"Due to the large size of the tail and thigh bones, we initially thought the remains belonged to a primate," said Isaac Casanovas-Vilar from the Institut Catala de Paleontologia Miquel Crusafont (ICP) in Spain.
Further excavation revealed that it was a large rodent skeleton with minuscule specialised wrist bones, identifying it as Miopetaurista neogrivensis -- an extinct flying squirrel.
Combining molecular and paleontological data to carry out evolutionary analyses of the fossil, Casanovas-Vilar and the team demonstrated that flying squirrels evolved from tree squirrels as far back as 31 to 25 million years ago, and possibly even earlier.
Their results showed that Miopetaurista is closely related to an existing group of giant flying squirrels called Petaurista.
The skeletons are in fact so similar that the large species that currently inhabits the tropical and subtropical forests of Asia could be considered living fossils, researchers said.
With 52 species scattered across the northern hemisphere, flying squirrels are the most successful group of mammals that adopted the ability to glide.
To drift between trees in distances of up to 150 metres, these small animals pack their own 'parachute': a membrane draping between their lower limbs and the long cartilage rods that extend from their wrists.
Their tiny, specialised wrist bones, which are unique to flying squirrels, help support the cartilaginous extensions.
However, the origin of these animals is highly debated. While most genetic studies point towards the group splitting from tree squirrels about 23 million years ago, some 36-million-year-old remains that could belong to flying squirrels have previously been found.
"The problem is that these ancient remains are mainly teeth," Casanovas-Vilar said.
"As the dental features used to distinguish between gliding and non-gliding squirrels may actually be shared by the two groups, it is difficult to attribute the ancient teeth undoubtedly to a flying squirrel," he said.
The study estimates that the split took place around 31 and 25 million years ago, earlier than previously thought, suggesting the oldest fossils may not belong to flying squirrels.
"Discovering even older fossils could help to retrace how flying squirrels diverged from the rest of their evolutionary tree," Casanovas-Vilar said.
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