Oldest skeleton of a fossil flying squirrel casts new light on the phylogeny of the group

摘要

Mammals can walk, hop, swim and fly; a few, like marsupial sugar gliders or colugos, can even glide. With 52 species scattered across the Northern hemisphere, flying squirrels are by far the most successful group that adopted this way of going airborne. 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, the digging of a dump site in Barcelona unearthed a peculiar skeleton first a tail and two thigh bones, big enough that the researchers thought it could be the fossil of a small primate. In fact, and much to the disappointment of paleoprimatologists, further excavating revealed that it was a rodent. As the specimen – nearly an entire skeleton – was being prepared, paleontologists insisted that all the ‘dirt’ attached to the bones had to be carefully screen-washed. From the mud emerged the minuscule specialized wrist bones the primate-turned-rodent was in fact Miopetaurista neogrivensis, an extinct flying squirrel. Here, Casanovas-Vilar et al. describe the 11.6 million years old fossil, the oldest ever found. The wrist bones reveal that the animal belongs to the group of flying squirrels that have large sizes. Evolutionary analyses that combined molecular and paleontological data demonstrated that flying squirrels evolved from tree squirrels as far back as 31 to 25 million years ago, and possibly even earlier. In addition, the results show that Miopetaurista is closely related to Petaurista, a modern group of giant flying squirrels. In fact, their skeletons are so similar that the large species that currently inhabit the tropical and subtropical forests of Asia could 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. Discovering older fossils, or even transitional forms, could help to retrace how flying squirrels took a leap from the rest of their evolutionary tree.