The study, led by the Complutense University of Madrid (UCM), with the participation of the University of Valencia, was published on Tuesday 14 April in the journal Palaeontology. Until now, the oldest weasel fossils had been found in Poland and Germany at Pliocene sites dating back to around 3.5 million years. This new discovery therefore doubles the species’ evolutionary history.

As explained by Juan Abella, lecturer of Palaeontology at the University of Valencia, the finding is the result of a review of the fossil collection at the National Museum of Natural Sciences (MNCN-CSIC), which highlights the importance of ongoing research activity within palaeontological collections themselves. “The fossils were found in Teruel but had not yet been studied. This discovery underlines the need to continue reviewing collections, even older ones”, he noted.

Analysis of the remains shows that this relative of modern weasels already had a very small body size – 134 grams – similar to that of the least weasel (Mustela nivalis), currently the smallest carnivore. However, the research team emphasises that its teeth already display adaptations associated with a highly carnivorous diet. The results help to clarify when and how weasels and their close relatives emerged, as well as the adaptations that enabled them to occupy new ecological niches and the evolution of ecosystems in the Northern Hemisphere.

“The finding suggests that body size reduction and adaptations for capturing small vertebrates appeared very early in the evolutionary history of this group, probably in response to environmental changes and the spread of certain rodents during the late Miocene”, explains Alberto Valenciano, assistant lecturer of Palaeontology at the UCM.

Advanced analytical methods
The fossils studied were recovered from excavations carried out by the National Museum of Natural Sciences (MNCN-CSIC) during the 1990s in the Teruel area, led by researcher Luis Alcalá. The study has combined classical comparative anatomy techniques with advanced analytical methods, including micro-computed tomography (micro-CT), which made it possible to reconstruct the internal structure of the teeth and jaw in three dimensions and observe anatomical structures invisible from the outside.

“This research is a clear example of the remarkable richness of mammal fossil deposits in Aragón, recognised worldwide: our team has spent decades contributing to the excavation of sites and the study of mammalian fossils”, says Daniel De Miguel, co-author and lecturer in the Department of Earth Sciences at the University of Zaragoza.

The work also revises the classification of another fossil mustelid from China of similar age, which they have named Zdanskyictis. “This new analysis allows clearer evolutionary relationships to be established within the family, placing it close to the lineages that gave rise to otters (Lutrinae) and ictonychines (Ictonychinae), a group that includes living species such as the marbled polecat, the African striped weasel and the grison”, notes Jorge Morales, researcher at the MNCN.

The next step, the researcher adds, will be to find new fossils that allow a more detailed reconstruction of the early evolution of weasels and their relatives. “At the same time, the team continues to investigate the fossil record of carnivorous mammals from the Miocene and Pliocene in the Mediterranean region and Africa, with the aim of reconstructing more precisely the early evolution of different carnivore families”, adds Valenciano.

In addition to the UCM, the University of Valencia and the MNCN, the study also involves the University of Zaragoza, the Institute of Evolution in Africa, the Miquel Crusafont Catalan Institute of Palaeontology, the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the University of Washington.

Future plans
New studies and excavations are currently under way at sites in Aragón, Madrid and the Valencian Community, as well as in various locations in Egypt, Kenya, Namibia, South Africa, Turkey and China.

Within the Valencian Community, for example, Abella highlights the projects in Castelló, with national involvement, and in Buñol (Valencia). The latter site was damaged by the storm of 29 October 2024. The University of Valencia lecturer explains that the palaeontological site was buried by overlying sediments and has not yet been restored. As a result, the Valencian team has begun surveying the surrounding area to continue field research in some capacity.

Article reference: Valenciano, A., Jiangzuo, Q., Law, C. J., Abella, J., Sánchez-Hernández, D., Fernández-Reyes, A., Alcalá, L., Morales, J. & DeMiguel, D. (2026). “Oldest evidence of a weasel reveals a Miocene origin of the Mustelinae (Mammalia, Carnivora)”. Palaeontology, e70055. https://doi.org/10.1111/pala.70055