FURRY KAMIKAZE PILOTS: FLYING TREE SQUIRRELS

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

With 52 species scattered across the Northern hemisphere, flying squirrels are by far the most successful group that took to the skies.

While not true flyers per se, these wee marsupials know how to skedaddle, coasting from tree to tree one giant leap of faith at a time. 

And they are pretty cute if you look past their teeth and claws. Think adorable kamikaze pilot snugged inside a paper aeroplane with just enough Freddie Kruger to keep it interesting.

Their airborne manoeuvres are made possible by a half-plane, half-parachute body design — a bat-suit style membrane that puffs up into a parachute their feet and hands. They have evolved teensy but sturdy wrists strong enough to support long cartilaginous rods, making their personal brand of flight possible. Their wee specialized wrist bones are a distinctive feature as they do not share them with their non-flying brethren. 

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. 

Troublesome are the findings from studies that show the dental features used to distinguish between gliding and non-gliding squirrels may actually be shared by the two groups.

In 2002, the digging of a dumpsite in Barcelona, Spain unearthed a peculiar skeleton. As the bones were extracted one by one they found first a tail, then two thigh bones — big enough that they initially thought they might belong to a small primate. 

Once they were all removed and re-articulated, you could see that they belonged to a rodent. 

As the specimen was being prepared and the associated matrix screen-washed for loose bits, they discovered the wee wrist bones. From the mud emerged the minuscule specialized wrist bones that confirmed this was the skeleton of Miopetaurista neogrivensis, an extinct flying squirrel.

Fossil flying squirrel Miopetaurista neogrivensis

Casanovas-Vilar et al. described the 11.6 million years old Spanish fossil — the oldest squirrel ever found. The wrist bones reveal that the animal belongs to the group of flying squirrels that have large sizes. 

Evolutionary analyses — the sexy science combining molecular and paleontological data — told a new story, flying squirrels evolved from tree squirrels as far back as 31 to 25 million years ago, and possibly even earlier. 

We also confirmed that Miopetaurista is closely related to Petaurista, a modern group of giant flying squirrels. 

Their skeletons are so similar, in fact, that the large extant species that inhabit the tropical and subtropical forests of Asia today should be considered living fossils.

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

There is still more to be done to tease out the lineage of these gliding mammals. Discovering older fossils, or even transitional forms could help to retrace how flying squirrels took a leap from the rest of their evolutionary tree.

Flying squirrels are the only group of gliding mammals with remarkable diversity and a wide geographical range. However, their evolutionary story is not well known. Thus far, the identification of extinct flying squirrels has been exclusively based on dental features, which, contrary to certain postcranial characters, are not unique to them. 

While best efforts are made, fossils attributed to this clade may indeed belong to other squirrel groups. The oldest fossil skeleton of a flying squirrel (11.6 Ma) displays the gliding-related diagnostic features shared by extant forms and allows for a recalibration of the divergence time between the tree and flying squirrels. 

Phylogenetic analyses combining morphological and molecular data generally support older dates than previous molecular estimates (~23 Ma), being congruent with the inclusion of some of the earliest fossils (~36 Ma) into this clade. They also show that flying squirrels experienced little morphological change for almost 12 million years.

Image: The fossil flying squirrel Miopetaurista neogrivensis

(a) Reconstruction of the skeleton based in the partial skeleton IPS56468 from Abocador de Can Mata. Missing elements are based on extant giant flying squirrel Petaurista petaurista and are coloured in blue. 

https://doi.org/10.7554/eLife.39270.001 / https://doi.org/10.7554/eLife.39270.002