 preserved additional skeletal details and supported placement on the stem lineage of salamanders. Both known individuals are larvae, so the adult body, mature size and changes at metamorphosis remain unknown. The genus marks an early point in salamander evolution within the ancient amphibian catalogue.

Quick facts

From an enigmatic fossil to a salamander relative

The first specimen was described in 1978 from Madygen in what is now southwestern Kyrgyzstan. Its small size and incomplete preservation made it difficult to decide whether it was a larval temnospondyl or an early salamander relative. A juvenile skeleton can lack the fully ossified features used to classify adults, so the initial uncertainty was reasonable.

Rainer Schoch, Ralf Werneburg and Sebastian Voigt described a second specimen in 2020. It preserved more of the skull and body, allowing the two fossils to be assessed together. Their analysis placed Triassurus on the stem lineage of salamanders, older than the better-known Jurassic record. The result makes it an early caudate relative, not a demonstrated direct ancestor of any living species.

A body preserved before adulthood

The known animals had broad heads, elongated trunks, short limbs and long tails. Their bones were incompletely ossified, consistent with a larval stage. External gills and a tail-fin margin are reconstructed from the preserved impressions and anatomy. These features indicate that the individuals were aquatic when they died, but they do not tell us how long the larval stage lasted.

Neither specimen records the adult after metamorphosis. The size of a larva cannot be scaled directly into adult length because the proportions of salamanders can change during growth. We do not know whether adults remained aquatic, moved onto land, retained external gills or developed a different skull shape. Any complete adult portrait would be speculative.

The skull retains a mixture of features. Some bones are more extensively ossified than in living salamander larvae, while other parts preserve an early tetrapod arrangement. The vertebral column and limbs add information about the evolving salamander body plan. These characters are informative in combination; no single trait alone makes the animal a “modern salamander”.

What the age and setting allow

The Madygen Formation is generally assigned to the Middle or Late Triassic. The often cited age near 230 million years is an approximate placement in a broad stratigraphic interval, not a direct date measured on the fossil. Madygen preserves a diverse terrestrial and freshwater community, but a formation includes deposits formed across places and time.

Larvae likely fed on small aquatic prey, as many living amphibian larvae do, but no gut contents identify a meal. The water body and vegetation shown in a reconstruction represent a plausible local environment. They are not proof that both known individuals lived in the same pool or encountered the same animals.

Why the second specimen matters

With only the first fossil, several competing identities remained possible. The second specimen did not provide an adult or a complete skeleton, but it supplied additional characters needed for a comparative analysis. The resulting placement pushes the documented salamander stem lineage back into the Triassic while making clear how sparse the record still is.

The Jurassic Marmorerpeton is much younger and has a different, more developed salamander-like anatomy. Comparing the two can show changes across deep time, but the gap between them does not reveal a direct evolutionary sequence. The fossils establish an early relative; they leave its mature life history unresolved.