It had water resistant fur and long, broad feet.
Despite subsequent efforts to find the species, named Nilopegamys, it is now feared extinct, with the specimen — the only one ever collected — now in the collection at Chicago’s field museum.
But scientists have now discovered two new species of mouse closely related to that specimen, which are also new to science.
“These two groups of mice have been confused with one another for a century,” said Julian Kerbis Peterhans, one of the paper's authors and a researcher at the Field Museum who's studied these rodents for over 30 years.
“They've been so elusive for so long, they're some of the rarest animals in the world, so it's exciting to finally figure out their family tree.”
The researchers focused on the Nilopegamys and another group, named Colomys.
Nilopegamys means “mouse from the source of the Nile”, while Colomys's name roughly translates to “stilt mouse” due to its elongated feet which let it wade in shallow streams to hunt for water-dwelling insects like caddisfly larvae.
While Nilopegamys has only been found in Ethiopia, Colomys have been found throughout the Congo Basin and into the western part of the African continent.
“It's underappreciated how little is known about the biodiversity of small mammals, especially in tropical parts of the world.
We're not discovering a whole lot of new lions, tigers, and bears, but there's an incredible potential for discovery of new species of small mammals because they're tough to find,” said Tom Giarla, the paper's lead author and an assistant professor of biology at Siena College in New York.
“They're sort of underappreciated animals - they're really cool when you start to learn about their ecology.
“These are semi-aquatic mice, so they're not just your average, everyday rodents.”
Professor Peterhans said the mouse’s features made it perfectly adapted to river life.
He said: “These mice are long-footed, kind of like a kangaroo. They sit up on their haunches, and they wade through shallow streams with their whiskers on the water's surface detecting movements, like sonar.
“They have unusually large brains in order to process this sensory information from their whiskers when they hunt.”
He added: “When I caught my first one some 30 years ago, it was the most beautiful African mouse I'd ever seen, it had water repellent fur that's very thick and lush and warm and cozy. They're incredibly soft, and they have this remarkable snow-white belly.”
For the researchers in the field, the mouse’s habitat does not make them easy to catch.
The species prefers shallow streams so that they can use their whiskers to help them hunt, but they've also been found in swampy areas and even rivers that are 3-4 feet deep in places, where they seek out the shallow edges.
“To cross one of the rivers where I caught a Colomys, you have to use walking sticks, the water's up to your waist,” said Terry Demos, a postdoctoral researcher at the Field Museum and another of the paper's authors.
“And you can have torrential rain in the tropics, so sometimes half the traps get swept away, and you have to go downriver to try to find them.”
The researchers evaluated the Colomys throughout its broad range, and also drew on new field work and museum collections.
They compared the animals’ physical traits and analysed their DNA.
The analyses revealed that within the Colomys genus, there were two new species that had not yet been described. They've been named Colomys lumumbai and Colomys wologizi, after, respectively, Congolese independence leader Patrice Lumumba and Liberia's Wologizi Mountains.
The researchers also found that one subspecies actually constituted its own separate species, and revised the range of another species.
Dr Giarla was also able to extract DNA from a piece of dried tissue on the skull of the 93-year-old specimen of Nilopegamys in the Field's collections.
“When you're working with ancient DNA or antique DNA you have to treat it differently. There can't be any contaminating DNA present, because that could ruin your whole study,” he said.
“I was stunned that I actually got it to work on my first try.”
The DNA showed that Nilopegamys is a sister genus to Colomys, and was its closest relative.
“The new species we named are part of a global effort to understand the biodiversity of African rainforests and highlight the critical areas to be preserved,“ said Dr Demos.
“There are vast areas of the Congo Basin that have barely been explored in the last seventy years, places that are hard to access due to political instability. We're not even completely sure how these animals are distributed, there are big gaps.”
The findings could even help inform public health efforts down the line, the researchers suggested.
“Covid is a zoonotic disease, and biodiversity research is essential to understanding zoonotic disease,” said Dr Giarla.
“We need to understand what species are present in natural areas, especially natural areas being changed by humans.”
The research is published in the Zoological Journal of the Linnean Society.