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 . But a fossil discovered in Teruel, Spain, has doubled that estimate, dating back to the late , around 6.5 million years ago.

The research team, including , a ÌìÃÀÓ°ÊÓ´«Ã½ 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 or Mustela nivalis. Much like the modern weasel, G. baskini was also likely a carnivore, based on its teeth.

The team 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 , 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 , 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 .

For more information, contact Law at cjlaw@uw.edu.Ìý

Adapted from a release from Complutense University of Madrid.

The ancestors of our furry cats and dogs once looked similar to today’s modern mongoose, a mammal with a long body and small, round ears. In fact, all members of the , which includes a variety of mammalian species, such as bears, wolves and even seals, evolved from these ‘mongoose-like’ creatures.

How did such a variety of body shapes emerge from one body type? New research led by the ÌìÃÀÓ°ÊÓ´«Ã½ suggests that two different climate transitions millions of years ago fueled this change.

The team, led by , a UW principal research scientist in biology, studied the skeletal shapes of more than 850 carnivoran specimens held at 17 different natural history museums. The specimens include almost 200 different species of carnivorans: 118 that currently exist and 81 that are extinct.

The researchers found that the Eocene-Oligocene Transition, which took place around 34 million years ago, led to changes in body shape between different carnivoran families — such as between cats and dogs. Then the Mid-Miocene Climate Transition, which took place around 15 to 13 million years ago, led to changes within families — such as changes between canid species.

The team Dec. 16 in Proceedings of the Royal Society B.

UW News reached out to Law, who is also an affiliate curator at the UW Burke Museum of Natural History and Culture, to learn more about these results and what they mean for carnivorans today.

Can you talk about the significance of these results?

Chris Law: Major transitions in climate can lead to tremendous changes in biodiversity on Earth. Here, we found that climate and environmental transitions over the past 56 million years facilitated the diversification of modern carnivorans and their body forms.

Before these climate transitions, early carnivorous mammals occupied most niches as the top predators and therefore prevented the ancestors of modern carnivorans from exhibiting much body shape diversity. But climate transitions contributed to the extinction of these early carnivorous mammals, releasing the ancestors of modern carnivorans from these constraints and enabling them to exploit new environments and resources. Thus, climate transitions enabled the ancestors of modern carnivorans to increase their phenotypic diversity and fill these new niches. Our work shows how the radiation of carnivorans — and probably other animal groups — occurs in sequential evolutionary phases triggered by multiple climatic and environmental transitions.

What was happening climate-wise during the Eocene-Oligocene and the Mid-Miocene Climate transitions?

CL: The Eocene-Oligocene Transition, which lasted for about 500,000 years, was characterized by plummeting global temperatures and the appearance of the first Antarctic ice sheets. The Earth’s climate transitioned from a warm ‘greenhouse’ with relatively consistent temperatures to a cooler, temperate ‘icehouse’ with increased seasonality, all of which led to habitat transitions from warm humid forest to dry temperate forests interspersed with grasslands.

The Mid-Miocene Climate Transition, which lasted around 2 million years, can also be characterized as another major period of rapid temperature decline, increased aridity and enhanced seasonality, which in turn facilitated further trends toward grasslands from forest habitats.

Why do you think one transition led to diversification between families and the other led to diversification within families?

CL: The Eocene-Oligocene Transition was the first release point for modern carnivorans. It eliminated most competing early carnivorous mammals and allowed early modern carnivorans to exploit these novel habitats, resources and other opportunities. These led to the appearance of all modern carnivoran families from the Early Oligocene to the Mid-Miocene.

The onset of the Mid-Miocene Climate Transition created even more novel habitats and resources, giving modern carnivorans even more opportunities to further diversity and exploit the new ecological niches during the Late Miocene to the Pleistocene. And the lack of other competing carnivorous groups may have helped fuel this period of diversification. As niche space is filled to capacity, additional skeletal diversification and evolution of skeletal innovations within families may have also been necessary to help partition species that are ecologically similar to each other.

Can you give some examples of some of the mammals that reside in the order Carnivora?

CL: Modern carnivorans are very phenotypically diverse. They range from dogs and cats to small elongated weasels and robust bears. Seals, sea lions and walruses are also carnivorans even though they spend the majority of their time in water and have flippers.

Extinct pan-carnivoran groups also include animals like saber-tooth cats, hyena-like dogs and bear-dogs — dog-like animals the size of bears.

There are also some surprises: Pandas, red pandas and kinkajous all belong in the carnivoran lineage even though they are not carnivorous.

Why did modern carnivorans all start out with mongoose-like body plans?

CL: As far as we know, the mongoose body plan is a very generalized body form. That is, they are not specialized to eat a specific food or move in a certain way, unlike a specialized runner like a cheetah or wolf, a specialized digger like a badger or a specialized climber like a panda. An issue with being a specialist over evolutionary time is that you may be prone to extinction if your resources or habitat change. Thus, being a generalist can be evolutionary advantageous.

So did mongooses just not change that much over time?

CL: Most likely, the mongoose — and the similarly shaped civets — retained their body types from the early carnivorans. It’s the other carnivoran groups like felids, canids and ursids that are the weird ones, because they evolved different body forms from the generalized mongoose body plan.

Do these findings have any bearing on our current understanding of these species and on our current climate situation?

CL: This study shows how major climate transitions can have profound impacts on the evolution of one group of mammals. For example, climate transitions can be detrimental to one group, leading to extinction, but can be advantageous to another group by eliminating competitors, which creates new habitats and facilitates diversification. So in the present, anthropogenic climate change may lead to the extinction of some species but we could see others take advantage of it.

at the National Research Center on Human Evolution and at the University of California, Berkeley are also co-authors on this paper. This research was funded by the National Science Foundation, a University of Texas early career provost fellowship, an Arthur James Boucot research grant through the Paleontological Society, a Vertebrate Paleontology Collections Study grant through the Burke Museum and the European Research Council within the European Union’s Horizon Europe.

For more information, contact Law at cjlaw@uw.edu.