Weasels are small carnivores with a long body and short legs. They also have a stout skull and sharp teeth. These creatures, along with ferrets and minks, make up the Mustelinae subfamily.
Until now, researchers believed that the oldest fossils from this family were from Poland and Germany, dating back to about 3.5 million years ago in the Pliocene epoch. But a fossil discovered in Teruel, Spain, has doubled that estimate, dating back to the late Miocene, around 6.5 million years ago.
The research team, including Chris Law, a University of Washington principal research scientist in the biology department, has identified this fossil as belonging to a new species, named Galanthis baskini. The researchers estimate that this creature was about 5 ounces, comparable in size to the smallest living carnivoran today, the least weasel or Mustela nivalis. Much like the modern weasel, G. baskini was also likely a carnivore, based on its teeth.
The team published these results April 13 in Palaeontology.
"This study begins to uncover the evolutionary history of modern weasels, specifically, why do they have unique small, elongated bodies compared to all other mammals?" said Law, who is also an affiliate curator at the UW Burke Museum of Natural History and Culture. "We had hypothesized that events during the mid- to late-Miocene - both the expansion of open habitats, such as grasslands, and the diversification of rodents - would have allowed weasels to evolve bodies that were small and flexible enough to chase rodent prey in small crevices underground. G. baskini is exciting because it confirms that weasels were present in the Late Miocene. And it's pretty cool that G. baskini was the size of the least weasel - that means small weasels were already around more than 6 million years ago."
To compare this fossil to other weasel family members, the researchers used a combination of classical comparative anatomy with advanced analytical techniques, such as micro-computed tomography, or micro-CT. Micro-CT allowed the team to three-dimensionally reconstruct the internal structure of teeth and jaws as well as observe anatomical features that were not externally visible.
"The new genus, Galanthis, is named after a figure from Greek mythology who was transformed into a weasel, symbolizing the fossil's significance as representing the origin of the weasel family and the lineage leading to modern species," said senior author Alberto Valenciano, assistant professor of paleontology at Complutense University of Madrid.
The fossils come from excavations carried out in the 1990s in the Teruel area of Aragón, Spain.
"This research is a clear example of the remarkable richness of Aragón's fossil record of mammals, recognized worldwide," said co-author Daniel De Miguel, professor at the University of Zaragoza. "Our team has been contributing for decades to excavations and the study of fossil mammals."
The study also revises the classification of another fossil of a similar age discovered in China. This fossil has now been assigned to the genus Zdanskyictis.
The next step, the researchers said, will be to find new fossils that help reconstruct in greater detail the early evolution of weasels and their relatives.
"Ideally, we will find an entire skeleton of a fossil weasel," Law said. "That way we can actually quantify just how elongate these ancient weasels were and when body elongation actually evolved."
A full list of co-authors and funding is included with the paper.
