Updated

Updated · WIRED · Jul 4

Mexico Researchers Identify 1st Fossil Axolotl Species, Pushing Lineage Back Millions of Years

Updated

Updated · WIRED · Jul 4

Mexico Researchers Identify 1st Fossil Axolotl Species, Pushing Lineage Back Millions of Years

Q: Why did the ancient axolotl's descendants lose a vertebra during their evolution?

There is no established answer yet. The fossil shows that *Ambystoma quetzalcoatli* had 17 trunk vertebrae, while living axolotls and related *Ambystoma* species have 16 or fewer, so the lineage appears to have lost one vertebra over millions of years. But the study and the supporting material do not identify the cause. What the research does show is an anatomical change, not the evolutionary mechanism behind it. Scientists documented the difference through CT scans and comparisons with 13 living *Ambystoma* species, confirming that the fossil represents an older body plan. That means the reduction happened sometime after this Late Pliocene species lived, but the available evidence does not say whether it was driven by natural selection, developmental changes, genetic drift, or adaptation to a different habitat or swimming style. In evolutionary biology, vertebral number can change through shifts in development, especially in how the embryo segments the spine. Such changes can persist if they are neutral or if they offer some advantage. But in this case, researchers have not yet linked the missing vertebra to a specific function or pressure. So the careful answer is that descendants did lose a vertebra relative to this fossil species, but why they did so remains unknown based on the current evidence.

3 articles · Updated · WIRED · Jul 4

Ambystoma quetzalcoatli was identified from 12 fossil salamander specimens in Hidalgo, making it Mexico’s first formally described fossil salamander species and the country’s oldest known Ambystoma record.
CT scans and anatomical comparisons with 13 living Ambystoma species showed the fossils were distinct, including an elongated skull opening and 17 trunk vertebrae—more than the 16 or fewer seen in modern axolotls.
The remains came from Atotonilco el Grande, where an ancient roughly 85-square-kilometer lake system once existed; the exceptionally preserved, articulated skeletons had been collected in the early 2000s but not formally studied.
The analysis indicates the species showed neoteny, suggesting that the axolotl life-history strategy was already established in Mexican lake systems during the Pliocene several million years ago.
UNAM said the find extends axolotls’ evolutionary history in Mexico and links today’s biodiversity to ancient freshwater ecosystems that later disappeared.