Fossil flying squirrel

They say that where there’s trash there’s treasure; and in the case of the next story that’s certainly true, because the excavation of a new rubbish dump nearby gave Isaac Casanovas-Vilar the world’s first intact skeleton of a flying squirrel. As he explains to Chris Smith, he’s been able to use the specimen to settle a long-standing dispute between molecular biologists - who said these were recent evolutionary spin-offs, and palaeontologists, who claimed flying squirrels are much more ancient…

Isaac - This started with the building of a rubbish dump of a landfill 40 kilometres outside Barcelona in 2008 and many fossils have been recovered from these landfill, including primate specimens, and they found a block with a couple of femora; and it was taken to our preparation lab because they thought, initially, it was a primate. But when preparing they found that it was a rodent - a very large rodent!

Chris - So they were disappointed on one hand but then excited on the other because this was something very unusual?

Isaac - Yeah yeah yeah. But the beginning the only one that was excited was me, because it was - wow - it's a large size rodent with very slender limb bones. What can this be? And we had found some dental remains of flying squirrels. So we initially thought that this limb bones could fit with a large-sized flying squirrel. And when preparing the block we found a skull with those teeth as well. So then we were for sure that was the skeleton of a flying squirrel. This is the first time, and the only time we know, that there was a skeleton so we could have a complete picture of of the animal.

Chris - Why is this one so well preserved and so intact compared to every other specimen ever uncovered?

Isaac - Aha. That's because it was buried extremely fast. The animal died and it was not scavenged at all and it was covered by a mud flow that, with time, became mudstone. So the animal is intact with the bones - some of the bones even preserved in the original anatomical position - so the femurs are still attached to the pelvis the tail is still attached to the pelvis.

Chris - Now what did we know before you found this particular specimen? What did we know about the family tree, if you'll excuse the pun, of flying squirrels?

Isaac - Yeah, well, there was a major disagreement between palaeontologists and molecular biologists: molecular biologists studying the DNA of extant squirrel species found the flying squirrels originated from tree squirrels very recently - about 23 million years ago; whereas palaeontologists reported flying squirrels as old as thirty six million years ago. So that was a major disagreement - of more than 10 million years between - palaeontological and molecular data.

Chris - So how did this new specimen change things?

Isaac - Well, to tell if a squirrel actually glides or not you need the bones: you need the bones of the skeleton, particularly the wrist bones, which tells you if the animal had this flying membrane - that we call a patagium - to glide from tree to tree.

Chris - These bones of the little projections off the wrist aren't they - they're like an extra digit that helps to extend that membrane to create the gliding surface?

Isaac - Well it's actually a cartilage. So it's the same as your nose; the cartilage supporting the gliding membrane that is attached to a particular wrist bone. And then, when the flying squirrel jumps, it moves the hands - it rotates the hands inwards - and the flying membrane is extended so it can glide for many metres - sometimes more than 100 metres until the next tree.

Chris - So if you had that particular wrist appendage, you'd know what you were dealing with unequivocally?

Isaac - Yeah sure sure sure. So we put an additional effort to recover these small wrist bones. These are a few millimetres long. So when preparing the skeleton we insisted that all the sediment attached to the bones had to be screen-washed in order to recover the smallest fragment.

Chris - So what does that mean putting it through a sort of sieve so that you make sure you don't lose anything?

Isaac - Yeah yeah. That's it. All the sediment you remove, you put it through a sieve and then you wash it with the little bit of water. And, when sorting, we've found two of the wrist bones that turned out to be two of the three that support the cartilage of the potagium, so we were extremely happy that day, because it was "we did it", It was Eureka moment! We found them!

Chris - So that gives you the diagnosis. What about dating it. Can you date the block and work out therefore the context in which this animal is and therefore where in the timeline it sits?

Isaac - Yeah yeah. We have very accurate datings using techniques based on changes in magnetic polarity that gives you a precision of one hundred thousand years in the dating, so we know it is eleven point six million years ago. Knowing that, if we put it in a family tree with the extant flying squirrels then we can recalculate the dates of divergence of the different squirrel species and families and groups.

Chris - And when you do that how old does it turn out that this family tree is, because you were saying there's a bit of a controversy between one school of thought based on palaeontological findings and one school of thought based on genetic molecular findings and they disagree. So, actually, what is the correct answer based on what this fossil tells you?

Isaac - This isn't going to be the definitive answer, but for the moment we get older ages than the molecular biologists; we found that 23 million years ago is a possible age. But an age as old as 31 million years old for those splitting between the two lineages is as likely as that one..