An international team of paleontologists has found the world’s oldest flying squirrel fossil — an 11.63-million-year-old specimen of an extinct species called Miopetaurista neogrivensis — at the Abocador de Can Mata site in Barcelona, Catalonia, Spain.

Mammals can walk, hop, swim and fly; a few, like marsupial sugar gliders or colugos, can even glide.

Flying squirrels are the only group of gliding mammals to have achieved a significant diversity (52 species in 15 genera) and wide geographical distribution across Eurasia and North America.

To drift from tree to tree, these small animals pack their own ‘parachute:’ a membrane draping between their lower limbs and the long cartilage rods that extend from their wrists. Tiny specialized wrist bones, which are unique to flying squirrels, help to support the cartilaginous extensions.

The origin of flying squirrels is a point of contention: while most genetic studies point towards the group splitting from tree squirrels about 23 million years ago, the oldest remains — mostly cheek teeth — suggest the animals were already soaring through forests 36 million years ago.

However, recent studies show that the dental features used to distinguish between gliding and non-gliding squirrels may actually be shared by the two groups.

In 2002, Dr. Isaac Casanovas-Vilar of the Universitat Autònoma de Barcelona and colleagues unearthed a peculiar skeleton: first a tail and two thigh bones, big enough that the paleontologists thought it could be the fossil of a small primate.

“Due to the large size of the tail and thigh bones, we initially thought the remains belonged to a primate,” Dr. Casanovas-Vilar said.

“In fact, and much to the disappointment of paleoprimatologists, further excavation revealed that it was a large rodent skeleton with minuscule specialized wrist bones, identifying it as Miopetaurista neogrivensis.”

Combining molecular and paleontological data to carry out evolutionary analyses of the fossil, the researchers demonstrated that flying squirrels evolved from tree squirrels as far back as 31 to 25 million years ago, and possibly even earlier.

In addition, their results showed that Miopetaurista neogrivensis is closely related to an existing group of giant flying squirrels called Petaurista.

Their 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.

“The problem is that these ancient remains are mainly teeth,” Dr. 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.”

“In our study, we estimate 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.”

“Molecular and paleontological data are often at odds, but this fossil shows that they can be reconciled and combined to retrace history.”

“Discovering even older fossils could help to retrace how flying squirrels diverged from the rest of their evolutionary tree.”

The study is published in the journal eLife.

_____

Isaac Casanovas-Vilar et al. 2018. Oldest skeleton of a fossil flying squirrel casts new light on the phylogeny of the group. eLife 7: e39270; doi: 10.7554/eLife.39270