Researchers at the Zaragoza School of Advanced Studies (FES Zaragoza) at the National Autonomous University of Mexico (UNAM) have identified a new fossil species of axolotl in the state of Hidalgo.

Named Ambystoma quetzalcoatli, it is the first fossil salamander species to be formally described in Mexico and the oldest known record of the genus Ambystoma ever documented in the country. According to the researchers, the discovery provides an important new piece of the puzzle for understanding the origin and evolution of the biodiversity that characterizes modern Mexican fauna.

The fossils were discovered in the municipality of Atotonilco el Grande, in Hidalgo, a region that was once home to an extensive freshwater lake system covering roughly 85 square kilometers. The lakes likely formed when the course of the Amajac River was temporarily blocked. This temperate, subhumid environment has yielded fossils of numerous plants, diatoms, gastropods, ostracods, beetles, and fish. Until now, however, the amphibian remains recovered from the site had never been formally studied or described.

The researchers examined a dozen fossil salamander specimens collected in the early 2000s by the FES Zaragoza Paleobotany Research Group. Many of the fossils were exceptionally well preserved, with complete, articulated skeletons that allowed for a detailed anatomical analysis and a precise assessment of their morphology.

The remains were initially identified as belonging to a species of the genus Ambystoma, the group that includes modern axolotls. However, a team led by researchers Jorge Herrera Flores and María Patricia Velasco de León revisited the material using modern techniques, including computed tomography (CT) scanning and detailed anatomical comparisons with living species, to determine its identity more accurately.

Their analysis revealed that the fossils collected nearly three decades ago belonged to an entirely new species, distinguished from modern axolotls by several significant anatomical differences.

According to the study, published in Palaeontologia Electronica, the fossils display distinctive features of the skull and skeleton that are absent in living species. Among the most notable are an elongated opening on the top of the skull, a differently structured palate, variations in the arrangement of several cranial bones, and the presence of 17 trunk vertebrae. This last characteristic is particularly significant, as modern axolotls have 16 or fewer trunk vertebrae.

To establish the fossils’ identity, the researchers compared them with 13 living Ambystoma species, including several endemic to Mexico, such as the Xochimilco axolotl (Ambystoma mexicanum), as well as tiger salamanders from Mexico and the United States. They relied on three-dimensional imaging and CT scans available through international scientific collections.