Paleontologists in Mexico have described a previously unknown fossil salamander species, Ambystoma quetzalcoatli, marking the first time a fossil salamander has been formally described in the country and the oldest confirmed record of the genus Ambystoma, the group that includes today's axolotls, ever found there. Researchers say the find fills in a missing piece of how Mexico's present day biodiversity took shape.
A Lost Lake System in Hidalgo
The fossils turned up in the municipality of Atotonilco el Grande, in the state of Hidalgo, a spot that once sat beside a sprawling freshwater lake system spread across roughly 85 square kilometers. Researchers believe the lakes formed after the Amajac River's course was temporarily blocked, creating a temperate, subhumid environment that trapped and preserved an unusually wide range of life. Over the years the site has produced fossils of plants, diatoms, gastropods, ostracods, beetles and fish. What it had not produced, until now, was a formal scientific description of its amphibian remains.
Fossils Waiting Decades for a Second Look
The specimens at the center of the new study, a dozen fossil salamanders in total, were actually collected in the early 2000s by the FES Zaragoza Paleobotany Research Group. Many were remarkably well preserved, with complete, articulated skeletons detailed enough to allow for a full anatomical breakdown. At the time, the remains were tentatively filed under the genus Ambystoma, the group modern axolotls belong to. It took a fresh look, led by researchers Jorge Herrera Flores and María Patricia Velasco de León, using computed tomography (CT) scanning alongside close anatomical comparisons with living species, to work out exactly what the fossils represented.
What Makes Ambystoma quetzalcoatli Different
That renewed analysis, published in the journal Palaeontologia Electronica, concluded that the nearly three decade old fossils actually belonged to a species science had never recorded. Several skull and skeletal features set it apart from any living axolotl: an elongated opening on the top of the skull, a differently shaped palate, a different arrangement of certain cranial bones, and, notably, 17 trunk vertebrae. That last detail matters because modern axolotls have 16 trunk vertebrae or fewer, making the vertebra count one of the clearest physical markers separating the fossil species from its living relatives.
Comparing Bones Across 13 Living Species
To confirm the fossils' identity, the team lined them up against 13 living Ambystoma species, among them axolotls endemic to Mexico such as the Xochimilco axolotl (Ambystoma mexicanum), as well as tiger salamanders found in both Mexico and the United States. The comparisons drew on three dimensional imaging and CT scans housed in international scientific collections. The researchers also obtained complete skeletons of the modern salamander Ambystoma velasci to use as a direct physical reference point, comparing bone shape and structure against what had been preserved in the fossil material. From there, they mapped the evolutionary relationships between the fossil salamanders and their living relatives, cross checking skeletal comparisons against earlier DNA based studies of modern salamanders.
A Trait Frozen in Time: Neoteny
One of the more striking findings is that Ambystoma quetzalcoatli shared a trait with several living axolotls, including the Xochimilco, Pátzcuaro and Alchichica axolotls: neoteny, the biological quirk that lets an animal keep its juvenile features into adulthood instead of undergoing full metamorphosis. Neoteny tends to show up in stable, isolated lake environments, where there is little evolutionary pressure to push through the complete transformation most other amphibians go through. Finding it in this fossil suggests that Mexican axolotls were already living this way during the Pliocene, several million years ago, long before anyone was around to record it.
Rewriting the Axolotl's Timeline
Taken together, the skeletal and physiological details convinced the researchers they were looking at a genuinely new species, one that pushes the known evolutionary history of axolotls back much further than scientists had previously assumed. It also confirms that these amphibians have been living in what is now Mexican territory for millions of years. In a statement, the National Autonomous University of Mexico (UNAM) said the discovery of Ambystoma quetzalcoatli "shows that the axolotl lineage has a much older evolutionary history than previously thought, with a presence in Mexico dating back to the Pliocene and an early diversification linked to ancient lake systems." The university added that beyond identifying a new species, the discovery "reinforces the idea that Mexico's modern biodiversity has deep roots in ecosystems that disappeared millions of years ago."
